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The transport of amino acids across cell membranes is believed to be mediated by integral membrane proteins with distinct substrate specificities. Using expression cloning in Xenopus oocytes and assaying for the uptake of 14C-labeled cystine, we isolated a 2.3-kilobase cDNA (D2) from a rat kidney library. D2 is expressed specifically in kidney and intestine and induces the transport of both neutral and cationic amino acids. The deduced amino acid sequence predicts a 78-kDa protein with a single transmembrane domain, a structure not typical of the known membrane transport proteins, which generally have multiple membrane-spanning regions. The putative extracellular region is highly similar to the 4F2 heavy-chain cell surface antigen and to a family of alpha-glucosidases, which raises the possibility that D2 encodes a transport activator or regulatory subunit.
Wells et al. (1992) studied this question.