Mutation of the 4F2 heavy-chain carboxy terminus causes y+ LAT2 light-chain dysfunction

Heteromeric amino acid transporters are composed of two subunits--a multipass membrane protein called the 'light chain'--and a single pass glycoprotein called the 'heavy chain'. The light chain contains the transport pore, while the heavy chain appears to be necessary for trafficking the light chain to the plasma membrane. In this study, the role of the 4F2hc heavy chain in the function of the y+ LAT2 light chain was investigated. Carboxy terminal truncations and site specific mutants of 4F2hc were co-expressed in Xenopus laevis oocytes with the y+ LAT2 light chain, and the oocytes were analysed for transport activity and surface expression. Truncations of the 4F2hc carboxy terminus ranging between 15 and 404 residues caused a complete loss of light chain function, although all heterodimers were expressed at the cell surface. This indicated that the 15 carboxy-terminal residues of 4F2hc are required for the transport function of the heterodimer. Mutation of the conserved residue leucine 523 to glutamine in the carboxy terminus reduced the Vmax of arginine and leucine uptake. The affinity of the transporter for both arginine and leucine remained unaltered, but the Km-value of Na+, being cotransported with leucine, increased about three-fold. The change of the Na+ Km caused a specific defect of leucine efflux, whereas uptake of leucine at high extracellular NaCl concentration was unaffected.