A cluster of five cell wall-associated receptor kinase genes, Wak1-5, are expressed in specific organs of Arabidopsis

WAK1 (wall-associated kinase 1) is a cytoplasmic serine/threonine kinase th at spans the plasma membrane and extends into the extracellular region to b ind tightly to the cell wall. The Wak1 gene was mapped and found to lie in a tight cluster of five highly similar genes (Wak1-5) within a 30 kb region . All of the Wak genes encode a cytoplasmic serine/threonine protein kinase , a transmembrane domain, and an extracytoplasmic region with several epide rmal growth factor (EGF) repeats. The extracellular regions also contain li mited amino acid identities to the tenascin superfamily, collagen, or the n eurexins. RNA blot analysis with gene-specific probes revealed that Wak1, W ak3 and Wak5 are expressed primarily in leaves and stems of Arabidopsis. Wa k4 mRNA is only detected in siliques, while Wak2 mRNA is found in high leve ls in leaves and stems, and in lower levels in flowers and siliques. A trac e amount of Wak2 can also be detected in roots. Wak1 is induced by pathogen infection and salicylic acid or its analogue INA and is involved in the pl ant's response, and Wak2, Wak3 and Wak5 also can be greatly induced by sali cylic acid or INA. The WAK proteins have the potential to serve as both lin kers of the cell wall to the plasma membrane and as signaling molecules, an d since Wak expression is organ-specific and the isoforms vary significantl y in the cell wall associated domain this family of proteins may be involve d in cell wall-plasma membrane interactions that direct fundamental process es in angiosperms.