Stimulation of Ca2+-Dependent Neurotransmitter Release and Presynaptic Nerve Terminal Protein Phosphorylation by Calmodulin and a Calmodulin-Like Protein Isolated from Synaptic Vesicles

Synaptic vesicles have a Ca²⁺-dependent protein kinase system that may play a role in mediating Ca²⁺-stimulated neurotransmitter release and vesicle function. Calcium's ability to initiate norepinephrine release and protein phosphorylation in synaptic vesicle preparations was shown to be stimulated by the presence of an endogenous heat-stable vesicle protein fraction. The heat stability and characteristics of this endogenous vesicle fraction were similar to those of calmodulin (Ca²⁺-dependent regular protein) isolated from rat and bovine brain. Calmodulin, like endogenous heat-stable vesicle factor, restored calcium's ability to stimulate vesicle neurotransmitter release and protein kinase activity. Calmodulin-like vesicle protein and purified calmodulin were also equally effective in stimulating cyclic nucleotide-dependent phosphodiesterase, further indicating that these two proteins are functionally equivalent. Depolarization-dependent Ca²⁺ uptake in intact synaptosomes simultaneously stimulated release of neurotransmitter and phosphorylation of particular synaptic vesicle proteins that were shown in the isolated vesicle preparation to be dependent on Ca²⁺ and calmodulin. The results suggest that calcium's effects on neurotransmitter release and presynaptic nerve terminal protein phosphorylation may be mediated by endogenous calmodulin-like proteins.