Abstract
MAPKs are universal eukaryotic signaling factors whose functioning is assumed to depend on the recognition of a common docking motif (CD) by its activators, substrates, and inactiva- tors. We studied the role of the CD domain of Arabidopsis MPK4 by performing interaction stu- dies and determining the ligand-bound MPK4 crystal structure. We revealed that the CD domain of MPK4 is essential for interaction and activation by its upstream MAPKKs MKK1, MKK2, and MKK6. Cys181 in the CD site of MPK4 was shown to become sulfenylated in response to reactive oxygen species in vitro. To test the function of C181 in vivo, we generated wild-type (WT) MPK4-C181, nonsulfenylatable MPK4-C181S, and potentially sulfenylation mimicking MPK4-C181D lines in the mpk4 knockout back- ground. We analyzed the phenotypes in growth, development, and stress responses, reveal- ing that MPK4-C181S has WT activity and complements the mpk4 phenotype. By contrast, MPK4-C181D cannot be activated by upstream MAPKK and cannot complement the pheno- types of mpk4. Our findings show that the CD motif is essential and is required for activation by upstream MAPKK for MPK4 function. Furthermore, growth, development, or immunity functions require upstream activation of the MPK4 protein kinase.