Title

A dileucine motif targets E-cadherin to the basolateral

cell surface in MDCK and LLC-PK1 epithelial cells.

Authors

Kevin C. Miranda 1,2, Tatiana Khromykh 1, Perpetina Christy 1, Tam Luan Le

1,2, Cara J.Gottardi 4, Alpha S.Yap 1,3, Jennifer L.Stow 1,2 and Rohan D.Teasdale

1,*.

Institutions

1. Institute for Molecular Bioscience, University of Queensland, Brisbane 4072 QLD,

Australia.

2. Department of Biochemistry, University of Queensland, Brisbane 4072 QLD, Australia.

3. Department of Physiology & Pharmacology, University of Queensland, Brisbane 4072

QLD, Australia.

4. Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA

* To whom correspondence should be addressed at

Institute for Molecular Bioscience, University of Queensland, Brisbane 4072 QLD,

Australia.

phone (61) 7 3365 8242

fax (61) 7 3665 4388

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Copyright 2001 by The American Society for Biochemistry and Molecular Biology, Inc.

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e-mail r.teasdale@imb.uq.edu.au

Running Title

Basolateral targeting of E-cadherin

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Summary

E-cadherin is a major adherens junction protein of epithelial cells, with a central role in cell-

cell adhesion and cell polarity. Newly-synthesised E-cadherin is targeted to the basolateral cell

surface. We analysed targeting information in the cytoplasmic tail of E-cadherin by utilising

chimeras of E-cadherin fused to the ectodomain of the IL-2α receptor expressed in MDCK and

LLC-PK1 epithelial cells. Chimeras containing the full-length or membrane-proximal half of

the E-cadherin cytoplasmic tail were correctly targeted to the basolateral domain. Sequence

analysis of the membrane-proximal tail region revealed the presence of a highly-conserved

dileucine motif which was analysed as a putative targeting signal by mutagenesis. Elimination of

this motif resulted in the loss of Tac/E-cadherin basolateral localisation, pinpointing this

dileucine signal as being both necessary and sufficient for basolateral targeting of E-cadherin.

Truncation mutants unable to bind β-catenin were correctly targeted, showing, contrary to

current understanding, that β-catenin is not required for basolateral trafficking. Our results also

provide evidence that dileucine-mediated targeting is maintained in LLC-PK1 cells despite the

altered polarity of basolateral proteins with tyrosine-based signals in this cell line. These results

provide the first direct insights into how E-cadherin is targeted to the basolateral membrane.

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Introduction

E-cadherin is expressed on the lateral membranes of epithelial cells where it accumulates as a

major component of the adherens junction. The cadherins in adherens junctions have central

roles in establishing and maintaining cell-cell adhesion and cell polarity in epithelia and

participate in morphogenesis during development (1-4). The continual expression and function

of E-cadherin is important in its role as a tumour suppressor and the loss of E-cadherin function

contributes to tumour invasion and progression in carcinomas (5).

Mature, human E-cadherin is a 728 amino acid, single pass transmembrane protein (6). The E-

cadherin ectodomain is involved in Ca2+-dependent homotypic binding to E-cadherins on

adjacent cell membranes and its cytoplasmic tail is involved in a series of protein interactions

providing a link to the actin cytoskeleton. The cytoplasmic tail of E-cadherin is bound directly

to β-catenin or plakoglobin, and thereby to α-catenin and actin (reviewed in 2). The binding

site for β-catenin has been mapped by deletion mutagenesis to the distal 76 amino acids of the

carboxy terminus of E-cadherin (7), with critical residues found in the last 30 amino acid

domain (8). Aside from its participation as a member of the cadherin-bound adherens junction

complex, β-catenin is involved in the Wnt signalling pathway through its interactions with a

cytoplasmic APC* complex and with the Lef/Tcf transcription factors in the nucleus (reviewed

in 9).

Epithelial cells are morphologically and functionally polarised with distinct complements of cell

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surface proteins and lipids at the apical or basolateral poles. Maintenance of this polarity requires

that newly-synthesised proteins are sorted and targeted to specific membrane domains (10-12).

Sorting of proteins to the apical domain of polarised cells occurs via lipid raft interactions or

through oligosaccharides (13-15), while specific amino acids motifs act as sorting signals to

target membrane proteins to the basolateral cell surface (reviewed in 16). For example,

tyrosine-based motifs target the LDL receptor to the basolateral domain of polarised cells (17),

while a dileucine motif is utilised by other proteins, like the Fc Receptor (18). Both types of

signals can also be used to direct endocytosis from the plasma membrane, although the structural

requirements for each pathway may differ (19-22). Yet other amino acid motifs can be used by

selected proteins for basolateral targeting (23-25). Previous studies on adhesion molecules have

defined a dileucine signal that functions in the basolateral targeting of the Lutheran glycoprotein

(26) and a similar dihydrophobic signal in CD44 (27). The basolateral targeting of N-CAM has

been attributed to a cytoplasmic tail sequence without classical motif homology (23). Detailed

information and further insights are needed into how other classes of adhesion proteins,

including cadherins, are sorted and trafficked in polarised cells.

Recently, the polarised epithelial cells of the pig kidney LLC-PK1 line have been distinguished

as having aspects of inverted polarity. Roush et. al. (1998) (28) noted that some typically

basolateral proteins, such as the H,K-ATPase β subunit, are delivered to the apical pole of LLC-

PK1 cells. Mis-sorting of basolateral proteins in LLC-PK1 cells led to subsequent analysis of

their AP-1 adaptor complexes. Most polarised epithelial cells express a special µ1B subunit

which directs polarised sorting to the basolateral surface (29). It has now been shown that LLC-

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PK1 cells express a µ1A subunit but not the µ1B subunit, and that this results in aberrant sorting

of proteins with tyrosine-based signals, a defect that can be overcome by expression of

recombinant µ1B protein (30). Cadherin staining in LLC-PK1 cells appears to be basolateral

(31) although the trafficking of these proteins in this cell line has not been studied in detail.

