A Tyrosine Phosphorylation Cycle Regulates Fungal Activation of a Plant Receptor Ser/Thr Kinase

Plants initiate immunity by cell-surface pattern-recognition receptors (PRRs), which perceive non-self molecules. PRRs are predominantly receptor serine/threonine (Ser/Thr) kinases that are evolutionarily related to animal interleukin-1 receptor-associated kinase (IRAK)/Pelle-soluble kinases. However, how the activity of these receptor kinases is modulated remains poorly understood. We report that the Arabidopsis PRR chitin elicitor receptor kinase 1 (CERK1) is autophosphorylated in unstimulated cells at tyrosine(428) (Tyr(428)), a modification that is required for CERK1 activation upon binding to the fungal cell wall component chitin. Upon chitin activation, CERK1 recruits the CERK1-interacting protein phosphatase 1 (CIPP1), a predicted Ser/Thr phosphatase, to dephosphorylate Tyr(428) and dampen CERK1 signaling. CIPP1 subsequently dissociates from Tyr(428)-dephosphorylated CERK1, allowing CERK1 to regain Tyr(428) autophosphorylation and return to a standby state. This work sheds light onto plant chitin signaling and shows that a receptor kinase and phosphatase can coordinately regulate signal transduction of a receptor kinase through a phosphorylation cycle.