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 overexpressed in different strains, and the ability to take up one or more of the 20 common L-alpha-amino acids was studied in order to obtain a complete picture of the substrate specificity for these permeases. Radiolabelled amino-acid uptake measurements showed that Agp1p is a general permease for most uncharged amino acids (Ala, Gly, Ser, Thr, Cys, Met, Phe, Tyr, Ile, Leu, Val, Gln and Asn). Gnp1p, which is closely related to Agp1p, has a somewhat less-broad specificity, transporting Leu, Ser, Thr, Cys, Met, Gln and Asn, while Bap2p and Bap3p, which are also closely related to Agp1p, are able to transport Ile, Leu, Val, Cys, Met, Phe, Tyr and Trp. All four permeases are transcriptionally induced by an extracellular amino acid, but differ in expression 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, which is a transporter for Glu, Asp, Gln, Asn, Ser, Ala and Gly, the rest of the permeases exhibit narrow specificity. Tat2p can take up Phe, Trp and Tyr; Put4p can transport Ala, Gly and Pro; while Can1p, Lyp1p and the previously uncharacterized Alp1p are specific for the cationic amino acids. These findings modify the prevalent view that S. cerevisiae only contains one general amino-acid permease, Gap1p, and a number of permeases that are specific for a single or a few amino acids.