WAKs: cell wall-associated kinases linking the cytoplasm to the extracellular matrix

There are only a few proteins identified at the cell surface that could dir ectly regulate plant cell wall functions. The cell wall-associated kinases (WAKs) of angiosperms physically link the plasma membrane to the carbohydra te matrix and are unique in that they have the potential to directly signal cellular events through their cytoplasmic kinase domain. In Arabidopsis th ere are five WAKs and each has a cytoplasmic serine/threonine protein kinas e domain, spans the plasma membrane, and extends a domain into the cell wal l. The WAK extracellular domain is variable among the five isoforms, and co llectively the family is expressed in most vegetative tissues. WAK1 and WAK 2 are the most ubiquitously and abundantly expressed of the five tandemly a rrayed genes, and their messages are present in vegetative meristems, junct ions of organ types, and areas of cell expansion. They are also induced by pathogen infection and wounding. Recent experiments demonstrate that antise nse WAK expression leads to a reduction in WAK protein levels and the loss of cell expansion. A large amount of WAK is covalently linked to pectin, an d most WAK that is bound to pectin is also phosphorylated. In addition, one WAK isoform binds to a secreted glycine-rich protein (GRP). The data suppo rt a model where WAK is bound to GRP as a phosphorylated kinase, and also b inds to pectin. How WAKs are involved in signaling from the pectin extracel lular matrix in coordination with GRPs will be key to our understanding of the cell wall's role in cell growth.