BIOCHEMISTRY AND MOLECULAR-BIOLOGY OF THE VESICULAR MONOAMINE TRANSPORTER FROM CHROMAFFIN GRANULES

Prior to secretion, monoamines (catecholamines, serotonin, histamine) are concentrated from the cytoplasm into vesicles by vesicular monoami ne transporters (VMAT). These transporters also carry non-physiologica l compounds, e.g. the neurotoxin methyl-4-phenylpyridinium. VMAT acts as an electrogenic antiporter (exchanger) of protons and monoamines, u sing a proton electrochemical gradient. Vesicular transport is inhibit ed by specific ligands, including tetrabenazine, ketanserin and reserp ine. The mechanism of transport and the biochemistry of VMAT have been analyzed with the help of these tools, using mainly the chromaffin gr anules from bovine adrenal glands as a source of transporter. Although biochemical studies did not suggest a multiplicity of VMATs, two homo logous but distinct VMAT genes have recently been cloned from rat, bov ine and human adrenal glands. The VMAT proteins are predicted to posse ss 12 transmembrane segments, with both extremities lying on the cytop lasmic side. They possess N-glycosylation sites in a putative luminal loop and phosphorylation sites in cytoplasmic domains. In rat, VMAT(1) is expressed in the adrenal gland whereas VMAT(2) is expressed in the brain. in contrast, we found that the bovine adrenal gland expressed both VMAT(1) and VMAT(2). VMAT(2) corresponds to the major transporter of chromaffin granules, as shown by partial peptidic sequences of the purified protein and by a pharmacological analysis of the transport o btained in transfected COS cells (COS cells are monkey kidney cells po ssessing the ability to replicate SV-40-origin-containing plasmids). W e discuss the possibility that VMAT(1) may be specifically addressed t o large secretory granules vesicles, whereas VMAT(2) may also be addre ssed to small synaptic vesicles; species differences would then reflec t the distinct physiological roles of the small synaptic vesicles in t he adrenal gland.