Multiple calmodulin (CaM) isoforms are expressed in plants, but their biochemical characteristics are not well resolved. Here we show the differential regulation exhibited by two soya bean CaM isoforms (SCaM-1 and SCaM-4) for the activation of five CaM-dependent enzymes, and the Ca2+ dependence of their target enzyme activation. SCaM-1 activated myosin light-chain kinase as effectively as brain CaM (Kact 1.8 and 1.7nM respectively), but SCaM-4 produced no activation of this enzyme. Both CaM isoforms supported near maximal activation of CaM-dependent protein kinase II (CaM KII), but SCaM-4 exhibited approx.12-fold higher Kact than SCaM-1 for CaM KII phosphorylation of caldesmon. The SCaM isoforms showed differential activation of plant and animal Ca2+-ATPases. The plant Ca2+-ATPase was activated maximally by both isoforms, while the erythrocyte Ca2+-ATPase was activated only by SCaM-1. Plant glutamate decarboxylase was activated fully by SCaM-1, but SCaM-4 exhibited an approx. 4-fold increase in Kact and an approx. 25% reduction in Vmax. Importantly, SCaM isoforms showed a distinct Ca2+ concentration requirement for target enzyme activation. SCaM-4 required 4-fold higher [Ca2+] for half-maximal activation of CaM KII, and 1.5-fold higher [Ca2+] for activation of cyclic nucleotide phosphodiesterase than SCaM-1. Thus these plant CaM isoforms provide a mechanism by which a different subset of target enzymes could be activated or inhibited by the differential expression of these CaM isoforms or by differences in Ca2+ transients.

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