Differential influence of the 4F2 heavy chain and the protein related to b0,+ amino acid transport on substrate affinity of the heteromeric b0,+ amino acid transporter

We provide evidence here that b(o,+) amino acid transporter (b(o,+)AT) interacts with 4F2 heavy chain (4F2hc) as well as with the protein related to b(o'+) amino acid transporter (rBAT) to constitute functionally competent b(o,+)-like amino acid transport systems. This evidence has been obtained by co-expression of b(o,+)AT and 4F2hc or b(o,+)AT and rBAT in human retinal pigment epithelial cells and in COS-l cells. The ability to interact with 4F2hc and rBAT is demonstrable with mouse b(o,+)AT as well as with human b(o,+)AT. Even though both the 4F2hc.b(o,+)AT complex and the rBAT.b(o,+)AT complex exhibit substrate specificity that is characteristic of system b(o,+), these two complexes differ significantly in substrate affinity. The 4F2hc.b(o,+) AT complex has higher substrate affinity than the rBAT.b(o,+)AT complex. In situ hybridization studies demonstrate that the regional distribution pattern of mRNA in the kidney is identical for b(o,+)AT and 4F2hc. The pattern of rBAT mRNA expression is different from that of b(o,+)AT mRNA and 4F2hc mRNA, but there are regions in the kidney where b(o,+)AT mRNA expression overlaps with rBAT mRNA expression as well as with 4F2hc mRNA expression.