Extensive sequence divergence between the human and rat brain vesicular monoamine transporter: Possible molecular basis for species differences in the susceptibility to MPP+

A cDNA encoding the human brain vesicular monoamine transporter (VMT) was isolated and sequenced using PCR. The cDNA contains an open reading frame encoding a hydrophobic polypeptide of 514 amino acids with twelve membrane spanning segments, a calculated molecular weight of 55,709 Da, and an estimated isoelectrical point of 5.62. A structurally identical transporter is expressed in human platelets. Two intraplasmatic consensus phosphorylation sites of cAMP-dependent protein kinase recognition and two potential protein kinase C phosphorylation sites may be central to the regulation of the VMT. Although the human brain VMT is 90.7% homologous to the rat protein, an extensive sequence divergence occurs in the large luminal loop located between the first two transmembrane domains. This loop displays a remarkably reduced homology of 64.0% with several deletions and insertions, although four putative glycosylation sites are conserved. Since functional vesicular monoamine transport supresses MPP+ toxicity and sequence divergence in the large luminal loop of the VMT expressed in rat brain and adrenal medulla may play a role in differential neurotoxic effects of MPP+, our findings indicate one possible molecular basis for the substantial species differences in the suceptibility to MPP+ demonstrated among humans, non-human primates, and rodents. They are also likely to facilitate molecular pharmacologic and genetic investigations of the human VMT in neurodegenerative disorders.