CDPKs are dual-specificity protein kinases and tyrosine autophosphorylation attenuates kinase activity

Although calcium-dependent protein kinases (CDPKs or CPKs) are classified as serine/threonine protein kinases, autophosphorylation on tyrosine residues was observed for soybean CDPK beta and several Arabidopsis isoforms (AtCPK4 and AtCPK34). We identified Ser-8, Thr-17, Tyr-24 (in the kinase domain), Ser-304, and Ser-358 as autophosphorylation sites of His(6)-GmCDPK beta. Overall autophosphorylation increased kinase activity with synthetic peptides, but autophosphorylation of Tyr-24 appears to attenuate kinase activity based on studies with the Y24F directed mutant. While much remains to be done, it is clear that several CDPKs are dual-specificity kinases, which raises the possibility that phosphotyrosine signaling may play a role in Ca2+/CDPK-mediated processes.