CDPK: Ca 2+-dependent protein kinase (soybean) (Calmodulin-like domain protein kinase)

Abstract

Ca2+-dependent protein kinases (CDPKs) are activated by the direct binding of Ca2+ to regulatory domains, which contain four EF-hand Ca2+-binding domains and are similar to calmodulin. This property distinguishes CDPKs from the Ca2+/calmodulin-dependent protein kinases and protein kinase C. CDPKs are activated by micromolar concentrations of free Ca2+ and are thought to play a role in Ca2+-mediated signal transduction pathways. CDPK is a monomer. Biochemical, immunological, and molecular genetic data indicate that there are at least four genes encoding CDPKs in soybean, with the enzymes ranging from 55 to 80 kDa. The complete sequence of one isoform (CDPKa encoded by cDNA SK5) has been determined. The predicted sequence of a second isoform encoded by a partial cDNA (SK2) is 70% identical to CDPKa. CDPKs have both catalytic and regulatory domains joined by an autoinhibitory domain. The regulatory domain is similar in sequence to calmodulin, especially within the four EF-hand Ca2+-binding domains. The molecular organization of the three functional domains is similar in the four CDPKs for which sequence data are available. Soybean CDPKa has short N- and C-terminal sequences of unknown function. Arabidopsis AK1 is 70% identical to CDPKa in the 492-residue overlap but is 116 residues longer at the N-terminus and 16 residues shorter at the C-terminus.