Effects of ecotropic murine retroviruses on the dual-function cell surface receptor/basic amino acid transporter

The widely expressed Na⁺-independent transporter for basic amino acids (system y⁺) is the cell surface receptor (ecoR) for ecotropic host-range mouse retroviruses (murine leukemia viruses (MuLVs)), a class of retroviruses that naturally infects only mice or rats. Accordingly, expression of mouse ecoR cDNA in mink CCL64 fibroblasts yields cells (CEN cells) that have y⁺ transporter activity above the endogenous background and that bind and are infected by ecotropic MuLVs. The effect of ecotropic MuLV infection on expression of y⁺ transporter was analyzed in mouse and in mink CEN fibroblasts. Chronic infection with ecotropic MuLVs caused 50-70% loss (down-modulation) of mouse y⁺ transporter in plasma membranes, detected as a reduced V_max for uptake and outflow of L-[³H]-arginine with no effect on K_m values. Down-modulation was specific for mouse y⁺ and did not affect other transporters or the endogenous mink y⁺, suggesting that it results from specific interaction between mouse y⁺ and the viral envelope glycoprotein gp70 in the infected cells. Because this partial loss of mouse y⁺ from cell surfaces is insufficient to explain the complete interference to superinfection that occurs in cells chronically infected with ecotropic MuLVs, alternative explanations for interference are proposed. In contrast to the y⁺ down-modulation caused by chronic infection, binding of extracellular envelope glycoprotein gp70 at 37°C resulted in noncompetitive inhibition of amino acid import by mouse y⁺ but had no effect on export through this same transporter or on any transporter properties of mink y⁺. The effects of gp70 on transport kinetics suggest that it slows the rate-limiting step of the amino acid import cycle, a conformational transition of the empty transporter in which the binding site moves from the inside back to the outside of the cell, and that gp70 has no effect on the rate-limiting step of the amino acid export cycle. Infected cells retain substantial y⁺ activity. Moreover, the virus binding site on ecoR is in a mobile region that changes conformation during the amino acid transport cycle.