Plant Molecular Biology 47: 197–206, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 197 WAKs: cell wall-associated kinases linking the cytoplasm to the extracellular matrix Catherine M. Anderson + , Tanya A. Wagner + , Mireille Perret, Zheng-Hui He 1 , Deze He and Bruce D. Kohorn ∗ Department of Biology, Rm B353, LSRC, Duke University, Durham, NC 27708, USA ( ∗ author for correspon- dence; e-mail kohorn@duke.edu); 1 present address: Department of Biology, San Francisco State University, San Francisco, CA 94132, USA; + these authors contributed equally to this paper Key words: cell expansion, GRP, pectin, wall-associated kinase, WAK Abstract 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 reductionin WAK protein levels and the loss of cell expansion. A large amount of WAK is covalently linked to pectin, and most WAK that is boundto 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. Abbreviations: ECM, extracellular matrix; EGF, epidermal growth factor; GA, gibberellic acid; GRP, glycine-rich protein; GST, glutathione S -transferase; GUS, β -glucuronidase; INA, 2,2-dichloroisonicotinic acid; KAPP, protein type 2C phosphatase; SA, salicylic acid; SAR, systemic acquired resistance; SRK, S -locus receptor kinase; WAK, wall-associated kinase Introduction As plant cells divide and expand they assemble and modify their extracellular matrix (ECM) to permit the subsequent change in cell size and shape. The se- cretion of an ECM by one cell can also influence neighboring cells, perhaps best exemplified by the in- teractions between the pollen and stigma. Throughout the plant, the production and regulation of the ECM architecture has the potential to influence many as- pects of development. The ECM of plant cells, often referred to as the cell wall, varies in composition throughout development. The primary wall is laid down during cell division and expansion, and the sec- ondary wall is deposited on the primary wall once growth has ceased (Cosgrove, 1997). The cell wall is primarily composed of an ordered array of cellulose microfibrils that are coated with cross-linking glycans and embedded in a gel of pectin. Within this arrange- ment are proteins with varying amounts of linked carbohydrates (Cosgrove, 1997; Reiter, 1998). This protein and carbohydrate matrix, continuous between cells, provides structure and may define tissues in an organ (Roberts, 1994).