Muscle-Like Contractile Proteins and Tubulin in Synaptosomes

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Abstract
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. Peptide mapping illustrated that the proteins in these bands have strong chemical similarities to actin, brain tropomyosin, myosin and tubulin. Synaptic membrane, vesicle and soluble fractions from synaptosomes were prepared. 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 MW 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.