The PR5K receptor protein kinase from Arabidopsis thaliana is structurally related to a family of plant defense proteins

We have isolated an Arabidopsis thaliana gene that codes for a receptor related to antifungal pathogenesis-related (PR) proteins, The PR5K gene codes for a predicted 665-amino acid polypeptide that comprises an extracellular domain related to the PR5 proteins, a central transmembrane-spanning domain, and an intracellular protein-serine/threonine kinase, The extracellular domain of PR5K (PR5-like receptor kinase) is most highly related to acidic PR5 proteins that accumulate in the extracellular spaces of plants challenged with pathogenic microorganisms. The kinase domain of PR5K is related to a family of protein-serine/threonine kinases that are involved in the expression of self-incompatibility and disease resistance, PR5K transcripts accumulate at low levels in all tissues examined, although particularly high levels are present in roots and inflorescence stems. Treatments that induce authentic PR5 proteins had no effect on the level of PR5K transcripts, suggesting that the receptor forms part of a preexisting surveillance system, When the kinase domain of PR5K was expressed in Escherichia coli, the resulting polypeptide underwent autophosphorylation, consistent with its predicted enzyme activity, These results are consistent with PR5K encoding a functional receptor kinase, Moreover, the structural similarity between the extracellular domain of PR5K and the antimicrobial PR5-proteins suggests a possible interaction with common or related microbial targets.