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 (Tyr)， 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 and dampen CERK1 signaling. CIPP1 subsequently dissociates from Tyr-dephosphorylated CERK1， allowing CERK1 to regain Tyr 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.