The correct placement of E-cadherin on the plasma membrane is required from an early stage to

help establish and maintain cell polarity (32). The mechanisms that mediate the sorting and

polarised delivery of E-cadherin to the surface have not been elucidated. Previous studies have

shown that β-catenin binds to E-cadherin early in the biosynthetic pathway, implying that the

two proteins, and perhaps others, are transported to the cell surface together as a complex (33).

More recently it was suggested that β-catenin plays an essential role in the trafficking of E-

cadherin, based on observations that mutagenised proteins, with reduced binding to β-catenin,

were not efficiently delivered to the surface (34). It has also been previously noted that the

cytoplasmic tail of E-cadherin does contain motifs with homology to known targeting signals

(34,35) which could potentially function to guide its trafficking. In this study we set out to test

putative signals in the cytoplasmic tail of E-cadherin for possible basolateral targeting

information. Our experiments also addressed the role of β-catenin in this targeting. We utilised

a series of chimeras to express the E-cadherin cytoplasmic tail, or mutagenised versions thereof,

using Tac (the α subunit of the IL-2 receptor) as an ectodomain marker. Tac is a 273 amino acid

protein that has previously been used as a reporter protein for trafficking studies (36,37). These

constructs were expressed in MDCK cells, in which the basolateral surface expression of E-

cadherin is well established, and in another epithelial cell line LLC-PK1 cells. Using these

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model systems we have identified a positive targeting signal in E-cadherin which is responsible

for basolateral delivery. Our results provide new insights into the trafficking of E-cadherin and

its accessory proteins (catenins) and the findings are also significant to our understanding of cell

polarity and sorting pathways in epithelia.

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Experimental Procedures

Cell culture

Madin-Darby canine kidney (MDCK) and pig kidney (LLC-PK1) cell lines were grown and

passaged as described previously (38) in Dulbecco’s modified Eagle’s medium (GibcoBRL,

Grand Island, NY) supplemented with 10% fetal calf serum and 2 mM glutamine in 5% CO2 and

95% air.

Antibodies

Mouse monoclonal antibodies (Transduction Laboratories, Lexington, KY) raised against a

conserved region of the cytoplasmic domain of human E-cadherin and against β-catenin were

used; Tac was recognized using a mouse monoclonal antibody (B-B10, Biosource International,

Camarillo, CA) and a rabbit polyclonal antibody directed against the green fluorescent protein

(GFP) (Molecular Probes Inc, Eugene, OR) was also used for staining. Secondary antibodies

included Cy3-conjugated sheep anti-mouse and goat anti-rabbit IgGs (Jackson

ImmunoResearch Labs, West Grove, PA) and a HRP-conjugated sheep anti-mouse IgG

(AMRAD, Victoria, Australia).

cDNA construction and expression.

Chimeric fusions between human E-cadherin cDNAs and the cDNA encoding for Tac were

generated in the pCDNA3 expression vector. Molecular cloning techniques were performed

according to Sambrook et. al. (39), using reagents from New England Biolabs (Beverly, MA).

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All constructs were confirmed by DNA sequencing. cDNAs encoding the transmembrane plus

cytoplasmic domains (residues 554-728), the cytoplasmic tail (residues 578-728), or the amino

terminal half of the cytoplasmic tail (578-653) of human E-cadherin, were amplified using PCR

with specific oligonucleotide primers. E-cadherin residue numbers correspond to the mature

protein as defined previously (6). The oligonucleotide primers contained restriction

endonuclease sites allowing generation of in-frame fusions with the Tac cDNA. Cloning the

respective PCR products into pCMV-IL2R (40) using the HindIII and XbaI sites generated the

pCMV-Tac/Ecad(578-728) and pCMV-Tac/Ecad(578-653) plasmids. The entire Tac/E-

cadherin cDNA constructs were then subcloned into the pCDNA3 expression vector (Invitrogen,

Carlsbad, CA) that also encodes for the neomycin selection marker. The resulting constructs

were termed Tac/Ecad(578-728) and Tac/Ecad(578-653). To generate the Tac/Ecad(554-728)

plasmid, an EcoRV restriction endonuclease site was introduced using PCR at the end of the

extracellular domain of the Tac cDNA within pCDNA3. This modification altered residue 239 of

the Tac cDNA from an Asp to a Gln. The final plasmid was generated by cloning the E-cadherin

PCR product into the EcoRV and XbaI sites.

E-cadherin-GFP encodes the full-length E-cadherin sequence with the green fluorescent

protein (GFP) fused to the carboxy terminus of the cytoplasmic domain. Initially, a SacII

restriction endonuclease site was introduced at the carboxy-terminus of the full-length cDNA of

E- cadherin. This was achieved by the PCR amplification of the entire E-cadherin cDNA using

specific oligonucleotide primers using pCDNA3-hECad (41) as a template. The 3’ primer

included the SacII site and a XbaI site. The resulting PCR product was digested with SgrA1 and

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XbaI and subcloned into pCDNA3-hECad using the same sites. The open reading frame of GFP

was amplified by PCR using specific oligonucleotide primers and pEGFP-N1 (Clontech, Palo

Alto, CA) as a template. The 5’ primer contained a SacII site that allowed the resulting PCR

product to be cloned into the SacII site introduced into in the carboxy-terminus of E-cadherin.

Mutagenesis of the two leucine residues at positions 587 and 588 of human E-cadherin to

alanines was performed using oligonucleotide cloning. The construct pCMV-Tac/Ecad(578-

728) was digested with HindIII and XmaI to remove a 55 bp fragment that included the codons

for the di-leucine residues. The digested plasmid was then ligated with the annealed,

phosphorylated, synthetic oligonucleotides 5’AGCTTCGTCGACGCGCGG

TGGTCAAAGAGCCCGCAGCACCCCCAGAGGATGACAC3’ and 5’CCGGGTGTCATC

CTCTGGGGGTGCTGCGGGCTCTTTGACCACCGCGCGTCGACGA3’. These

complementary oligonucleotides contain a 5’ HindIII end and a 3’ XmaI end, as well as codons

encoding for LRRRAVVKEPAAPPEDD (altered residues underlined). The resulting construct

was termed Tac/Ecad(578-728)∆S1.

