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 directly regulate plant cell wall functions. The cell wall-associated kinases (WAKs) of angiosperms physically link the plasma membrane to the carbohydrate matrix and are unique in that they have the potential to directly signal cellular events through their cytoplasmic kinase domain. In Arabidopsis there are five WAKs and each has a cytoplasmic serine/threonine protein kinase domain, spans the plasma membrane, and extends a domain into the cell wall. The WAK extracellular domain is variable among the five isoforms, and collectively the family is expressed in most vegetative tissues. WAK1 and WAK2 are the most ubiquitously and abundantly expressed of the five tandemly arrayed genes, and their messages are present in vegetative meristems, junctions of organ types, and areas of cell expansion. They are also induced by pathogen infection and wounding. Recent experiments demonstrate that antisense 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, and 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 support a model where WAK is bound to GRP as a phosphorylated kinase, and also binds to pectin. How WAKs are involved in signaling from the pectin extracellular matrix in coordination with GRPs will be key to our understanding of the cell wall's role in cell growth.