Molecular characterization of synaptophysin, a major calcium-binding protein of the synaptic vesicle membrane.

Abstract

Synaptophysin, a mol. wt 38 000 glycopolypeptide of the synaptic vesicle membrane, was solubilized using Triton X‐100 and purified by immunoaffinity or ion‐exchange chromatography. From gel permeation and sucrose‐density centrifugation in H2O/D2O, a Stokes radius of 7.3 nm, a partial specific volume of 0.830 and a total mol. wt of 119 000 were calculated for the native protein. Cross‐linking of synaptic vesicles with glutaraldehyde, dimethylsuberimidate, or Cu2+ ‐o‐phenantroline, resulted in the formation of a mol. wt 76 kd dimer of synaptophysin. Crosslinking of the purified protein in addition produced tri‐ and tetrameric adducts of the polypeptide. Native synaptophysin thus is a homooligomeric protein. Synaptophysin is N‐glycosylated, since cultivation of the rat phaeochromocytoma cell line PC12 in the presence of tunicamycin reduced its mol. wt by about 6 kd. Upon transfer to nitrocellulose and incubation with 45Ca2+, synaptophysin behaved as one of the major calcium‐binding proteins of the synaptic vesicle membrane. Pronase treatment of intact synaptic vesicles abolished this 45Ca2+ binding indicating that the Ca2+ binding site of synaptophysin must reside on a cytoplasmic domain of the transmembrane polypeptide. Based on these data, we propose that synaptophysin may play an important role in Ca2+‐dependent neurotransmitter release.

Presynaptic plasticity: The regulation of Ca2+-dependent transmitter release

“…Synaptophysin and synaptoporin are two isoforms of a ubiquitous protein, present in all brain areas and co-localizing on synaptic vesicles (Fykse et al, 1993). Synaptophysin was found to bind Ca 2 ÷ (Rehm et al, 1986) and to form pores in lipid bilayers as a multimeric structure comparable to the formation of gap junctions by the related protein connexin (see Thomas et al, 1988;). It has been suggested that synaptophysin is able to form the actual fusionpore, probably in concert with a plasma membrane counterpart physophilin as a docking protein or receptor (Thomas and Betz, 1990).…”

Quantification of p38/synaptophysin in highly purified adrenal medullary chromaffin vesicles

“…p38/synaptophysin is a membrane protein present in clear (synaptic) vesicles of neurons and endocrine cells [1][2][3][4]. From the amino acid sequence deduced from cDNAs encoding p38/synaptophysin, a model with several membrane spanning polypeptide segments and a carboxy-terminal protein domain exposed to the cytoplasmic surface has been constructed [5][6][7].…”

“…It has been suggested to form a transmembrane channel for ions, or to interact with cytoplasmic factors via its cytoplasmic domain [7]. Since synaptophysin binds Ca 2+, it may also play a role in the release of neurotransmitters stored in clear (synaptic) vesicles [3].…”

Differential regulation of synaptic vesicle proteins by antidepressant drugs

“…Keywords: depression; psychopharmacology; psychosis; SSRI; synapsin; synaptophysin INTRODUCTION Synaptic vesicle proteins (SVP) such as synapsin I-III, synaptophysin, synaptotagmin, and synaptobrevin (VAMP -vesicle-associated membrane protein) play a critical role in synaptic plasticity, and are required for vesicle fusion and neurotransmitter release. [1][2][3][4][5][6] Hence, changes in the expression of these proteins may contribute to the molecular effects of antidepressant treatment and associated behavioral and cognitive alterations. Previous studies have shown that antidepressants modulate de novo gene transcription and synthesis of proteins involved in neural and synaptic plasticity; in a recent transcriptomic study, a decreased expression of VAMP was found with imipramine and sertraline.…”