Evolution of functional and sequence variants of the mammalian XPR1 receptor for mouse xenotropic gammaretroviruses and the human-derived retrovirus XMRV.

Genetic conflicts between retroviruses and their receptors result in the evolution of novel host entry restrictions and novel virus envelopes, and such variants can influence trans-species transmission. We screened rodents and other mammals for sequence variation in the Xpr1 receptor for the mouse xenotropic or polytropic mouse leukemia viruses (X-MLVs or P-MLVs, respectively) of the gammaretrovirus family and for susceptibility to mouse-derived X/P-MLVs and to XMRV (xenotropic murine leukemia virus-related virus), an X-MLV-like virus isolated from humans with prostate cancer and chronic fatigue syndrome. We identified multiple distinct susceptibility phenotypes; these include the four known Xpr1 variants in Mus and a novel fifth Xpr1 gene found in Mus molossinus and Mus musculus. We describe the geographic and species distribution of the Mus Xpr1 variants but failed to find the X-MLV-restrictive laboratory mouse allele in any wild mouse. We used mutagenesis and phylogenetic analysis to evaluate the functional contributions made by constrained, variable, and deleted residues. Rodent Xpr1 is under positive selection, indicating a history of host-pathogen conflicts; several codons under selection have known roles in virus entry. All non-Mus mammals are susceptible to mouse X-MLVs, but some restrict other members of the X/P-MLV family, and the resistance of hamster and gerbil cells to XMRV indicates that XMRV has unique receptor requirements. We show that the hypervariable fourth extracellular XPR1 loop (ECL4) contains three evolutionarily constrained residues that do not contribute to receptor function, we identify two novel residues important for virus entry (I579 and T583), and we describe a unique pattern of ECL4 variation in the three virus-restrictive Xpr1 variants found in MLV-infected house mice; these mice carry different deletions in ECL4, suggesting either that these sites or loop size affects receptor function.