The host restriction factor TRIM5␣ mediates species-specific, early blocks to retrovirus infection; susceptibility to these blocks is determined by viral capsid sequences. Here we demonstrate that TRIM5␣ variants from Old World monkeys specifically associate with the HIV type 1 (HIV-1) capsid and that this interaction depends on the TRIM5␣ B30.2 domain. Human and New World monkey TRIM5␣ proteins associated less efficiently with the HIV-1 capsid, accounting for the lack of restriction in cells of these species. After infection, the expression of a restricting TRIM5␣ in the target cells correlated with a decrease in the amount of particulate capsid in the cytosol. In some cases, this loss of particulate capsid was accompanied by a detectable increase in soluble capsid protein.Inhibiting the proteasome did not abrogate restriction. Thus, TRIM5␣ restricts retroviral infection by specifically recognizing the capsid and promoting its rapid, premature disassembly.

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Tripartite motif (TRIM) proteins are composed of RING, B-box 2, and coiled coil domains. Some TRIM proteins, such as TRIM5␣, also possess a carboxy-terminal B30.2(SPRY) domain and localize to cytoplasmic bodies. TRIM5␣ has recently been shown to mediate innate intracellular resistance to retroviruses, an activity dependent on the integrity of the B30.2 domain, in particular primate species. An examination of the sequences of several TRIM proteins related to TRIM5 revealed the existence of four variable regions (v1, v2, v3, and v4) in the B30.2 domain. Species-specific variation in TRIM5␣ was analyzed by amplifying, cloning, and sequencing nonhuman primate TRIM5 orthologs. Lineage-specific expansion and sequential duplication occurred in the TRIM5␣ B30.2 v1 region in Old World primates and in v3 in New World monkeys. We observed substitution patterns indicative of selection bordering these particular B30.2 domain variable elements. These results suggest that occasional, complex changes were incorporated into the TRIM5␣ B30.2 domain at discrete time points during the evolution of primates. Some of these time points correspond to periods during which primates were exposed to retroviral infections, based on the appearance of particular endogenous retroviruses in primate genomes. The results are consistent with a role for TRIM5␣ in innate immunity against retroviruses.

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TRIM5alpha and TRIMCyp are retroviral restriction factors that, like other members of the tripartite motif (TRIM) family, contain RING, B-box 2 and coiled-coil domains. We found that both proteins are rapidly turned over, with half-lives of 50-60 min. Polyubiquitylation and rapid degradation of TRIM5alpha depended upon intact RING and B-box 2 domains. A chimera consisting of monkey TRIM5alpha with a RING domain of human TRIM21 exhibited a half-life of 210 min, yet potently restricted human immunodeficiency virus; therefore, rapid turnover of TRIM5alpha is not required for its antiretroviral activity. TRIM5alpha forms cytoplasmic bodies that contain other polyubiquitylated proteins, heat shock proteins and dynein, and thus resemble aggresome precursors. Consistent with this interpretation, proteasomal inhibitors triggered the formation of TRIM5alpha(rh)-containing aggresomes in a microtubule-dependent manner. Thus, TRIM5alpha levels in the cell are maintained by continuous synthesis and rapid proteasome-mediated degradation, imbalances in which result in the formation of pre-aggresomal cytoplasmic bodies.

TRIM5alpha acts on several retroviruses, including human immunodeficiency virus (HIV-1), to restrict cross-species transmission. Using natural history cohorts and tissue culture systems, we examined the effect of polymorphism in human TRIM5alpha on HIV-1 infection. In African Americans, the frequencies of two non-coding SNP variant alleles in exon 1 and intron 1 of TRIM5 were elevated in HIV-1-infected persons compared with uninfected subjects. By contrast, the frequency of the variant allele encoding TRIM5alpha 136Q was relatively elevated in uninfected individuals, suggesting a possible protective effect. TRIM5alpha 136Q protein exhibited slightly better anti-HIV-1 activity in tissue culture than the TRIM5alpha R136 protein. The 43Y variant of TRIM5alpha was less efficient than the H43 variant at restricting HIV-1 and murine leukemia virus infections in cultured cells. The ancestral TRIM5 haplotype specifying no observed variant alleles appeared to be protective against infection, and the corresponding wild-type protein partially restricted HIV-1 replication in vitro. A single logistic regression model with a permutation test indicated the global corrected P value of <0.05 for both SNPs and haplotypes. Thus, polymorphism in human TRIM5 may influence susceptibility to HIV-1 infection, a possibility that merits additional evaluation in independent cohorts.

