The characterization and phosphorylation of an actin-like protein in synaptosomal membranes.

A protein of 43,000 daltons, named protein 'C', is a component of synaptosomal plasma membranes, vesicular and microsomal membranes, as well as synaptosomal and cellular cytoplasm. Protein 'C' undergoes endogeneous phosphorylation in synaptosomal plasma membranes but not in other subcellular fractions. This phosphorylation is stimulated by papaverine and calcium, inhibited by magnesium and not affected by cyclic nucleotides. Protein 'C' and muscle actin were shown to be very similar by isoelectric focusing, two dimensional gel electrophoresis, and by peptide mapping. This suggests that protein 'C' is an actin-like protein which undergoes endogenous phosphorylation specifically in synaptosomal plasma membranes. Phosphorylation of protein 'C' may be involved in neurotransmitter release.