Research- Receptor-like Kinases in Plants
Receptor-like kinases in plants

Development of multicellular organisms relies on coordinated cell proliferation and differentiation. In animals, growth factor receptor kinases play key roles in cell differentiation and development, either by stimulating or inhibiting cell growth. Recent studies revealed that higher plants also possess genes coding for putative receptor kinases (Receptor-like Kinases, RLK). For instance, a completely sequenced Arabidopsis genome contains over 600 genes encoding RLKs (Shiu and Bleecker, 2001), suggesting that higher plants, like animals, use receptor kinase signaling commonly and broadly in responding to vast arrays of stimuli to modulate gene expressions. Although only a handful of RLKs thus far are shown to have defined biological functions, their roles in development, self-incompatibility response, and defense against pathogens illustrate important and versatile function of the RLK superfamily. However, given that only a few RLKs have been shown to regulate developmental processes, it is far from being understood how receptor-kinase signaling control cell proliferation in plants.

A common feature of these putative receptor kinases (RLKs), is that each has an N-terminal signal sequence, an extracellular domain that varies in structure, a single membrane-spanning region, and a cytoplasmic protein kinase catalytic domain. Unlike animals, where a majority of the receptor kinases possess tyrosine kinase activity, all of the plant RLKs thus far are shown to phosphorylate serine-and threonine residues, except one that displays dual specificity in vitro (Walker, 1994; Torii and Clark, 2000,; many other great reviews are available!).

Plant RLKs are classified into subfamilies based on the structural feature of the extracellular domain, which is thought to act as a ligand-binding site.

Receptor-like kinases in plants Figure.1

1. S-domain class: S-RLKs possess an extracellular S-domain homologous to the self-incompatibility-locus glycoproteins (SLG) of Brassica oleracea (Nasrallah et al. 1988). The S-domain consists of 12 conserved cysteine residues (ten of which are absolutely conserved). In addition, the S-domain possesses the PTDT-box, which has a conserved WQSFDXPTDTFL sequcnce (X=non conserved amino acid; F=aliphatic amino acid). In Brassica, the S-RLK gene is physically linked to the S locus (Nasrallah et al. 1988). It has been shown that the S-RLK primarily functions as a receptor for the pollen-derived ligand, SCR (S-locus cysteine rich protein) during the self-incompatibility recognition process between pollen and stigma. The SLG protein is required for a full manifestation of the self-incompatibility response (Takasaki et al., 2000). However, isolation of several S-RLK genes from self-compatible plant species and their expression in vegetative tissues indicate that S-RLKs may play a developmental role in addition to self-incompatibility. In addition, one of the S-RLKs of Brassical is implicated in plant defense response (Pastugalia et al., 1997).

2. LRR class: To date, LRR-RLKs comprise the largest class of plant RLKs. LRRs (leucine-rich repeats) are tandem repeats of approximately 24 amino acids with conserved leucines. LRRs have been found in a variety of proteins with diverse functions, from yeast, flies, human, and plants, and are implicated in protein-protein interactions. Several LRR-RLKs have been shown to play critical roles in development. Those include ERECTA which regulates organ shape, CLAVATA1 which controls cell differentiation at the shoot meristem, HAESA, which regulates floral abscission process, and BRI1, which is involved in brassinosteroid perception (Torii et al., 1996; Clark et al., 1997; Li and Chory 1997; Jinn et al., 2000). On the other hand, the rice gene Xa21 confers race-specific resistance to Xanthomonas oryzae pv oryzae (Song et al., 1995). Therefore, LRR-RLKs also play a role in disease resistance. Interestingly, the tomato Cf disease resistance gene products, which confer a race-specific resistance to Cladosprium fulvum, contain extracellular LRR domains but lack the cytoplasmic protein kinase domain (Jones and Jones 1997).

3. TNFR class: The maize CRINKLY4 (CR4) gene product possess TNFR (tumor-necrosis factor receptor)-like repeats, that has a conserved arrangement of six cyeteines, and seven repeats of ~39 amino acids that display a weak similarity to the RCC GTPase (Becraft et al., 1996, McCarthy and Chory, 2001). CR4 is required for a normal cell differentiation of the epidermis (Becraft et al., 1996). The Aragbidopsis genome contains several genes related to CR4 (McCarthy and Chory, 2001).

4. EGF class: The cell wall associated receptor kinases (WAKs) represent the EGF (epidermal growth factor) class. The EGF-like repeat motif is characterized by a conserved arrangement of six cysteines. The EGF-like repeats are found in variety of animal extracytoplasmic receptor domains and are known to play a role in protein-protein interactions. In Arabidopsis, four WAKs (WAK1 to WAK4) have been identified, and all of them have extracellular EGF-like repeats (He et al., 1996; Ellard-Ivey et al., 1997). Reverse-genetic experiments suggest that WAKs may be involved in pathogenic responses (He et al., 1998).

5. PR class: The Arabidopsis PR5K (PR5-like receptor kinase) is the known example of this class. The extracellular domain of PR5K exhibits sequence similarity to PR5 (pathogenesis related protein 5), whose expression is induced upon pathogen attack (Wang et al., 1996). The structural similarity between the PR5K receptor domain and PR5 suggests a role for PR5K in pathogenesis response.

6. Lectin class: The Arabidopsis LecRK1 gene product possesses an extracellular domain homologous to carbohydrate-binding proteins of the legume family (Harvé et al., 1996). Although biological function of LecRK1 is yet known, its structural feature suggests that LecRK1 may be involved in a perception of oligosaccharide-mediated signal transduction. The Arabidopsis genome contains >30 genes belonging to Lectin-RLKs several genes coding for Lectin-RLKs (McCarthy and Chory, 2001).

7. Others: The RLKs, which possess extracellular domains sharing no homology to known motifs, are classified as "Others".

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 © Keiko U. Torii, 2004