Modulation of ecotropic murine retroviruses by N-linked glycosylation of the cell surface receptor/amino acid transporter

Abstract

The cell surface receptor for ecotropic host-range (infection limited to mice or rats) murine leukemia viruses (MuLVs) is the widely expressed system y+ transporter for cationic amino acids (CAT-1). Like other retroviruses, ecotropic MuLV infection eliminates virus-binding sites from cell surfaces and results in complete interference to superinfection. Surprisingly, infection causes only partial (ca 40 to 60%) loss of mouse CAT-1 transporter activity. The NIH/Swiss mouse CAT-1 (mCAT-1) contains 622 amino acids with 14 hydrophobic potential membrane-spanning sequences, and it is known that the third extracellular loop from the amino terminus is required for virus binding. Although loop 3 is hypervariable in different species and mouse strains, consistent with its proposed role in virus-host coevolution, loop 3 sequences of both susceptible and resistant species contain consensus sites for N-linked glycosylation. Both of the consensus sites in loop 3 of mCAT-1 are known to be glycosylated and to contain oligosaccharides with diverse sizes (J. W. Kim and J. M. Cunningham, J. Biol. Chem. 268:16316-16320, 1993). We confirmed by several lines of evidence that N-linked glycosylation occludes a potentially functional virus-binding site in the CAT-1 protein of hamsters, thus contributing to resistance of that species. To study the role of receptor glycosylation in animals susceptible to infection, we eliminated loop 3 glycosylation sites by mutagenesis of an mCAT-1 cDNA clone, and we expressed wild-type and mutant receptors in mink fibroblasts and Xenopus oocytes. These receptors had indistinguishable transport properties, as determined by kinetic and voltage-jump electrophysiological studies of arginine uptake in oocytes and by analyses Of L-[3H]arginine uptake in mink cells. Bindings of ecotropic envelope glycoprotein gp7O to the accessible receptor sites on surfaces of mink cells expressing wild-type or mutant mCAT-1 were not significantly different in kinetics or in equilibrium affinities (i.e., K(D) approximately 3.7 X 10(-10) to 7.5 X 10(-10) M). However, when values were normalized to the same levels of mCAT-1 transporter expression, cells with wild-type glycosylated mCAT-1 had only approximately 50% as many sites for gp70 binding as cells with unglycosylated mCAT-1. Although infection with ecotropic MuLV had no effect on activity of the mink CAT-1 transporter that does not bind virus, it caused partial down-modulation of wild-type mCAT-1 and complete down-modulation of unglycosylated mutant mCAT-1. These results suggest that N-linked glycosylation causes wild-type mCAT-1 heterogeneity and that a significant proportion is inaccessible to virus. In part because only the interactive fraction of mCAT-1 can be down-modulated, infected murine cells conserve an amino acid transport capability that supports their viability.