Isolation of Synaptosomal Plasma Membranes from Cholinergic Nerve Terminals and a Comparison of Their Proteins with Those of Synaptic Vesicles

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Abstract
Plasma membranes were purified from purely cholinergic nerve endings (synaptosomes) isolated from the electric organ of Torpedo marmorata. Synaptosomes were lysed, membranes recovered and further separated by density gradient centrifugation. A fraction was obtained enriched in 5''-nucleotidase, Na+, K+-activated ATPase and acetylcholinesterase. Morphological examination showed abundant membrane fragments of the size range of synaptosomes and few of vesicle size. The fraction has a characteristic protein composition upon gel electrophoresis. Five reproducible major bands with apparent Mr [MW ratio] of 100,000, 75,000, 52,000, 42,000 and 35,000-33,000 are found. A gel-electrophoretic comparison with proteins from synaptic vesicles from the same source (major bands Mr 160,000, 147,000, 42,000, 34,000 and 25,000) was made. Comigration of major bands was detected in one-dimensional gel electrophoresis with the 42,000-Mr, 35,000-33,000-Mr and 34,000-Mr components. Upon 2-dimensional gel electrophoresis the 42,000-Mr component comigrates with a similar component in vesicles, recently characterized as actin; the other components are different. The presence of tubulin-like polypeptides is unlikely. Beside actin, all major vesicle proteins are often detected in small amounts in the plasma membrane preparation. It cannot be decided if they result from fused or contaminating vesicle membranes, but since they are essentially absent in some preparations, it seems that the plasma membrane does not contain vesicle proteins.