Retroviruses encounter dominant postentry restrictions in cells of particular species. Human immunodeficiency virus type 1 (HIV-1) is blocked in the cells of Old World monkeys by TRIM5␣, a tripartite motif (TRIM)protein composed of RING, B-box 2, coiled-coil, and B30.2(SPRY) domains. Rhesus monkey TRIM5␣ (TRIM5␣ rh ) more potently blocks HIV-1 infection than human TRIM5␣ (TRIM5␣ hu ). Here, by studying chimeric TRIM5␣ proteins, we demonstrate that the major determinant of anti-HIV-1 potency is the B30.2(SPRY) domain. Analysis of species-specific variation in TRIM5␣ has identified three variable regions (v1, v2, and v3) within the B30.2 domain. The TRIM5␣ proteins of Old World primates exhibit expansion, duplication, and residue variation specifically in the v1 region. Replacement of three amino acids in the N terminus of the TRIM5␣ hu B30.2 v1 region with the corresponding TRIM5␣ rh residues resulted in a TRIM5␣ molecule that restricted HIV-1 nearly as efficiently as wild-type TRIM5␣ rh . Surprisingly, a single-amino-acid change in this region of TRIM5␣ hu allowed potent restriction of simian immunodeficiency virus, a phenotype not observed for either wild-type TRIM5␣ hu or TRIM5␣ rh . Some of the chimeric TRIM5␣ proteins that are >98% identical to the human protein yet mediate a strong restriction of HIV-1 infection may have therapeutic utility. These observations implicate the v1 variable region of the B30.2(SPRY) domain in TRIM5␣ rh antiviral potency.The primate lentiviruses include human immunodeficiency virus type 1 (HIV-1) and HIV-2 and simian immunodeficiency viruses (SIVs) (2,5,7,8,11). HIV-1 and HIV-2 infect humans, HIV-1-like viruses infect chimpanzees, and SIV variants infect African monkeys. Humans infected by HIV-1 and HIV-2 and Asian macaques infected by certain SIV strains (SIV mac ) often develop life-threatening immunodeficiency due to depletion of CD4-positive T lymphocytes (9,19).HIV and SIV tropism is determined by cell-type-specific and species-specific host factors. Following entry into the host cell, uncoating of the viral core, reverse transcription, nuclear access, and integration of the viral DNA into the host genome must occur to establish a permanent infection (1,10,33). Early postentry restrictions to retrovirus infection can determine tropism at the species level. HIV-1 encounters a postentry block in Old World monkeys, whereas SIV mac is blocked in most New World monkey cells (15,16,27). These species-specific restrictions occur prior to or concurrent with reverse transcription; at most, low levels of early reverse transcripts are made in restricted cells (6,15,20,27). The viral determinant of susceptibility to these blocks is the capsid protein (6,13,18,22,23,30). The early restriction of HIV-1 and SIV is mediated by dominant host factors, the activities of which can be titrated by the introduction of virus-like particles containing proteolytically processed capsid proteins of the restricted viruses (3,4,6,12,22,31,32). Thus, in the cells of specific monkey species, host restricti...

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.

In owl monkeys, a retrotransposition event replaced the gene encoding the retroviral restriction factor TRIM5alpha with one encoding TRIMCyp, a fusion between the RING, B-box 2 and coiled-coil domains of TRIM5 and cyclophilin A. TRIMCyp restricts human immunodeficiency virus (HIV-1) infection by a mechanism dependent on the interaction of the cyclophilin A moiety and the HIV-1 capsid protein. Here, we show that infection by retroviruses other than HIV-1 can be restricted by TRIMCyp, providing an explanation for the evolutionary retention of the TRIMCyp gene in owl monkey lineages. The TRIMCyp-mediated block to HIV-1 infection occurs before the earliest step of reverse transcription. TRIMCyp-mediated restriction involves at least two functions: (1) capsid binding, which occurs most efficiently for trimeric TRIMCyp proteins that retain the coiled-coil and cyclophilin A domains, and (2) an effector function that depends upon the B-box 2 domain.