Members of the tripartite motif (TRIM) protein family of RING E3 ubiquitin (Ub) ligases promote innate immune responses by catalyzing synthesis of polyubiquitin chains linked through lysine 63 (K63). Here we investigate the mechanism by which the TRIM5α retroviral restriction factor activates Ubc13, the K63-linkage specific E2. Structural, biochemical and functional characterization of the TRIM5α:Ubc13-Ub interactions reveals that activation of the Ubc13-Ub conjugate requires dimerization of the TRIM5α RING domain. Our data explain how higher-order oligomerization of TRIM5α, which is promoted by the interaction with the retroviral capsid, enhances the E3 Ub ligase activity of TRIM5α and contributes to its antiretroviral function. This E3 mechanism, in which RING dimerization is transient and depends on the interaction of the TRIM protein with the ligand, is likely to be conserved in many members of the TRIM family and may have evolved to facilitate recognition of repetitive epitope patterns associated with infection.
Rhesus TRIM5␣ (TRIM5␣ rh ) is a cytosolic protein that potently restricts HIV-1 at an early postentry stage, prior to reverse transcription. The ability of TRIM5␣ rh to block HIV-1 infection has been correlated with a decrease of pelletable HIV-1 capsid during infection. To genetically dissect the ability of TRIM5␣ to block reverse transcription, we studied a set of TRIM5␣ rh RING domain mutants that potently restrict HIV-1 but allow the occurrence of reverse transcription. These TRIM5␣ rh RING variants blocked HIV-1 infection after reverse transcription but prior to integration, as suggested by the routing of nuclear viral DNA to circularization in the form of 2-long terminal repeat (2-LTR) circles. The folding of RING domain variants was similar to that of the wild type, as evaluated by nuclear magnetic resonance. RING domain changes that allowed the occurrence of reverse transcription were impaired in their ability to decrease the amount of pelletable capsid compared with wild-type TRIM5␣. Similar effects of this particular group of mutations were observed with human TRIM5␣ inhibition of N-tropic murine leukemia virus (N-MLV). Interestingly, TRIM5␣ rh RING domain variants also prevented the degradation of TRIM5␣ rh that occurs following cell entry of HIV-1. These data correlated the block of reverse transcription with the ability of TRIM5␣ to accelerate uncoating. Collectively, these results suggest that TRIM5␣ rh blocks HIV-1 reverse transcription by inducing premature viral uncoating in target cells. Several newly discovered proteins endogenously expressed in primates show the ability to dominantly block retroviral infection and cross-species transmission by interfering with the early phase of viral replication (27,46,51). Of particular interest are members of the tripartite motif (TRIM) family of proteins (43). The splicing variant alpha of TRIM5 from rhesus macaque (TRIM5␣ rh ) is an ϳ53-kDa cytosolic protein that potently restricts HIV-1 (24, 49). TRIM5␣ rh blocks HIV-1 and certain other retroviruses soon after viral entry but prior to reverse transcription (24, 51). The retroviral capsid protein (CA) is the viral determinant for susceptibility to restriction by TRIM5␣ (38). Studies on the fate of the HIV-1 capsid in the cytosol of infected cells have correlated restriction with a decreased amount of cytosolic particulate capsid (10, 13, 41, 52), suggesting that TRIM5␣ rh acts by inducing premature uncoating in target cells.TRIM5␣ rh is composed of four distinct domains: RING, B-box 2, coiled-coil, and B30.2(SPRY) (43). The RING domain of TRIM5␣ rh is an E3 ubiquitin ligase (12,23,26,28,31,32,57). The E3 ligase activity of TRIM5␣ rh correlates with the ability of TRIM5␣ rh to block HIV-1 (31). The B-box 2 domain of TRIM5␣ rh and other TRIM proteins, such as TRIM63, self-associates into dimeric complexes that are important for TRIM5␣ higher-order self-association (HOSA) and capsid binding avidity; these B-box 2 domain functions are essential for full and potent restriction of 14,20,22,34,39). The coiled-c...
Recent findings suggested that the SUMO-interacting motifs (SIMs) present in the human TRIM5α (TRIM5αhu) protein play an important role in the ability of TRIM5αhu to restrict N-MLV. Here we explored the role of SIMs in the ability of rhesus TRIM5α (TRIM5αrh) to restrict HIV-1, and found that TRIM5αrh SIM mutants IL376KK (SIM1mut) and VI405KK (SIM2mut) completely lost their ability to block HIV-1 infection. Interestingly, these mutants also lost the recently described property of TRIM5αrh to shuttle into the nucleus. Analysis of these variants revealed that they are unable to interact with the HIV-1 core, which might explain the reason that these variants are not active against HIV-1. Furthermore, NMR titration experiments to assay the binding between the PRYSPRY domain of TRIM5αrh and the small ubiquitin-like modifier 1(SUMO-1) revealed no interaction. In addition, we examined the role of SUMOylation in restriction, and find out that inhibition of SUMOylation by the adenoviral protein Gam1 did not altered the retroviral restriction ability of TRIM5α. Overall, our results do not support a role for SIMs or SUMOylation in the antiviral properties of TRIM5α.
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