Identification of a Novel Divergent Calmodulin Isoform from Soybean Which Has Differential Ability to Activate Calmodulin-dependent Enzymes (∗)

Calmodulin plays pivotal roles in the transduction of various Ca2+-mediated signals and is one of the most highly conserved proteins in eukaryotic cells. In plants, multiple calmodulin isoforms with minor amino acid sequence differences were identified but their functional significances are unknown. To investigate the biological function of calmodulins in the regulation of calmodulin-dependent enzymes, we cloned cDNAs encoding calmodulins in soybean. Among the five cDNAs isolated from soybean, designated as SCaM-1to −5, SCaM-4and −5encoded very divergent calmodulin isoforms which have 32 amino acid substitutions from the highly conserved calmodulin, SCaM-1 encoded by SCaM-1and SCaM-3. SCaM-4 protein produced in Escherichia colishowed typical characteristics of calmodulin such as Ca2+-dependent electrophoretic mobility shift and the ability to activate phosphodiesterase. However, the extent of mobility shift and antigenicity of SCaM-4 were different from those of SCaM-1. Moreover, SCaM-4 did not activate NAD kinase at all in contrast to SCaM-1. Also there were differences in the expression pattern of SCaM-1and SCaM-4. Expression levels of SCaM-4were approximately 5-fold lower than those of SCaM-1in apical and elongating regions of hypocotyls. In addition, SCaM-4transcripts were barely detectable in root whereas SCaM-1transcripts were as abundant as in apical and elongating regions of hypocotyls. In conclusion, the different biochemical properties together with differential expression of SCaM-4 suggest that this novel calmodulin may have different functions in plant cells.