We recently identified a gene (TPO1, YLL028w) that encodes a polyamine transport protein on the vacuolar membrane in yeast [Tomitori, Kashiwagi, Sakata, Kakinuma and Igarashi (1999) J. Biol. Chem. 274, 3265–3267]. Because the existence of one or more other genes for a polyamine transport protein on the vacuolar membrane was expected, we searched sequence databases for homologues of the protein encoded by TPO1. Membrane proteins encoded by the open reading frames YGR138c (TPO2), YPR156c (TPO3) and YOR273c (TPO4) were postulated to be polyamine transporters and, indeed, were subsequently shown to be polyamine transport proteins on the vacuolar membrane. Cells overexpressing these genes were resistant to polyamine toxicity and showed an increase in polyamine uptake activity and polyamine content in vacuoles. Furthermore, cells in which these genes were disrupted showed an increased sensitivity to polyamine toxicity and a decrease in polyamine uptake activity and polyamine content in vacuoles. Resistance to polyamine toxicity in cells overexpressing the genes was overcome by bafilomycin A1, an inhibitor of the vacuolar H+-ATPase. Among the four polyamine transporters, those encoded by TPO2 and TPO3 were specific for spermine, whereas those encoded by TPO1 and TPO4 recognized spermidine and spermine. These results suggest that polyamine content in the cytoplasm of yeast is elaborately regulated by several polyamine transport systems in vacuoles. Furthermore, it was shown that Glu-207, Glu-324 (or Glu-323) and Glu-574 of TPO1 protein were important for the transport activity.
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