ENDOPLASMIC

RETICULUM

Who discovered the ER? discovered by Belgian biologist Albert Claude (1899-1983).

used the newly developed electron microscope to explore the interior of cells and along with his associate Keith Porter in 1945 observed the presence of a “lace-work structure.”

How does the ER looks like?

rough ER appears as lumpy sheets of folded membranes; the lumps are the ribosomes. They are not permanently attached, adhering only when there is protein manufacturing work to be done.

smooth ER which isn't coated with ribosomes looks like smooth tubes.

Where is the ER Located? no specific base or point in a cell, as

it extends a network of membranes, tubules vesicles, and sacs throughout the cell.

ER membranes are folded and stacked on top of each other and connected to the nucleus.

rough ER with ribosomes attached is closest to the nucleus.

Endoplasmic Reticulum

endoplasmic - “within the cytoplasm” reticulum - Latin for a “a little net”- extensive network of folded

membranes that extends from the nuclear envelope to which it is connected, throughout the cytoplasm.

- divided into two sub-compartments, rough ER and smooth ER

Rough ER - defined by the presence of ribosomes bound to its cytosolic

surface

- typically composed of a network of flattened sacs (cisternae)

Rough ERcontinuous with

the outer membrane of the nuclear envelope, which also bears ribosomes on its cytosolic surface.

Functions of the Rough ER

secrete large quantities of proteins,

such as the:

a. acinar cells of the pancreas

b. mucus secreting cells of the lining of the digestive tract

Functions of the Rough ER

Functions of the Rough ER

Rough ER is the starting point of the biosynthetic pathways of:proteins carbohydrate chains phospholipids

that journey through the membranous compartments of the cell.

lacks associated ribosomes. membranous elements are highly curved and tubular, forming an interconnecting system of pipelines curving through the cytoplasm

Smooth ER

Smooth ER

extensively developed in a number of cell types, including those of skeletal muscle, kidney tubules, and steroid-producing endocrine glands

FIGURE 1. The smooth ER (SER). Electron micrograph of a Leydig cell from the testis showing the extensive smooth ER where steroid hormonesare synthesized.

Functions of the Smooth ER

Synthesis of fatty acids and steroid hormones,

such as estrogens and testosterone.

Detoxification in the liver of a wide variety of

organic compounds, including barbiturates and ethanol.

Sequestering calcium ions within the cytoplasm

of cells.

Rough ER vs. Smooth ERRibosomes on its outer surface

Not associated with ribosomes

Site of synthesis of proteins destined for secretion

Involved in lipid metabolism

Protein Synthesis

On membrane-bound ribosomes:a. Secreted proteinsb. Integral membrane proteinsc. Soluble proteins in ER, Golgi complex, lysosomes,

endosomes, vesicles and plant vacuoles

On free ribosomes:d. Cytosolic proteinse. Peripheral proteinsf. Proteins transported to the nucleusg. Proteins to be incorporated in peroxisomes,

chloroplasts and mitochondria

What determines the location in a cell where a protein is synthesized?

What determines the location in a cell where a proteinis synthesized? In the early 1970s, Günter Blobel, in collaborationwith David Sabatini and Bernhard Dobberstein ofRockefeller University, first proposed, and then demonstrated,that the site of synthesis of a protein was determined by thesequence of amino acids in the N-terminal portion of thepolypeptide, which is the first part to emerge from the ribosomeduring protein synthesis. They suggested the following:1. Secretory proteins contain a signal sequence at theirN-terminus that directs the emerging polypeptide andribosome to the ER membrane.2. The polypeptide moves into the cisternal space of theER through a protein-lined, aqueous channel in the ERmembrane. It was proposed that the polypeptide movesthrough the membrane as it is being synthesized, that is,cotranslationally

Proteins destined for incorporation into the plasma membrane or the membranes of the ER, Golgi, or lysosomes are initially inserted into the ER membrane instead of being released into the lumen.

Cotranslational targeting of secretory proteins to the ER

Posttranslational translocation of proteins into the ER

Synthesis of Secretory, Lysosomal, or Plant Vacuolar Proteins on Membrane-Bound Ribosomes

Step 1. As the signal sequence emerges from the ribosome, it is bound by a signal recognition particle (SRP) that arrests further synthesis.Step 2. This mediates the binding of the complex to the RER membrane by the SRP receptor.Step 3. The SRP is released from the membrane.Step 4. The nascent polypeptide passes through a protein-lined pore in the ER membrane and into the ER lumen.

Synthesis of Integral Membrane Proteins on Membrane-Bound

Ribosomes

Step 1. The nascent polypeptide enters the translocon just as if it were a secretory protein. Steps 2–3 show the synthesis of a transmembrane protein whose N-terminus is in the lumen of the ER and C-terminus is in the cytosol. In step 2, the translocon has opened laterally and expelled the transmembrane segment into the bilayer.Step 3 shows the final disposition of the protein. Steps 2a–4a show the synthesis of a transmembrane protein whose C-terminus is in the lumen and N-terminus is in the cystosol. In step 2a, the translocon has reoriented the transmembrane segment, in keeping with its reversed positively and negatively charged flanks. In step 3a, the translocon has opened laterally and expelled the transmembrane segment into the bilayer.Step 4a shows the final disposition of the protein.

Membrane Biosynthesis in the ERAs each protein is synthesized in the rough ER, it becomes inserted into the lipid bilayer in a predictable orientation determined by its amino acid sequence. This orientation is maintained throughout its travels in the endomembrane system, as illustrated in this figure.

Misfolded proteins are recognized by a glucosyltransferase (GT) which adds a glucose to the end of the oligosaccharide chains.

Glycoproteins containing monoglucosylated oligosaccharides are recognized by the membrane-bound chaperone calnexin and given an opportunity to achieve their correctly folded state.

If this does not occur after repeated attempts, the protein is translocated to the cytosol and destroyed.

Quality control: ensuring that misfolded proteins do not proceed forward

Ora et Labora.

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