For transfection and expression of cDNAs, sub-confluent LLC-PK1 or MDCK cells were

transfected with plasmid DNA (2µg) in complex with Lipofectamine Plus Reagent (Gibco/BRL,

Grand Island, NY) according to manufacturer’s guidelines. For stably-expressing lines,

transfected cells were passaged and maintained in media containing G418 (Geneticin

(Gibco/BRL, Grand Island, NY)); cells were kept under selection for 7-10 days and then plated

at low density for ring cloning of surviving cells. Clonal cell lines were generated and then

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assessed by indirect immunofluorescence and immunoblotting to select lines with different levels

of recombinant protein expression.

Indirect immunofluorescence

Confluent monolayers of cells grown on glass coverslips or on Transwell polycarbonate filters

(Corning Costar, Cambridge, MA) were generally fixed in 4% paraformaldehyde in PBS for 90

min and then permeabilised in PBS containing 0.1% Triton X-100 for 5 min. In one experiment

LLC-PK1 cells were fixed in ice cold methanol for 10 mins. Cells were then incubated

sequentially with primary antibody (1 h) and then secondary antibodies (30 min) using PBS

containing BSA (Sigma Chemical Co, St. Louis, MO) as a blocking buffer. Cells were mounted

on slides in PBS/glycerol (50/50) containing 1% n-propyl-galate. For some experiments, cells

were treated with 10µM cycloheximide (Sigma Chemical Co, St. Louis, MO), which was added

to the medium for various times up to 4 h prior to fixation. Cells on coverslips were examined

by epifluorescence using an Olympus Provis AX-70 microscope and images were collected with

a CCD300ET-RCX camera (DageMTI, Michigan City, IN) using NIH Image software. Cells

growing on Transwell filters were examined using a Bio-Rad MRC-600 confocal laser-

scanning microscope mounted on a Zeiss Axioskop and XY and XZ sections were generated

using Bio-Rad MRC-600 CoMOS software.

Immunoprecipitation and immunoblotting.

Confluent monolayers of transfected MDCK and LLC-PK1 cells were solubilised in cold RIPA

buffer (1% Triton X-100, 1% deoxycholate, 0.1% SDS, 0.15 NaCl, 5mM EDTA, 25mM Tris-

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HCl, pH7.4) containing protease inhibitors (Boehringer Mannheim, Germany) on ice. Post-

nuclear supernatants were incubated with the Tac antibody for 2 h and then with washed protein

G beads (Sigma Chemical Co, St. Louis, MO) for a further 2 h. Precipitates were recovered by

centrifugation then washed through several rounds of RIPA buffer and 20mM Tris-HCl (pH 7.4)

prior to solublisation in concentrated SDS-PAGE sample buffer. Proteins in cell extracts and

immunoprecipitates were separated on 8% SDS-PAGE reducing gels and then transferred to

PVDF Immunobilon-P membranes (Millipore, Bedford, MA) and stained with 0.1% Coomassie

brilliant blue to ensure even protein transfer and protein loading. Membranes were

immunoblotted by sequential incubations in primary antibody, horseradish peroxidase-

conjugated secondary antibody followed by chemiluminescent detection with Supersignal West

Pico (Pierce Chemical Co, Rockford, IL). Different luminescence exposures were collected and

exposures in the linear range were used.

Surface Biotinylation

Confluent monolayers of LLC-PK1 cells stably expressing Tac/Ecad(578-728) or

Tac/Ecad(578-653) grown on filters were incubated in media containing 1.5mg/mL Sulfo-

NHS-SS-biotin (Pierce Chemical Co, Rockford, IL), applied to either the apical or basal side of

the filter, for 60 min on ice. Filters were then washed several times in cold PBS before cells

were scraped off and lysed in cold RIPA buffer. Soluble cell fractions were incubated with

streptavidin beads (Sigma Chemical Co, St. Louis, MO) in RIPA buffer pH 7.4 for 2h with

rotation. Beads were then washed in several rounds of RIPA buffer and 20mM Tris-HCl (pH

7.4). Biotinylated proteins bound to the streptavidin beads and unlabelled proteins in the

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supernatants were analysed by SDS-PAGE, immunoblotting and densitometry to quantitate the

relative amounts of biotinylated Tac/Ecad proteins.

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Results

Basolateral targeting of E-cadherin.

E-cadherin is delivered to the basolateral surface of polarised MDCK cells, where it gives a

typical and widely-documented staining pattern using specific antibodies (Fig. 1a). The same

antibody did not stain E-cadherin in paraformaldehyde-fixed LLC-PK1 cells (Fig. 1c), but it

did produce cell surface staining of E-cadherin in methanol-fixed LLC-PK1 cells (31 and Fig.

1b). Hence E-cadherin is expressed endogenously in both cell lines and is found in a polarised

distribution. A tagged construct of human GFP-E-cadherin was expressed in MDCK cells,

generating a clear basolateral surface staining pattern with GFP antibodies, showing that GFP-

E-cadherin is targeted in a manner analogous to the endogenous protein (Fig. 1d). GFP-E-

cadherin was also expressed in epithelial LLC-PK1 cells, where it was also targeted in a

polarised fashion to the basolateral membrane (Fig. 1e).

Targeting of Tac/E-cadherin chimeras.

