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+-med iated signals and is one of the most highly conserved proteins in euka ryotic cells. In plants, multiple calmodulin isoforms with minor amino acid sequence differences were identified but their functional signif icances are unknown. To investigate the biological function of calmodu lins in the regulation of calmodulin-dependent enzymes, we cloned cDNA s encoding calmodulins in soybean. Among the five cDNAs isolated from soybean, designated as SCam-1 to -5, SCaM-4 and -5 encoded very diverg ent calmodulin iso forms which have 32 amino acid substitutions from t he highly conserved calmodulin, SCaM-1 encoded by SCaM-1 and SCaM-3. S CaM-4 protein produced in Escherichia coli showed typical characterist ics of calmodulin such as Ca2+-dependent electrophoretic mobility shif t and the ability to activate phosphodiesterase. However, the extent o f 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 cont rast to SCaM-1. Also there were differences in the expression pattern of SCaM-1 and SCaM-4. Expression levels of SCaM-4 were approximately 5 -fold lower than those of SCaM-1 in apical and elongating regions of h ypocotyls. In addition, SCaM-4 transcripts were barely detectable in r oot whereas SCaM-1 transcripts were as abundant as in apical and elong ating regions of hypocotyls. In conclusion, the different biochemical properties together with differential expression of SCaM-4 suggest tha t this novel calmodulin may have different functions in plant cells.