TRIM5α from the rhesus macaque (TRIM5αRh) is a restriction factor that shows strong activity against HIV-1. TRIM5αRh binds specifically to HIV-1 capsid (CA) through its B30.2/PRYSPRY domain shortly after entry of the virus into the cytoplasm. Recently, three putative SUMO interacting motifs (SIMs) have been identified in the PRYSPRY domain of human and macaque TRIM5α. However, structural modeling of this domain suggested that two of them were buried in the hydrophobic core of the protein, implying that interaction with SUMO was implausible, while the third one was not relevant to restriction. In light of these results, we re-analyzed the TRIM5αRh PRYSPRY sequence and identified an additional putative SIM ((435)VIIC(438)) which we named SIM4. This motif is exposed at the surface of the PRYSPRY domain, allowing potential interactions with SUMO or SUMOylated proteins. Introducing a double mutation in SIM4 (V435K, I436K) did not alter stability, unlike mutations in SIM1. SIM4-mutated TRIM5αRh failed to bind HIV-1CA and lost the ability to restrict this virus. Accordingly, SIM4 undergoes significant variation among primates and substituting this motif with naturally occurring SIM4 variants affected HIV-1 restriction by TRIM5αRh, suggesting a direct role in capsid recognition. Interestingly, SIM4-mutated TRIM5αRh also failed to activate NF-κB and AP-1-mediated transcription. Although there is no direct evidence that SIM4 is involved in direct interaction with SUMO or a SUMOylated protein, mutating this motif strongly reduced co-localization of TRIM5αRh with SUMO-1 and with PML, a SUMOylated nuclear protein. In conclusion, this new putative SIM is crucial for both direct interaction with incoming capsids and for NF-κB/AP-1 signaling. We speculate that the latter function is mediated by interactions of SIM4 with a SUMOylated protein involved in the NF-κB/AP-1 signaling pathways.
Keywords: Cellular aspects of innate immunity; Innate immune system; Proteins; Virology; Viruses.