For targeting studies we utilised chimeras consisting of the ectodomain of Tac fused to the

cytoplasmic tail of E-cadherin (Fig. 2A). Chimeric cDNAs expressed in epithelial cells all

produced proteins of the expected molecular masses (Fig. 2B). Tac typically localises to the

apical membrane in polarised cells (36 and see Fig. 3a and 3b), therefore any basolateral signal

in the E-cadherin cytoplasmic domain is predicted to redirect Tac from apical to basolateral

membranes. cDNAs for Tac alone or chimeric proteins were transfected into MDCK and LLC-

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PK1 cells and clonal, stably-transfected cell lines were selected. Antibodies against Tac were

used to detect the chimeric proteins and determine their localisation by indirect

immunofluorescence and confocal microscopy.

Chimeras containing the full cytoplasmic tail of E-cadherin were expressed and found to redirect

Tac to the basolateral domain in both MDCK and LLC-PK1 cells. Both Tac/Ecad(578-728)

and Tac/Ecad(554-728), which additionally encodes the transmembrane domain of E-cadherin,

were localised by epifluorescence and by confocal imaging to the basolateral membranes of

MDCK and LLC-PK1 cells (Fig. 3). Thus the presence or absence of the E-cadherin

transmembrane domain had no effect on targeting. These results indicate that the Tac/E-

cadherin chimeras are efficiently synthesised and transported to the cell surface and that the

cytoplasmic tail of E-cadherin contains positive sorting information, capable of rerouting Tac to

a basolateral trafficking pathway. MDCK cells expressing Tac/Ecad(578-728) showed no

concomitant loss of endogenous E-cadherin staining on the basolateral surface (not shown)

suggesting that the sorting and targeting machinery in these cells has not been saturated or

subverted by the overexpressed protein.

Basolateral targeting mediated by the membrane-proximal E-cadherin tail.

As the first step in a more detailed analysis of the cytoplasmic tail of E-cadherin, a truncation

mutant was created to effectively bisect the cytoplasmic tail, leaving only the membrane-

proximal portion of the tail fused to Tac. The resulting Tac/Ecad(578-653) construct was

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expressed in MDCK and LLC-PK1 cells (Fig. 4). Immunofluorescence staining and confocal

analysis showed that it was distributed in a polarised fashion. There was no staining of apical

membranes when antibody was applied to either unpermeabilised cells (not shown) or

permeabilised cells, however, there was staining of the basolateral membranes in LLC-PK1 and

MDCK cells expressing Tac/Ecad(578-653) (Fig. 4a and 4c). Thus, chimeras containing either

the full-length tails or only the membrane-proximal tails, are trafficked similarly and have the

same polarised surface distribution.

We noted that, in cells from several different clones stably expressing Tac/Ecad(578-653), there

was intracellular staining of Tac/Ecad(578-653) in a perinuclear, Golgi-like pattern in addition

to basolateral surface staining (Fig. 4a). This pattern was not regularly seen in cell lines

expressing chimeras with full-length tails (Tac/Ecad(554-728) or Tac/Ecad(578-728)). LLC-

PK1 cells expressing Tac/Ecad(578-653) were treated with cycloheximide to stop protein

synthesis and fixed and stained at various times after addition of the drug. There was a

sequential loss of intracellular staining followed at longer times by a diminution of cell surface

staining. Figure 4 shows that after 2hrs of treatment, all of the intracellular staining had

disappeared, leaving only staining of Tac/Ecad(578-653) at the basolateral surface. From this it

was concluded that intracellular staining in these cells represents a transient accumulation of

newly-synthesised Tac/Ecad(578-653) in the biosynthetic pathway and that the membrane-

proximal chimera is transported to the basolateral membrane at a slower rate than Tac/Ecad(578-

728).

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The targeting of Tac/Ecad(578-653) was finally tested using a surface biotinylation assay to

measure and quantify its appearance on the plasma membrane domains of confluent, polarised

cells. Cell surface biotinylation was performed on cell lines stably expressing Tac/Ecad(578-

728) and Tac/Ecad(578-653). Addition of biotin reagents to the basal sides of the monolayers

labelled most of the Tac/Ecad(578-728) and Tac/Ecad(578-653) proteins, while from the apical

side almost no labelling occurred in either case (see Table 1). Together these results show that a

chimeric protein, containing only the membrane-proximal half of the E-cadherin cytoplasmic

tail, has sufficient information to direct efficient sorting and targeting to the basolateral cell

surface, albeit perhaps at a slower rate than constructs with the full-length cytoplasmic tail.

A dileucine motif is responsible for basolateral targeting of E-cadherin.

Sequence analysis of the membrane-proximal E-cadherin tail encoded by the region in

Tac/Ecad(578-653), revealed the presence of two putative targeting motifs. Sequence alignment

of members of the Type I cadherins, of which E-cadherin is the prototype, and Type II cadherins

(42), revealed that a dileucine motif at position 587 is highly conserved across species and

preserved in almost all members of the family (Fig. 5A). To test this dileucine motif for targeting

information, the leucines at positions 587 and 588 were changed to alanines in the chimeras

encoding the full-length tail and membrane-proximal tail, using oligonucleotide cloning. The

resulting mutated chimeras, termed Tac/Ecad(578-728)∆S1 and Tac/Ecad(578-653)∆S1, were

transfected into LLC-PK1 and MDCK cells and then localised by immunofluorescence (Fig.

5B). Tac/Ecad (578-728)∆S1 and Tac/Ecad(578-653)∆S1 were localised at the apical surfaces

of transfected LLC-PK1 and MDCK cells, in patterns similar to the apical Tac (Fig. 3a and 3b)

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and distinct from the basolateral non-mutated chimeras. Thus removal of the dileucine motif at

587 from the E-cadherin cytoplasmic domain resulted in a loss of basolateral targeting

information. We conclude that this motif is a positive sorting signal for the basolateral membrane

localisation of E-cadherin and that it is necessary to direct sorting. The high level of

conservation of this dileucine motif throughout the cadherins family further suggests that it has a

key functional role. A second potential targeting signal of the type NPXY is present in the

sequence of Tac/Ecad(578-653). This tyrosine-based signal at position 600 was not tested here

and is not considered a likely candidate for targeting, based on experimental data from another

study (34) and our observation that the motif is not conserved in cadherins across different

species.

