Specificity and control of XA21-mediated signal transduction: the role of the juxtamembrane domain, WRKY factors and a PP2C phosphatase
Perception of extracellular signals by cell surface receptors is of central importance to eukaryotic development and immunity. Many of these receptors possess intrinsic protein kinase activity in their cytoplasmic domains (RKs) and regulate transcription of target genes through phosphorylation events.
The rice Xa21 gene, encodes an RK with leucine rich repeats (LRRs) in the extracellular domain and is a key recognition and signaling determinant in the innate immune response, a pathogen defense pathway widely conserved between plants and animals. The plant response to attempted infection by microbial pathogens is often accompanied by rapid cell death in and around the initial infection site, a reaction known as the hypersensitive response, a form of programmed cell death (PCD).
In contrast to most protein kinases that are regulated by autophosphorylation of the activation segment, a centrally located loop that sits close to the catalytic center, an emerging theme for some RKs is that the juxtamembrane (JM) domain plays a key regulatory role in their activation. In this case, residues in the JM domain perform both positive and negative regulatory functions. In their unphosphorylated state they repress catalytic activity and upon phosphorylation they serve as high affinity binding sites for downstream signaling proteins. Regulation of RK signaling through the JM domain is so far unique to only a few receptor families controlling important cellular processes and consequently there is great interest in exploring the molecular basis for this control.
XA21 is not autophosphorylated in the activation segment and at least four XA21 binding proteins (Xbs) require the JM region for interaction. These include Xb10, encoding a putative transcriptional regulator of the WRKY class and Xb15 encoding a PP2c phosphatase-like protein. Based on these results we hypothesize that the XA21 JM domain plays a key role in XA21 kinase-mediated signal transduction and that JM-binding proteins act as positive or negative regulators to control transcription of defense related genes.
We are testing this hypotheses using transgenic, genomic and proteomic tools. We have recently identified a rice line containing an insertion in the phosphatase domain of Xb15 indicating that this line likely lacks Xb15 function. Seedlings from this line display a PCD phenotype suggesting the intriguing possibility that Xb15 is a negative regulator of PCD. Although many cell-death regulators that have been characterized in humans, C. elegans and Drosophila the same genes have not been identified in the Arabidopsis and rice genomes. We are therefore quite interested in further investigating the possibility that the balance between XA21 kinase and XB15 phosphatase activities controls the expression of PCD, a key biological process in both plants and animals.
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