Differential influence of the 4F2 heavy chain and the protein related to b^0,+ amino acid transport on substrate affinity of the heteromeric b^0,+ amino acid transporter

We provide evidence here that b^0,+ amino acid transporter (b^0,+ AT) interacts with 4F2 heavy chain (4F2hc) as well as with the protein related to b amino acid transporter (rBAT) to constitute functionally competent b^0,+-like amino acid transport systems. This evidence has been obtained by co-expression of b^0,+ AT and 4F2hc or b^0,+AT and rBAT in human retinal pigment epithelial cells and in COS-1 cells. The ability to interact with 4F2hc and rBAT is demonstrable with mouse b^0,+AT as well as with human b^0,+AT. Even though both the 4F2hc.b^0,+AT complex and the rBAT·b^0,+ AT complex exhibit substrate specificity that is characteristic of system b^0,+, these two complexes differ significantly in substrate affinity. The 4F2hc·b^0,+AT complex has higher substrate affinity than the rBAT·b^0,+AT complex. In situ hybridization studies demonstrate that the regional distribution pattern of mRNA in the kidney is identical for b^0,+ AT and 4F2hc. The pattern of rBAT mRNA expression is different from that of b^0,+AT mRNA and 4F2hc mRNA, but there are regions in the kidney where b^0,+AT mRNA expression overlaps with rBAT mRNA expression as well as with 4F2hc mRNA expression.