Transport of sulfonium compounds - Characterization of the S-adenosylmethionine and S-methylmethionine permeases from the yeast Saccharomyces cerevisiae

We report here the characterization and the molecular analysis of the two h igh affinity permeases that mediate the transport of S-adenosylmethionine ( AdoMet) and S-methylmethionine (SMM) across the plasma membrane of yeast ce lls. Mutant cells unable to use AdoMet as a sulfur source were first isolat ed and demonstrated to lack high affinity AdoMet transport capacities. Func tional complementation cloning allowed us to identify the corresponding gen e (SAM3), which encodes an integral membrane protein comprising 12 putative membrane spanning regions and belonging to the amino acid permease family. Among amino acid permease members, the closest relative of Sam3p is encode d by the YLL061w, open reading frame. Disruption of YLL061w was shown to sp ecifically lead to cells unable to use SMM as a sulfur source. Accordingly, transport assays demonstrated that YLL061w disruption mutation impaired th e high affinity SMM permease, and YLL061w was therefore renamed MMP1. Furth er study of sam3 Delta and mmp1 Delta mutant cells showed that in addition to high affinity permeases, both sulfonium compounds are transported into y east cells by low affinity transport systems that appear to be carrier-faci litated diffusion.