Proteins of cholinergic synaptic vesicles from the electric organ of Torpedo: Characterization of a low molecular weight acidic protein

Abstract

A study has been made, using gel filtration, of the membrane-bound and soluble proteins obtained when cholinergic synaptic vesicles isolated from the electric organ of Torpedo marmorata and related species are exposed to water. The usual technique for releasing protein was to dialyze suspensions of vesicles separated by zonal centrifuging in distilled water; the freeze-dried residue was then filtered through columns of Biol-Gel A-5 m or Sephadex G-200. About 60% of the protein was recovered in a void volume fraction. On disc-gel electrophoresis in the presence of sodium dodecyl sulfate (SDS), this fraction gave 3 prominent high molecular weight components. Over 40% of the protein was a low molecular weight, acidic protein which we have called vesiculin. Hydrolysates were rich in hydroxy amino acids as well as glutamate and aspartate. Vesiculin is loosely associated with a small molecule, probably a nucleotide or mixture of nucleotides. Estimates of molecular weight by amino acid analysis, disc-gel electrophoresis, gel filtration and sedimentation coefficient all indicate a value close to 10,000. Vesiculin differs in composition and molecular weight from other acidic brain proteins. It is suggested that vesiculin may constitute the counter-ion to acetylcholine within the vesicle core.