Substrate specificity and gene expression of the amino-acid permeases in Saccharomyces cerevisiae

All known amino-acid permeases (AAPs) in Saccharomyces cerevisiae belong to a single family of homologous proteins. Genes of 15 AAPs were overexpresse d in different strains, and the ability to take up one or more of the 20 co mmon L-alpha-amino acids was studied in order to obtain a complete picture of the substrate specificity for these permeases. Radiolabelled aminoacid u ptake measurements showed that Agp1p is a general permease for most uncharg ed amino acids (Ala, Gly, Ser, Thr, Cys, Met, Phe, Tyr, lie, Leu, Val, Gin and Asn). Gnp1p, which is closely related to Agp1p, has a somewhat less-bro ad specificity, transporting Leu, Ser, Thr, Cys, Met, Gin and Asn, while Ba p2p and Bap3p, which are also closely related to Agp1p, are able to transpo rt Ile, Leu, Val, Cys, Met, Phe, Tyr and Trp. All four permeases are transc riptionally induced by an extracellular amino acid, but differ in expressio n with respect to the nitrogen source. On a non-repressive nitrogen source, AGP1 is induced, while GLN1, BAP2 and BAP3 are not. Except for Dip5p, whic h is a transporter for Glu, Asp, Gin, Asn, Ser, Ala and Gly, the rest of th e permeases exhibit narrow specificity. Tat2p can take up Phe, Trp and Tyr, Put4p can transport Ala, Gly and Pro; while Can1p, Lyp1p and the previousl y uncharacterized Alp1p are specific for the cationic amino acids. These fi ndings modify the prevalent view that S. cerevisiae only contains one gener al amino-acid permease, Gap1p, and a number of permeases that are specific for a single or a few amino acids.