Muscle-Like Contractile Proteins and Tubulin in Synaptosomes

Material in major bands with molecular weights corresponding to those of actin, brain tropomyosin, and myosin is present in purified rat synaptosomes dissolved in sodium dodecyl sulfate and subjected to electrophoresis on dodecyl sulfate-acrylamide gels. A band corresponding to tubulin appears to be the major constituent of synaptosomes, confirming the work of Feit and his coworkers. We have demonstrated by peptide mapping that the proteins in these bands have strong chemical similarities to actin, brain tropomyosin, myosin, and tubulin. We have prepared synaptic membrane, vesicle, and soluble fractions from synaptosomes. The polypeptide composition of synaptic membranes, as determined by dodecyl sulfate-acrylamide gel electrophoresis, is similar to that of synaptosomes, with tubulin, actin, and tropomyosin being major constituents. Synaptic vesicles have as their major polypeptide an unidentified protein with a molecular weight of 50,000; they also have many bands in common with synaptosomes. The soluble fraction predominantly contains actin and tubulin. The possibility that the muscle-like contractile proteins and tubulin are membrane-associated in various cell types is discussed, as is their possible role in neurotransmitter release.