Binding of β-catenin.

β-catenin is known to bind to the cytoplasmic tail of E-cadherin early in the biosynthetic

pathway and has previously been implicated in trafficking to the cell surface (34). Therefore,

Tac/E-cadherin chimeras were tested for their ability to bind to β-catenin. The interaction of

endogenous E-cadherin in MDCK cells with β-catenin was demonstrated by co-

immunoprecipitation of the two proteins using an E-cadherin antibody (Fig. 6A).

Tac/Ecad(578-728) was immunoprecipitated with the Tac antibody from extracts of transfected

MDCK and LLC-PK1 cells and β-catenin co-precipitating in the complex was detected by

immunoblotting. In extracts of both MDCK and LLC-PK1 cells, β-catenin was bound to

Tac/Ecad(578-728) (Fig. 6B). This finding suggests that the full length cytoplasmic tail in

Tac/Ecad(578-728) is correctly folded and processed in a manner conducive to effective

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trafficking and surface delivery.

The membrane-proximal Tac/Ecad(578-653) construct is missing the carboxy terminal β-

catenin binding domain, and co-immunoprecipitation experiments confirm that, as expected, β-

catenin was not co-precipitated with this truncated chimeric protein (Fig. 6B). Endogenous E-

cadherin and chimeras with full-length E-cadherin tails were able to efficiently bind and co-

immunoprecipitate β-catenin (Fig. 6A and 6B). Deletion of the 587 dileucine targeting signal

had no effect on binding of β-catenin, which was efficiently co-precipitated with

Tac/Ecad(578-728)∆S1 (Fig. 6B). The correct basolateral targeting of Tac/Ecad(578-653) in

the absence of bound β-catenin now demonstrates that β-catenin is not essential for basolateral

sorting and targeting in either MDCK or LLC-PK1 cells. The complexing of β-catenin with

newly-synthesised E-cadherin may be required for other roles in biosynthetic processing or

trafficking, for instance, the loss of β-catenin binding may account for the increased intracellular

accumulation and apparently slower trafficking of the Tac/Ecad(578-653) mutants.

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Discussion

E-cadherin is one of the proto-typical polarised membrane proteins in epithelia. It is delivered

to the basolateral membrane and is concentrated in adherens junctions where it participates in

cell-cell adhesion. In order to address how E-cadherin is trafficked and targeted in polarised

cells, we made use of Tac/E-cadherin chimeras expressed in two epithelial cell lines. Our

findings show that chimeras containing full-length or truncated cytoplasmic tails of E-cadherin

were effectively sorted and trafficked to the basolateral surface domain in MDCK and LLC-PK1

cells. Furthermore, this polarised targeting was lost after deletion of a single, dominant targeting

motif in the proximal region of the tail. Three main conclusions emerged from these

experiments; i) that a single dileucine motif at 587 is able to convey basolateral targeting

information in Tac/E-cadherin chimeras and that this signal is necessary for basolateral sorting,

ii) LLC-PK1 cells are able to correctly sort and traffic E-cadherin using a dileucine-based

mechanism, in contrast to proteins sorted by tyrosine-based signals that are mis-trafficked in

these cells, and iii) β-catenin is not required for basolateral targeting and surface delivery of E-

cadherin.

Chimeras containing the full cytoplasmic tail of E-cadherin, Tac/Ecad(578-728) and

Tac/Ecad(554-728), were efficiently targeted and trafficked to the basolateral domain of MDCK

and LLC-PK1 cells in a similar manner to that of endogenous E-cadherin or GFP-E-cadherin

in either cell line. The Tac ectodomain did not interfere with the intracellular trafficking or

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processing in the full-length tail constructs as there was no evidence of less efficient membrane

delivery or retention within the secretory pathway. The transmembrane domain has been

implicated in mediating lateral association and adhesive strength of cadherins in the plasma

membrane, without affecting surface delivery (43). Amino acid sequences within

transmembrane segments can however be involved in targeting as exemplified by the apical

targeting signal in one of the transmembrane segments of the α-subunit of the gastric parietal

H,K-ATPase (44). Our experiments directly tested the E-cadherin transmembrane domain, with

the finding that this region of the protein has no role in basolateral targeting nor in sorting or

membrane delivery of Tac/E-cadherin chimeras. Overall the results obtained with the Tac/E-

cadherin chimeras point to the cytoplasmic tail of E-cadherin as having positive basolateral

sorting information, in keeping with many other Type I membrane proteins.

The correct basolateral targeting of the Tac/Ecad(578-653) construct first indicated that the

membrane-proximal tail region alone is sufficient for membrane targeting, and that distal

regions of the tail are not required for targeting or surface delivery. In our hands, mutants with

the truncated tail of E-cadherin, Tac/Ecad(578-653), were processed, targeted and delivered to

the basolateral cell surface, albeit at a slower rate than constructs with the full cytoplasmic tail.

Transient accumulation of Tac/Ecad(578-653) in the biosynthetic pathway, seen as intracellular

staining in LLC-PK1 cells, indicated that it was trafficked less efficiently than the full length

tail. Partial accumulation of Tac/Ecad(578-653) was similarly noted in MDCK cells, although

the truncated tail mutants did eventually reach the cell surface. Our findings are in contrast to

those of a previous study in which a series of chimeras representing truncation mutants of the E-

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cadherin tail fused to a GP-2 ectodomain, were found to be very poorly trafficked (34). Only

about ten percent of the truncated GP-2 chimeric proteins in that study reached the surface of

MDCK cells and were randomly sorted, whereas the majority of the proteins were blocked or

degraded early in the secretory pathway (34). On the basis of the correct targeting and delivery

of Tac/Ecad(578-653), which contains similar tail regions to some of the mutants in the previous

study, we hypothesise that the poor processing and trafficking of those chimeras may have been

due to the influence of the GP-2 ectodomain rather than being a function of E-cadherin

trafficking or accessory proteins (see later).

