Identification of myosin in isolated synaptic junctions

A monospecific antibody prepared against chicken gizzard myosin reacted with only one peptide corresponding to myosin heavy chain (Mr = 200,000) in gels of synaptic plasma membranes (SPM) and synaptic junctions (SJ) prepared from several species. Preadsorption of antisera with purified brain myosin eliminated antibody reactivity to SPMs and SJs. SJs were found to contain approximately 3 times the concentration of myosin found in SPMs when assayed by an indirect immunoradiometric assay. Postsynaptic density and myelin fractions contained no myosin detectable by immunoradiometric assay, antibody binding to gels, or Coomassie blue staining. The band identified as myosin in SJ fractions yielded peptide fingerprints indistinguishable from fingerprints of purified brain myosin but distinct from fingerprints of purified smooth and skeletal muscle myosins. The distribution of exogenous [ 125I]myosin during subcellular fractionation indicated that myosin in isolated synaptic junctions could not have resulted from artifactual re-distribution of soluble myosin. Together these results show that a non-muscle myosin is an endogenous component of CNS asymmetric synapses.