Amino-acid transport across cellular plasma membranes depends on several parallel-functioning (co-)transporters and exchangers＇. The widespread transport system L accounts for a sodium-independent exchange of large, neutral amino acids, whereas the system y(+)L exchanges positively charged amino acids and/or neutral amino acids together with sodium(2,3). The molecular nature of these transporters remains unknown, although expression of the human cell-surface glycoprotein 4F2 heavy chain (h4F2hc; CD98 in the mouse)(4,5) is known to induce low levels of L- and/or y(+)L-type transport(6-9). This glycoprotein is found in activated lymphocytes, together with an uncharacterized, disulphide-linked lipophilic light chain with an apparent relative molecular mass of 40,000 (M-r 40 K)(10,11). Here we identify the permease-related protein E16 (ref. 12) as the first light chain of h4F2hc and show that the resulting heterodimeric complex mediates L-type amino-acid transport. The homologous protein from Schistosoma mansoni, SPRM1, also associates covalently with coexpressed h4F2hc glycoprotein, although it induces amino-acid transport of different substrate specificity. The coexpression of h4F2hc is required for surface expression of these permease-related light chains, which belong to a new family of amino-acid transporters that form heterodimers with cell-surface glycoproteins.