The membrane-proximal tail region of E-cadherin has been implicated in a number of functions.

Cadherin clustering and adhesive strength were shown to be affected by mutations in the

membrane-proximal region of the C-cadherin tail (45). Binding of the p120ctn protein has also

been shown to occur in this region (45,46) and some of the amino acid sequences mapped by two

hybrid analysis as being specifically involved in the binding of p120ctn (47), overlap with the

dileucine targeting signal identified in this study. It is therefore likely that this membrane-

proximal region of the tail is involved in multiple, temporally regulated protein interactions that

are important, initially for E-cadherin transport, and then for adhesive function at the cell

surface. Positive sorting mediated by the Tac/Ecad(578-653) membrane-proximal tail allowed

us to focus on putative sorting signals in this region. The dileucine motif at 587 was chosen here

as a candidate targeting signal. Despite being in the non-conserved region of the tail, sequence

alignments revealed that the dileucine consensus motif is highly conserved throughout cadherins

of Type I or Type II cadherin families (42). We therefore predict that this dileucine will function

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to target all similar transmembrane cadherins to basolateral domains of polarised epithelial cells.

Cadherins in which the dileucines are not conserved include cadherins 5 (VE-cadherin) and 20.

The GPI-anchored cadherin 13 (T-cadherin), as expected, does not have a dileucine motif (48).

Our experimental results then confirmed that the dileucine motif in E-cadherin does function as

a targeting signal. Replacing the leucines with alanines in the full-length or truncated tail

constructs resulted in a complete loss of basolateral targeting. The dileucine signal in E-

cadherin has an acidic amino acid cluster on its carboxy terminal side that is highly conserved

throughout dileucine-containing cadherins and is similar to targeting motifs in other basolateral

proteins including furin (49), invariant chain (50) and LDL receptor (22). In some cases, such as

in furin, these acidic clusters have been shown to be important for the function of dileucine

signals in basolateral targeting (49). Dileucine signals functioning in endocytosis typically have

an acidic residue at the -4 position (D/EXXXLL) (21,51,52). In contrast, the cadherin dileucine

signal typically has a basic lysine or arginine in the -4 position. It is therefore unlikely that this

motif will also function as an endocytosis motif.

There are additional motifs sharing consensus with targeting signals encoded in the E-cadherin

tail. Tyrosines at two places within the tail, one being in the membrane-proximal region and

another at the carboxy terminus, were deleted in a previous study and found to have no role in

targeting of E-cadherin (34). There is a combined YXXφ tyrosine and dileucine motif (673-

677), similar in structure to the overlapping motif which has been shown to be responsible for

basolateral targeting of the pIg F receptor in MDCK cells (53). There is also a motif belonging

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to the YXXφ group, with a tyrosine at the second X position (705-708). Both of these latter

signals are adjacent to, or within, the β-catenin binding domain and are thus predicted to be

sequestered when E-cadherin is complexed to β-catenin. The possibility remains open

however, that any of these additional signals might be uncovered during dynamic protein

interactions and therefore could act as targeting signals during further trafficking of surface E-

cadherin. One or more of these signals could, for instance, target E-cadherin to clathrin-coated

vesicles for endocytosis and recycling (35) after it reaches the basolateral plasma membrane.

Overall, analysis of targeting motifs points to a dileucine signal rather than tyrosine-based

signals being responsible for the polarised sorting and basolateral delivery of E-cadherin.

Both MDCK and LLC-PK1 cell lines form polarised epithelial monolayers in culture which

show patent basolateral trafficking and secretion of soluble proteoglycans (54,55). Due to the

reported inverse polarity of LLC-PK1 (28,30), it was of interest to also analyse E-cadherin

trafficking in these cells. Our findings verify that E-cadherin is trafficked correctly to the

basolateral surface of the LLC-PK1 cells and we show that that this targeting, as in MDCK cells,

is directed by a dileucine motif. This provides new evidence to confirm that basolateral targeting

via dileucine-based mechanisms functions correctly in LLC-PK1 cells. The correct targeting of

endogenous E-cadherin, GFP-E-cadherin and Tac/Ecad constructs suggests that dileucine-

based sorting is fully sufficient to direct basolateral trafficking in these cells. LLC-PK1 cells are

defective in targeting of tyrosine-based motifs due to the absence of the µ1B chain (30).

However the sorting of dileucine motifs occurs through interaction with the β subunits of the

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adaptor complex (56), allowing correct sorting of proteins such as T cell receptor sub-unit

(CD3γ), Fc receptor and E-cadherin. The correct targeting of endogenous E-cadherin and Tac/Ecad

constructs in LLC-PK1 cells acts as further evidence that this targeting does, in fact, rely on

dileucine rather than tyrosine motifs. The full nature of the adaptor complexes or that required

for post-Golgi transport of E-cadherin in either LLC-PK1 or MDCK epithelial cells have yet to

be characterised.

