Svp25, a synaptic vesicle membrane glycoprotein from Torpedo electric organ that binds calcium and forms a homo-oligomeric complex

Abstract

Svp25 is a major glycoprotein of cholinergic synaptic vesicles isolated from the Torpedo electric organ. On SDS-PAGE svp25 migrates as a protein of Mr 25,000 and on two dimensional gel electrophoresis separates into several isoforms around a pI of 6.0. It binds concanavalin A and on phase separation with Triton X-114 behaves as an integral membrane protein. Svp25 represents a major vesicular 45Ca2(+)-binding protein. Under non-reducing conditions svp25 forms complexes of higher molecular weight which are multiples of 25,000. Svp25 is contained in the dense web of nerve terminal ramifications at the ventral side of the electroplaque cells. Colloidal gold labelling using a monospecific antibody confirms the selective association of the protein with synaptic vesicles. Although the function of the vesicular svp25 glycoprotein is not known, its ability to bind Ca2+ suggests that it is regulated by activation of the nerve terminal.