Characterization of a second calcium-dependent protein kinase from winged bean

In plants, Ca2+ has emerged as the predominant second messenger for signal transduction, as cyclic nucleotides are not known to play any significant role in this system. Earlier, we characterized an interesting Ca(2+)-dependent protein kinase, WbCDPK (winged bean calmodulin-like domain protein kinase), from the soluble fraction of winged bean (Psophocarpus tetragonolobus) shoot extract. Here an isoform of WbCDPK is purified to apparent homogeneity from the same winged bean shoot extract. It is a single polypeptide chain protein-serine kinase, having an M(r) of about 70,000 and like WbCDPK, its preferred substrates are histone H1, syntide 2 and MLC-peptide (a synthetic myosin light chain related peptide) and it is totally dependent on Ca2+ for its activity, but exogenous calmodulin (CaM) does not stimulate it. However, it is strongly inhibited by CaM antagonists, indicating the presence of a CaM-like domain, as in WbCDPK. The two enzymes do not cross react immunologically and the isoform differs significantly from WbCDPK in its apparent inability to catalyse the autophosphorylation reaction, which is known to cause down-regulation of substrate phosphorylation in the case of WbCDPK.