Finally, the current study also provides new insights into the role of β↑catenin in E-cadherin

trafficking. β-catenin is a cytoplasmic protein with affinity for the cytoplasmic tail of E-

cadherin, it binds to E-cadherin early in the biosynthetic pathway, forming a stable complex that

is transported to the cell surface (33). Chen and colleagues (34) concluded that β-catenin is

required for the biosynthetic processing and trafficking of E-cadherin, based on a correlation

between deletion of residues within or near the β-catenin binding domain and loss of surface

delivery in GP-2-E-cadherin chimeras and other constructs. However, our current results

suggest a different scenario. We showed that full length tail chimeras co-precipitated β-catenin

in similar proportion to that seen in endogenous E-cadherin-β-catenin complexes. Upon

expressing the Tac/Ecad(578-653) chimera, which clearly did not bind β-catenin, we found it

was correctly targeted and transported to the cell surface , suggesting that β-catenin is not

required for sorting or delivery under these conditions. β-catenin may have a role in facilitating

or optimising the transport, processing or folding of newly-synthesised E-cadherin. All of these

factors might well contribute to the slower processing and transient accumulation of

Tac/Ecad(578-653) that we noted in the absence of β-catenin. Recent biophysical and

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biochemical analyses further suggest that β-catenin binding might also serve to protect the E-

cadherin tail from degradation (57). Our finding that β-catenin is not required for basolateral

targeting or surface delivery, sheds new light on β-catenin as having a role in facilitating, but not

directing, cadherin trafficking.

The polarised targeting of E-cadherin is of seminal importance to the maintenance of epithelial

polarity and function. Targeting signals and mechanisms must ensure the accurate basolateral

delivery of newly-synthesised E-cadherin and of internalised and recycled E-cadherin for its

incorporation into adherens junctions. In this study we demonstrate one such mechanism,

basolateral sorting directed via a dileucine signal. Future studies will address specific roles for

additional signals and perhaps for some of the accessory proteins in cadherin/catenin complexes.

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Footnotes

Abbreviations

*The abbreviations used are: APC, adenomatous polyposis coli; BSA, bovine serum albumin;

DMEM, Dulbecco’s modified eagle’s medium; GFP, green fluorescent protein; IL2-R,

interleukin-2 receptor; LDL, low density lipoprotein; LLC-PK1, pig kidney proximal tubular

epithelial cell line; MDCK, Madin-Darby canine kidney cells; PAGE, polyacrylamide gel

electrophoresis; PBS, phosphate-buffered saline; PCR, polymerase chain reaction; pIgR,

polymeric immunoglobulin receptor; PVDF, polyvinylidene difluoride; Tac, IL2-R α chain

Acknowledgments

The authors would like to thank Susan La Flamme for providing materials and to acknowledge

Darren Brown, Marita Goodwin and Juliana Venturato for their technical support. This work was

funded by grants from the NHMRC (to JLS & ASY - Wellcome Trust International Senior

Medical Research Fellow) and from the ARC as part of the Special Research Center for

Functional Genomics.

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Figure Legends

Figure 1

Localisation of E-cadherin. MDCK and LLC-PK1 cells were generally fixed with

paraformaldehyde and labelled with a mouse monoclonal antibody to localise endogenous E-

cadherin by immunofluorescence, a) E-cadherin staining in MDCK cells on cell boundaries is at

the basolateral domain; b) in LLC-PK1 cells fixed in methanol, there is typical basolateral

surface staining, c) in LLC-PK1 cells fixed in paraformaldehyde the E-cadherin antibody gave

no staining. GFP-E-cadherin is localised by the GFP antibody on the basolateral membrane of

transfected MDCK cells (d) and LLC-PK1 cells (e). There is also some intracellular,

perinuclear staining of newly-synthesised GFP-E-cadherin in LLC-PK1 cells (arrows). Mag

Bar = 2.5 µm

Figure 2

A. Diagrammatic representation of chimeras. Chimeras were made by fusing amino terminal

regions of Tac, encompassing the extracellular domain and transmembrane domain to the full-

length carboxy terminal cytoplasmic tail of E-cadherin (shaded) (Tac/Ecad(578-728)). In some

constructs the transmembrane domain of Tac was replaced with that of E-cadherin

(Tac/Ecad(554-728)). In Tac/Ecad(578-653) the E-cadherin tail was truncated leaving only the

membrane-proximal region.

B. Extracts of MDCK (lane 1) and LLC-PK1 cells (lanes 2-5) were probed by immunoblotting

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with the E-cadherin antibody to detect Tac/Ecad chimeras. Proteins of the expected molecular

weight were detected for each of the Tac/Ecad chimeras (Tac/Ecad(578-728) – 72 kDa;

Tac/Ecad(554-728) 72 kDa; Tac/Ecad(578-653) 62 kDa; Tac/Ecad(578-728)∆S1 72 kDa (see

also Fig 5)).

Figure 3

Localisation of Tac/E-cadherin chimeras. Immunofluorescence staining of Tac or Tac/E-

cadherin chimeras in stably-transfected cell lines using the Tac antibody. Upper views show

cross sections of monolayers and lower views in each panel represent XZ sections of filter-

grown cells, a) MDCK and b) LLC-PK1 cells overexpressing full-length Tac on their apical

domains, c) Tac/Ecad(554-728) expressed in LLC-PK1 cells is localised on the basolateral

membrane. The Tac/Ecad(578-728) construct is also on the basolateral domains of transfected

LLC-PK1 cells (d) and MDCK cells (e). Mag.bars = 2.5 µm

Figure 4.

Immunofluorescence localisation of the membrane-proximal chimera. LLC-PK1 (a,b) and

MDCK (c) cells overexpressing Tac/Ecad(578-653) were stained with the Tac antibody. The

truncated chimera encoding only the membrane-proximal tail localised to the basolateral

domains of confluent cells. There was also some intracellular, perinuclear staining in untreated

cells (a) that disappeared after 2 hours of treatment with cycloheximide (b). Mag bars = 2.5 µm

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Figure 5.

A. Sequence alignment of cadherins with conserved cytoplasmic tails (Type I and Type II

families as defined by Nollet et. al. (42)). Sequence alignment of the membrane-proximal tail

regions of cadherin family members, including E-cadherin (cadherin 1), N-cadherin (cadherin

2) and VE-cadherin (cadherin 5). A dileucine motif at position 587 (shaded) in E-cadherin

(cadherin 1) is highly-conserved across members of the family and is relatively close to the

transmembrane domain (underlined) in the membrane-proximal region of the tail.

