Cell Wall Associated, Receptor Protein Kinases

Duke University, Durham NC

Investigators

Abstract

Cell walls are carbohydrate and protein structures that surround and separate plant cells. As plant cells expand in a regulated fashion they must necessarily modify and enlarge their cell walls to permit the subsequent increase in volume. The cell wall itself might influence this expansion process. Thus the cell has the potential to influence almost every aspect of plant function simply because of its position and physical properties. For this reason many have speculated on its role in a plant's development and response to the outside world. Cell growth can occur in many dimensions, such as the polarized expansion of a pollen tube tip, the creation of elongated cells characteristic of many vegetative tissues, the establishment of a meristem, or even the jigsaw-like arrangement of cells at the leaf surface. Currently the understanding of how cell wall architecture is coordinated with programmed and hormone regulated cell expansion events during development is not well understood in angiosperms. Recent studies have identified a number of proteins at the cell surface that could directly regulate cell wall functions, and the Cell Wall Associated Kinases, or WAKs, are among these. There are five cell wall associated kinases in Arabidopsis and representatives in other angiosperm families. WAKs each have a cytoplasmic serine/threonine protein kinase domain, span the plasma membrane and extend a domain into the cell wall. WAKs 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 kinase domain. The WAK extracellular domain is variable between the five isoforms, and collectively the family is expressed in all organs. 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. The WAK1 but not WAK2 cell wall domain binds to a glycine rich protein (GRP) of the cell wall in in vitro assays. WAK1 and GRP can be co-immunoprecipitated from leaf or seedling extracts, and this WAK is phosphorylated. Recent experiments demonstrate that antisense WAK expression leads to the loss of cell expansion, and preliminary results indicate that antisense GRP production causes an enhancement of cell expansion (and eventual cell death). Thus GRP may be a negative regulator of WAK activity. A large amount of WAK is also covalently linked to pectin, and most of WAK that is bound to pectin is also phosphorylated. The data support a model where WAK1 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 the understanding of the cell wall's role in cell growth. WAKs are poised to provide a direct signal from the cell wall to the cytoplasm of many cell types during the development of Arabidopsis. In order to characterize the associations and roles of GRPs and WAKs in plants it will be necessary to generate additional (partial) loss of function alleles of the two predominant WAKs and of GRP. These alleles can then be used to determine the relationship of WAK mediated cell expansion to other known signaling pathways. WAK kinase substrates will be also identified and used to assay the activation of WAK by wall components and developmental cues, and eventually mutant alleles of these substrates will help in the analysis of WAKs and cell elongation. The goal is to understand how WAKs in association with GRP and pectins regulate cytoplasmic components so that the cell wall is coordinated with processes of cell growth.