A Cell Wall-associated, Receptor-like Protein Kinase

Physical connections between higher plant cell walls and the plasma membrane have been identified visually, but the molecules involved in the contact are unknown. We describe here an Arabidopsis thaliana protein kinase, designated Wak1 for wall-associated kinase, whose predicted extracytoplasmic domain contains several epidermal growth factor repeats and identity with a viral movement protein. Wak1 fractionates with insoluble material when plant tissue is ground in a variety of buffers and detergents, suggesting a tight association with the plant extracellular matrix. Immunocytochemistry confirms that Wak1 is associated with the cell wall. Enzymatic digestion of the cell wall allows the release of Wak1 from the insoluble cell wall fraction, and protease experiments indicate that Wak1 likely has a cytoplasmic kinase domain, and the EGF containing domain is extracellular. Wak1 is found in all vegetative tissues of Arabidopsis, and has relatives in other angiosperms, but not Chlamydomonas. We suggest that Wak1 is a good candidate for a physical continuum between the cell wall and the cytoplasm, and since the kinase is cytoplasmic, it also has the potential to mediate signals to the cytoplasm from the cell wall.