Protein Kinases

- Primary elements in signal transduction

- Phosphorylated residues by protein kinases: S, T, Y

-A Protein kinase can ⓟ hundreds of target proteins, thereby amplifying signals.

- Activator of Protein Kinases: light, pathogen, growth regulators, temperature change, nutrition deprivation,water stresses (drought, salt), etc

The various classes of protein kinases found in plants.

Ex) PKA, PKC

GSK3, CKII

Cell Surface Receptors May Have Catalytic Activity

1) Receptor Tyrosine Kinases

2) Receptor Tyrosine Phosphatases

3) Receptor Ser/Thr Kinases

4) Tyrosine linked receptors

5) Receptor guanylate cyclases

6) cell surface proteases

Ligand Binding to Receptor Tyrosine Kinases Induces autophosphorylation

Signaling Proteins That Bind to RTKs Are Activated by phosphorylation

Receptor-Like Kinases (RLKs)

1. S-domain RLKs

2. Leucine rich repeat (LRR) RLKs

3. Epidermal growth factor (EGF)-like RLKs

4. Lectin receptor kinases

Plant Receptor-like Kinases Are Structurally Similar to Animal Receptor Tyrosine Kinases

1.S receptor kinase (SRK)

The first class is characterized by an extracellular S domain

The S domain was identified in S locus glycoproteins (SLGs) which regulate self-incompatibility in Brassica species.

Fig. 18.59. General model for pollen tube incompatibility as a result of two way signaling between pollen tube and stigma.

An example of S-domain RLK

2. Leucine-rich repeat (LRR) receptors

They were first identified as disease resistance genes that may play key roles in the cell surface recognition of ligands

Plant LRR receptors have been implicated in normal developmental functions as well. (in cell–cell recognition during pollination, in shape and size of organs)

Brassinosteroid receptor has been identified as an LRR receptor

Fig. 18.60 Model of a homodimeric receptor-like protein kinase, RLK5, binding its ligand and undergoing autophosphorylation

An example of LRR RLKs

3. Receptors with EGF (epidermal growth factor)–like repeat

This type of RLK that contains an epidermal growth factor–like repeat has been identified in Arabidopsis.

Receptor PRO25 is localized in the chloroplast and interacts with a light harvesting chlorophyll a/b–binding protein (LHCP).

An example of EGF-Like RLK

An example of Lectine RLK

Figure 14.24 The MAPK cascade. Hormonal stimulation of the receptor tyrosine kinase leads to the activation of Raf, also known as MAP kinase kinase kinase (MAPKKK).

Fig 18.63 Many signals are transduced by protein kinase cascades that regulate gene expression

Siganls: Auxin, GA, ABA, touch, wind, osmotic stress, wounding, elicitor

Raf-1L kinase: CTR1 in ethylene signaling

Fig18.64Structure of FK506, rapamycin and cyclosporine.

Protein Phosphatase

◆ PP1◆ PP2A◆ PP2B (calcineurin, CaM-dependent)◆ PP2C: abi in ABA signaling

◆ PP inhibitor can prevent red light, proliferation, growth factors, pollen tube signaling

Fig. 18.65Ethylene-signaling mutants have striking phenotypes.

ETR1: the receptor of ethyleneCTR1: negative regulator of ethylene signaling

Fig 18.22 Transgenic plants expressing variants of the etr1-1 gene.

Fig18. 66Comparison of ethylene signaling pathway and animal Ras-signaling pathway

ABA signaling in plants

ABA

NADPH Oxidase

ROS (H2O2)

Ca2+ Channel open

[Ca2+]

ABI1 (PP2C)

K+ channel, Cl- channel

Reduction of Turgor pressure

NO

Fig 18.71 Some of the known interactions in the plant cell signal transduction network.