Examination of the completed genome sequences of Arabidopsis and rice reveals very large multigene families predicted to encode proteins with an organization of functional domains similar to that of animal receptor ki-nases, such as the mammalian insulin and epidermal growth factor receptor tyrosine kinases and the transforming growth factor P (TGF-P)receptor serine/threonine kinases (Shiu et al. 2004). The three most prominent structural features of this class of proteins are a putative extracellular ligand-binding domain, a single-pass transmembrane sequence, and an intracellular cyto-plasmic domain consisting of a juxtamembrane region, a catalytic kinase domain with 12 conserved subdomains, and a short carboxy terminal region. Such structural features suggest a role for receptor kinases in extracellular signal perception followed by intracellular signal transduction via phospho-rylation of specific targets.
More than 220 of these receptor-like kinases in Arabidopsis and nearly 400 in rice have an extracellular domain with multiple leucine-rich repeats and are classified as LRR RLKs (Shiu and Bleecker 2001, 2003; Shiu et al. 2004).
Only a small subset of LRR RLKs have been characterized in any detail by genetic and biochemical analyses, but those that have been so studied are known to play essential roles in regulating plant growth and development, as well as in defense responses to various pathogens (Torii 2004). Examples of Arabidopsis LRR RLKs that have been well characterized include CLAVATA1, controlling meristematic cell fate (Clark et al. 1997; Trotochaud et al. 1999); ERECTA, specifying organ shape (Torii et al. 1996); HAESA, involved in organ abscission (Jinn et al. 2000); FLS2, which binds the flagellin peptide ligand (Gomez-Gomez and Boller 2000); and AtSERK1 (SOMATIC EMBRYOGENESIS RECEPTOR KINASE), associated with early embryogenesis (Hecht et al. 2001). Besides the critical role of BRI1 in BR signaling, another LRR RLK, BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1), has been shown to associate with BRI1 in planta and modulate BR signaling outputs (Li et al. 2002; Nam and Li 2002).
To fully characterize LRR RLK function in BR signaling, it is essential to understand the role of ligand-dependent BRI1/BAK1 dimerization and cytoplasmic domain phosphorylation, including identification of specific in vivo phosphorylation sites and their functional significance. Moreover, identification of BRI1 and BAK1 kinase domain substrates and cataloging of structural requirements for substrate recognition and phosphorylation are also necessary for completing the picture of LRR RLK action in BR signal transduction. Several studies published in the last few years have addressed these issues and suggest that plant LRR RLKs such as BRI1 and BAK1 share several conserved features with the mechanism of animal receptor kinase action, including ligand-dependent oligomerization, followed by phosphorylation-dependent activation of the kinase domain, leading ultimately to changes in the expression level of numerous BR-responsive genes (Nam and Li 2004; Ehsan et al. 2005; Wang et al. 2005a,b; Wang and Chory 2006).
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