The host cell factors TRIM5␣ hu and Fv-1 restrict N-tropic murine leukemia virus (N-MLV) infection at an early postentry step before or after reverse transcription, respectively. Interestingly, the identity of residue 110 of the MLV capsid determines susceptibility to both TRIM5␣ hu and Fv-1. In this study, we investigate the fate of the MLV capsid in cells expressing either the TRIM5␣ hu or Fv-1 restriction factor. The expression of TRIM5␣ hu , but not Fv-1, specifically promoted the premature conversion of particulate N-MLV capsids within infected cells to soluble capsid proteins. The TRIM5␣ hu -mediated disassembly of particulate N-MLV capsids was dependent upon residue 110 of the viral capsid. Furthermore, the deletion or disruption of TRIM5␣ hu domains necessary for potent N-MLV restriction completely abrogated the disappearance of particulate N-MLV capsids observed with wild-type TRIM5␣ hu . These results suggest that premature disassembly of the viral capsid contributes to the restriction of N-MLV infection by TRIM5␣ hu , but not by Fv-1.Intracellular restriction factors can govern the species-specific tropism of a broad range of retroviruses. Early studies identified the Friend virus susceptibility factor 1 (Fv-1) locus as a mouse determinant governing the ability of mouse strains to be infected by murine leukemia viruses (MLV) and to subsequently develop leukemia (17). Two major alleles of Fv-1 have been identified: Fv-1 n , which renders NIH mice resistant to B-tropic MLV (B-MLV) infection but susceptible to N-tropic (N-MLV) infection, and Fv-1 b , which renders BALB/c mice resistant to N-MLV but susceptible to B-MLV infection (4, 10). Fv-1 is a dominant, saturable restriction factor that inhibits MLV infection following reverse transcription of the viral RNA but prior to the integration of viral DNA into the cellular genome (14). The Fv-1 protein is related to retroviral capsid proteins (14). The viral determinant of susceptibility to Fv-1 restriction has been mapped to residue 110 of the MLV capsid (7, 16). Currently, the mechanism by which Fv-1 restricts MLV infection is unknown.More recently, rhesus monkey TRIM5␣ (TRIM5␣ rh ) and human TRIM5␣ (TRIM5␣ hu ) were identified as intracellular restriction factors capable of blocking human immunodeficiency virus type 1 (HIV-1) and N-MLV infection, respectively, in the cells of these primate species (11,15,24,33,38). TRIM5␣ restricts HIV-1 and N-MLV at an early postentry step prior to reverse transcription (2,3,6,12,19,30,36). Interestingly, arginine 110 of the N-MLV capsid determines sensitivity to TRIM5␣ hu -mediated restriction (24, 36). Replacing the arginine residue at position 110 of the N-MLV capsid with the corresponding glutamic acid residue from B-MLV generates a virus that can partially overcome TRIM5␣ hu restriction. Conversely, replacement of glutamic acid 110 of the B-MLV capsid with arginine generates a virus that is susceptible to TRIM5␣ hu restriction.TRIM5␣ is a member of the tripartite motif family of proteins and contains RING, B-box 2,...

The coiled-coil domain of the tripartite motif (TRIM) family protein TRIM5alpha is required for trimerization and function as an antiretroviral restriction factor. Unlike the coiled-coil regions of other related TRIM proteins, the coiled coil of TRIM5alpha is not sufficient for multimerization. The linker region between the coiled-coil and B30.2 domains is necessary for efficient TRIM5alpha trimerization. Most of the hydrophilic residues predicted to be located on the surface-exposed face of the coiled coil can be altered without compromising TRIM5alpha antiviral activity against human immunodeficiency virus (HIV-1). However, changes that disrupt TRIM5alpha trimerization proportionately affect the ability of TRIM5alpha to bind HIV-1 capsid complexes. Therefore, TRIM5alpha trimerization makes a major contribution to its avidity for the retroviral capsid, and to the ability to restrict virus infection.