The Amino Acid Transport System y⁺L Induced in Xenopus laevis Oocytes by Human Choriocarcinoma Cell (JAR) mRNA Is Functionally Related to the Heavy Chain of the 4F2 Cell Surface Antigen

Injection of mRNA isolated from human placental choriocarcinoma cells (JAR) into Xenopus laevis oocytes induced the transport of the neutral amino acid leucine as well as the transport of the cationic amino acid arginine. The induced transport of leucine was predominantly Na⁺-dependent, whereas that of arginine was Na⁺-independent. The ratio of transport activity for these amino acids in mRNAinjected oocytes versus water-injected oocytes was much greater if the transport activity was measured at pH 5.5 instead of at pH 7.5. Leucine transport in mRNA-injected oocytes was inhibited to a marked extent by arginine and lysine. The bicyclic amino acid BCH and the N-methyl amino acid MeAIB had no effect. Arginine transport in mRNA-injected oocytes was insensitive to N-ethylmaleimide and inhibited markedly by micromolar concentrations of leucine in the presence of Na⁺. The inhibitory potency of leucine was reduced severalfold in the absence of Na⁺. These results indicated that the arginine transport activity induced in Xenopus oocytes by JAR cell mRNA was due to an amino acid transport system which is identical with or highly similar to y⁺L and that this system was responsible for a major portion of leucine transport activity measured in these oocytes. Northern blot analysis showed that normal placenta and JAR cells do not possess detectable levels of mRNA transcripts for D2, a protein closely related to the function of another transport system, namely b°⁺, which is also involved in the cellular uptake of neutral and cationic amino acids. On the other hand, the mRNA transcripts for the heavy chain of the 4F2 cell surface antigen are expressed at high levels in placenta and JAR cells. This protein is also known to induce the transport of neutral as well as cationic amino acids in Xenopus oocytes. Hybrid depletion of JAR cell mRNA using an antisense oligomer specific for the mRNA of this protein completely abolished the induction of the y⁺L-like activity in oocytes. It is concluded that the 4F2 cell surface antigen is responsible for or is a component/inducer of an amino acid transport system which is most likely identical with system y⁺L.