DISCRIMINATION OF 2 AMINO-ACID-TRANSPORT ACTIVITIES IN 4F2 HEAVY EXPRESSING XENOPUS-LAEVIS OOCYTES

Expression of the type II membrane proteins of the rbAT/4F2hc family i n Xenopus laevis oocytes results in the induction of amino acid transp ort activity. To elucidate the mechanism of action, amino acid transpo rt was investigated in oocytes expressing the surface antigen 4F2hc. L eucine transport was mediated by a Na+-independent and a Na+-dependent transport mechanism. Both systems could be further discriminated by t heir stereochemical constraints. Isoleucine, with a branch at the beta -position, shared only the Na+-independent transport system with leuci ne. Both transport systems were sensitive to inhibition by arginine, b ut only the Na+-independent system was sensitive to inhibition by 2-am inobicyclo[2,2, 1]heptane-2-carboxylic acid. When compared with known transport systems the two transport activities could be described as s imilar to, but not identical with, mammalian systems b(0,+) and y(+)L. The Na+-independent b(0,+)-like transport system was found both in rb AT and 4F2hc expressing oocytes, indicating that both proteins act in a similar way.