Cell Wall Associated, Receptor Protein Kinases

细胞壁是包围和分隔植物细胞的碳水化合物和蛋白质结构。当植物细胞以有规律的方式扩张时，它们必须改变和扩大细胞壁以允许随后体积的增加。细胞壁本身可能会影响这一扩张过程。因此，细胞有可能影响植物功能的几乎每个方面，仅仅是因为它的位置和物理特性。由于这个原因，许多人推测它在植物发育和对外界反应中的作用。细胞生长可以在许多方面发生，如花粉管顶端的极化扩张，许多营养组织特有的伸长细胞的产生，分生组织的建立，甚至是细胞在叶表面的锯齿状排列。细胞壁相关激酶（Cell Wall Associated Kinases，WAKs）是细胞壁结构与细胞发育过程中细胞膨胀之间的协调机制之一。目前，被子植物对细胞壁结构与细胞发育过程中的程序化和激素调控的细胞膨胀之间的关系还不十分清楚。最近的研究发现，细胞表面的许多蛋白质可以直接调控细胞壁的功能，WAKs就是其中之一。在拟南芥中有五种细胞壁相关激酶，在其它被子植物科中有五种细胞壁相关激酶。WAK都有一个胞质丝氨酸/苏氨酸蛋白激酶结构域，跨越质膜并延伸到细胞壁中。WAK将质膜与碳水化合物基质物理连接起来，其独特之处在于它们具有通过其激酶结构域直接向细胞事件发出信号的潜力。WAK胞外结构域在五种亚型之间可变，并且该家族在所有器官中共同表达。WAK 1和WAK 2是五个串联基因中表达最广泛和最丰富的，它们的信息存在于营养分生组织、器官类型的连接处和细胞扩展区域。病原体感染和创伤也可诱发。在体外试验中，WAK 1而不是WAK 2细胞壁结构域与细胞壁的富含甘氨酸的蛋白（GRP）结合。WAK 1和GRP可以从叶片或幼苗提取物中共沉淀，并且该WAK是磷酸化的。最近的实验表明，反义WAK表达导致细胞扩增的损失，并且初步结果表明，反义GRP产生导致细胞扩增的增强（和最终的细胞死亡）。因此，GRP可能是WAK活性的负调节因子。大量的WAK也与果胶共价连接，并且与果胶结合的大多数WAK也被磷酸化。这些数据支持WAK 1作为磷酸化激酶与GRP结合，并与果胶结合的模型。WAK如何参与果胶细胞外基质蛋白与GRP的协同作用是理解细胞壁在细胞生长中作用的关键，WAK在拟南芥发育过程中为许多细胞类型提供了从细胞壁到细胞质的直接信号。为了研究植物中GRP和WAK的关联和作用，有必要产生两种主要WAK和GRP的额外的（部分）功能丧失等位基因，然后利用这些等位基因来确定WAK介导的细胞扩张与其他已知信号通路的关系。WAK激酶底物也将被鉴定并用于检测细胞壁成分和发育线索对WAK的激活，最终这些底物的突变等位基因将有助于WAK和细胞伸长的分析。我们的目标是了解WAKs与GRP和果胶是如何调节细胞质成分，使细胞沃利斯与细胞生长过程相协调的。

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 enlargetheir cell walls to permit the subsequent increase in volume. The cell wall itself mightinfluence this expansion process. Thus the cell has the potential to influence almost everyaspect of plant function simply because of its position and physical properties. For thisreason many have speculated on its role in a plant's development and response to theoutside world. Cell growth can occur in many dimensions, such as the polarizedexpansion of a pollen tube tip, the creation of elongated cells characteristic of manyvegetative tissues, the establishment of a meristem, or even the jigsaw-like arrangementof cells at the leaf surface. Currently the understanding of how cell wall architecture iscoordinated with programmed and hormone regulated cell expansion events duringdevelopment is not well understood in angiosperms.Recent studies have identified a number of proteins at the cell surface that coulddirectly regulate cell wall functions, and the Cell Wall Associated Kinases, or WAKs, areamong these. There are five cell wall associated kinases in Arabidopsis andrepresentatives in other angiosperm families. WAKs each have a cytoplasmicserine/threonine protein kinase domain, span the plasma membrane and extend a domaininto the cell wall. WAKs physically link the plasma membrane to the carbohydrate matrixand are unique in that they have the potential to directly signal cellular events throughtheir kinase domain.The WAK extracellular domain is variable between the five isoforms, and collectivelythe family is expressed in all organs. WAK1 and WAK2 are the most ubiquitously andabundantly expressed of the five tandemly arrayed genes, and their messages are presentin vegetative meristems, junctions of organ types, and areas of cell expansion. They arealso induced by pathogen infection and wounding. The WAK1 but not WAK2 cell walldomain binds to a glycine rich protein (GRP) of the cell wall in in vitro assays. WAK1and GRP can be co-immunoprecipitated from leaf or seedling extracts, and this WAK isphosphorylated. Recent experiments demonstrate that antisense WAK expression leadsto the loss of cell expansion, and preliminary results indicate that antisense GRPproduction causes an enhancement of cell expansion (and eventual cell death). Thus GRPmay be a negative regulator of WAK activity. A large amount of WAK is also covalentlylinked to pectin, and most of WAK that is bound to pectin is also phosphorylated. Thedata support a model where WAK1 is bound to GRP as a phosphorylated kinase, and alsobinds to pectin. How WAKs are involved in signaling from the pectin extracellular matrixin coordination with GRPs will be key to the understanding of the cell wall's role in cellgrowth.WAKs are poised to provide a direct signal from the cell wall to the cytoplasm of manycell types during the development of Arabidopsis. In order to characterize theassociations and roles of GRPs and WAKs in plants it will be necessary to generateadditional (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 cellexpansion to other known signaling pathways. WAK kinase substrates will be alsoidentified and used to assay the activation of WAK by wall components anddevelopmental cues, and eventually mutant alleles of these substrates will help in theanalysis of WAKs and cell elongation. The goal is to understand how WAKs inassociation with GRP and pectins regulate cytoplasmic components so that the cell wallis coordinated with processes of cell growth.