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We have identified a new human cDNA (y⁺L amino acid transporter-1 (y⁺LAT-1)) that induces system y⁺L transport activity with 4F2hc (the surface antigen 4F2 heavy chain) in oocytes. Human y⁺LAT-1 is a new member of a family of polytopic transmembrane proteins that are homologous to the yeast high affinity methionine permease MUP1. Other members of this family, theXenopus laevis IU12 and the human KIAA0245 cDNAs, also co-express amino acid transport activity with 4F2hc in oocytes, with characteristics that are compatible with those of systems L and y⁺L, respectively. y⁺LAT-1 protein forms a ≈135-kDa, disulfide bond-dependent heterodimer with 4F2hc in oocytes, which upon reduction results in two protein bands of ≈85 kDa (i.e. 4F2hc) and ≈40 kDa (y⁺LAT-1). Mutation of the human 4F2hc residue cysteine 109 (Cys-109) to serine abolishes the formation of this heterodimer and drastically reduces …