B. Targeting of dileucine deletion mutants. MDCK and LLC-PK1 cells expressing

Tac/Ecad(578-728)∆S1 or Tac/Ecad(578-653)∆S1 were stained with the Tac antibody. Both

constructs with deleted dileucine motifs, now show apical targeting. Prominent apical staining

only, is now seen in both cell lines expressing Tac/Ecad(578-728)∆S1 or Tac/Ecad(578-

653)∆S1. Mag bars =2.5 µm..

Figure 6.

Co-immunoprecipitation of β-catenin.

A. Proteins immunoprecipitated from untransfected MDCK cells with a monoclonal antibody to

E-cadherin were probed by immunoblotting with the E-cadherin antibody (top) or with a β-

catenin antibody (bottom). Proteins at 120 kDa and 92 kDa, respectively, were detected.

B. Τhe Tac antibody was used to immunoprecipitate chimeras from transfected LLC-PK1 cells.

The β↑catenin antibody was then used for immunoblotting supernatants (SN lanes 1, 3 and 5)

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and immunoprecipitates (IP lanes 2, 4 and 6). β-catenin was co-precipitated with

Tac/Ecad(578-728) (lane 2), but not with the truncated Tac/Ecad(578-653) chimera (lane 4).

Deletion of the di-leucine targeting motif (Tac/Ecad(578-728)∆S1) did not affect co-

precipitation of β-catenin (lane 6).

Tables

Cell surface subjected to biotinylation

Basal ApicalTac/Ecad(578-728) 95% 5%

Tac/Ecad(578-653) 96% 4%

Table 1

Surface expression of Tac/Ecad constructs. Cells stably overexpressing Tac/Ecad(578-728) and

Tac/Ecad(578-653) were incubated with Sulfo-NHS-SS-biotin added to apical or basal side.

Biotinylated proteins were isolated and analysed by immunoblotting and densitometry to

calculate the proportion of each construct accessed from each side of the cell layer.

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d

c

Figure 1 Miranda

MDCK LLC-PK1ba

e

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Figure 2 Miranda

578 653

578 728

554 728

TACTac

Tac/Ecad (554-728)

Tac/Ecad (578-728)

Tac/Ecad (578-653)

TAC

TAC

TAC

extracellular intracellularA

B

Tac/Ecad(554-728)

Tac/Ecad(578-653)

Tac/Ecad(578-728)∆S1

Tac/Ecad(578-728)

80

70

60

kDa

Tac/Ecad(578-728)

1 2 3 4 5

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b

e

c

Figure 3 Miranda

a

d

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Figure 4 Miranda et

cba

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MDCK LLC-PK1

A

B

cadherin 1 PAILGILGGILALLILILLLLLFLRRRAV--VKEP LLPPE-DDTRDNVYYYDEEGGGEEDcadherin 2 GAIIAILLCIIILLILVLMFVVWMKRRDKERQAKQ LLIDPEDDVRDNILKYDEEGGGEEDcadherin 3 GFILPVLGAVLALLFLLLVLLLLVRKKRK--IKEP LLLPE-DDTRDNVFYYGEEGGGEEDcadherin 4 GAIVAILICILILLTMVLLFVMWMKRREKERHTKQ LLIDPEDDVREKILKYDEEGGGEEDcadherin 5 QAVVAILLCILTIT-VITLLIFLRRRL---RKQARAHGKSVPEIHEQLVTYDEEGGGEMDcadherin 6 GALVAILLCIVILLVTVVLFAALRRQR----KKEP LIISK-EDIRDNIVSYNDEGGGEEDcadherin 7 GALIAILACVLTLLVLILLIVTMRRR-----KKEP LIFDEERDIRENIVRYDDEGGGEEDcadherin 8 GALIAILACIILLLVIVVLFVTLRRHQ----KNEP LIIKDDEDVRENIIRYDDEGGGEEDcadherin 9 GALVAILLCVLILLILVVLFAALKRQR----KKEP LIISK-DDVRDNIVTYNDEGGGEEDcadherin 10 GALIAILLCIIILLVIVVLFAALKRQR----KKEP LILSK-EDIRDNIVSYNDEGGGEEDcadherin 11 GALIAILACIVILLVIVVLFVTLRR-Q----KKEP LIVFEEEDVRENIITYDDEGGGEEDcadherin 12 GALIAILLCIVILLAIVVLYVALRRQK----KKHT LMTSK-EDIRDNVIHYDDEGGGEEDcadherin 13 NAAGALRFSLPSVLLLSLFSLACL------------------------------------cadherin 15 GALVIVLASALLLLVLVLLVALRARFWKQS-RGKG LLHGPQDDLRDNVLNYDEQGGGEEDcadherin 18 GALIAILLCVLILLAIVVLFITLRRS-----KKEP LIISE-EDVRENVVTYDDEGGGEEDcadherin 19 EVIIAILICIMIIFGFIFLTLGLKQR-----RKQI LFPEKSEDFRENIFQYDDEGGGEEDcadherin 20 GPLIAILACIFVLLVLVLLILSMRRH-----RKQPYIIDDEENIHENIVRYDDEGGGEED

Figure 5 Miranda et al

Tac/Ecad(578-728)

∆S1

Tac/Ecad(578-653)

∆S1

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β-catenin

Tac/Ecad(578-653)

Tac/Ecad(578-728)∆S1

Tac/Ecad(578-728)

E-cadherin

β-catenin

Endogenous

A B

Figure 6 Miranda

1 2

SN IP

3 4

SN IP

5 6

SN IP

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Alpha S. Yap, Jennifer L. Stow and Rohan D. TeasdaleKevin C. Miranda, Tatiana Khromykh, Perpetina Christy, Tam Luan Le, Cara J. Gottardi,

LLC-PK1 epithelial cellsA dileucine motif targets E-cadherin to the basolateral cell surface in MDCK and

published online April 18, 2001J. Biol. Chem. 

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