Retroviral Capsid Determinants of Fv1 NB and NR Tropism

Abstract

The specificity determinants for susceptibility to resistance by the Fv1 n and b alleles map to amino acid 110 of the murine leukemia virus CA protein. To study the interaction between Fv1 and CA, we examined changes in CA resulting in the loss of susceptibility to Fv1 resistance in naturally occurring NB-and NR-tropic viruses. A variety of amino acid changes affecting Fv1 tropism were identified, at CA positions 82, 92 to 95, 105, 114, and 117, and they all were mapped to the apparent exterior of virion-associated CA. These amino acids may form a binding surface for Fv1.The Fv1 gene is one of a series of mouse genes, originally described in the early 1970s, that control the susceptibility of mice to murine leukemia virus (MLV) infection (32,34 nr , found in a few inbred strains of mice and apparently also in some wild mice, restricts B-tropic MLV and some, but not all, N-tropic viruses (28,48). In this paper, we will refer to N-tropic viruses that are not restricted by Fv1 nr as being NR tropic.Fv1 acts in a cell-autonomous manner to restrict virus replication (44), but the precise mechanism for restriction is unclear. It has been shown that viral replication is blocked at a stage after virus entry into the cell and before the integration of newly synthesized viral DNA into the host genome (22, 41). The block to infection is not absolute in vitro, but the number of infected cells is reduced by a factor of 50 to 1,000 (17). When expressed at natural levels, e.g., in mouse fibroblast lines, neither Fv1 n nor Fv1 b shows significant in vitro restriction of NB-tropic MLV.Genetic studies initially suggested that the target for Fv1 restriction is the MLV capsid (CA) protein (20, 43). Subsequent studies indicated that viral tropism is determined by a pair of adjacent amino acids, at positions 109 and 110, in CA (10, 40). A more recent study has shown that position 110 is the most important residue for N and B tropism (29). N-tropic MLV has an Arg residue at this position, and B-tropic MLV has a Glu residue. The determinants for NB and NR tropism have not been fully characterized.The Fv1 gene was cloned a few years ago (3) and was found to have sequence similarity to a family of endogenous retroviruses called HERV-L (60% identity over 1.3 kb) or MuERV-L (1, 3). Based on its position within the element and the presence of a major homology region (3), Fv1 apparently encodes a Gag-related protein. Gag proteins bind tightly to each other via interaction domains during virus assembly (35), which suggests a possible mechanism for Fv1's action on MLV CA (16). To date, however, there is no direct evidence for the binding of Fv1 to CA. Biochemical analyses are greatly complicated by the extremely low natural expression levels of Fv1 in vivo. We have therefore taken a genetic approach to analyzing the viral determinants of NB and NR tropism, using a rapid fluorescence-activated cell sorting (FACS)-based approach for Fv1 testing (5), in an attempt to delineate the region(s) of CA that interacts with Fv1. MATERIALS AND METHODS Cel...

Origin, antiviral function and evidence for positive selection of the gammaretrovirus restriction gene Fv1 in the genus Mus

“…The differences include a 9-base in-frame deletion near the N terminus, and 9-aa differences at the C terminus due to the B1 insertion. The rest of the differences are scattered and notably include differences at the 3 amino acid positions (352, 358, and 399) that distinguish Fv1 n , Fv1 b , and Fv1 nr and have been shown to modulate resistance (3,5,6). Residues at these positions for the known restrictive Fv1 alleles are: SKV (Fv1 n ), SER (Fv1 b ), and FKV (Fv1 nr ).…”

“…The 6 codons identified as being under positive selection included 3 at positions 261, 265, and 270, clustered at the 5Ј end of the MHR, a highly conserved region known to be involved in capsid interactions (18)(19)(20)(21)(22). The other 3 codons identified as being under positive selection were at positions 352, 399, and 401; 2 of these codons, 352 and 399, distinguish the laboratory mouse Fv1 restriction genes (3,5), and substitutions at both of these codons have been shown to contribute to virus restriction (5,6). A third codon, 358, previously implicated in Fv1 restriction in laboratory mice was not identified as being under positive selection; all Fv1 genes except for Fv1 b have K at this site suggesting that the laboratory mouse Fv1 b variant K358E arose fairly recently in Mus.…”

“…The major resistance variants of Fv1 differ from one another at 3 amino acid sites in its C-terminal region, and Fv1 b additionally differs from Fv1 n and Fv1 nr at its C terminus due to a 1.3-kb indel (3). Substitutions at the 3 sites and variation at the C terminus all contribute to resistance (5,6). The mechanism of resistance is unknown, but Fv1 typically blocks replication after reverse transcription and before integration.…”

“…The mechanism of resistance is unknown, but Fv1 typically blocks replication after reverse transcription and before integration. Fv1 is known to target the virus capsid gene; a single amino acid substitution at position 110 distinguishes N-and B-tropic viruses (7), and substitutions at additional residues in the capsid N-terminal domain are responsible for NR-and NB-tropism (5,8).…”

Infectivity and insertional mutagenesis of endogenous retrovirus in autoimmune NZB and B/W mice

“…Therefore, once induced, NZW-1 may be able to spread in both NZW and B/W mice. However, if it is restricted (Stevens et al, 2004), it may need some mutational/recombinational event to modify the capsid protein.…”