Tripartite motif (TRIM) proteins make up a large family of coiledcoil-containing RING E3 ligases that function in many cellular processes, particularly innate antiviral response pathways. Both dimerization and higher-order assembly are important elements of TRIM protein function, but the atomic details of TRIM tertiary and quaternary structure have not been fully understood. Here, we present crystallographic and biochemical analyses of the TRIM coiled-coil and show that TRIM proteins dimerize by forming interdigitating antiparallel helical hairpins that position the Nterminal catalytic RING domains at opposite ends of the dimer and the C-terminal substrate-binding domains at the center. The dimer core comprises an antiparallel coiled-coil with a distinctive, symmetric pattern of flanking heptad and central hendecad repeats that appear to be conserved across the entire TRIM family. Our studies reveal how the coiled-coil organizes TRIM25 to polyubiquitylate the RIG-I/viral RNA recognition complex and how dimers of the TRIM5α protein are arranged within hexagonal arrays that recognize the HIV-1 capsid lattice and restrict retroviral replication.antiparallel dimer | disulfide crosslinking | X-ray crystallography T ripartite motif (TRIM) proteins make up the largest superfamily of RING E3 ubiquitin ligases, with more than 100 members in the human proteome (1, 2). TRIM proteins function in a variety of cellular pathways, and many regulate innate immunity and/or mediate antiviral responses. Antiviral TRIMs include TRIM25, which regulates the IFN response to RNA viruses (3, 4), and TRIM5α, which senses and inhibits early stages of retroviral replication (5, 6).TRIM proteins share a common N-terminal domain organization, termed the tripartite or RBCC (RING, B-box, coiled-coil) motif, followed by variable C-terminal protein recognition domains ( Fig. 1 A and B). "Linker" segments of unknown structure typically separate both the RING and B-box domains (L1) and the coiledcoil and terminal effector domains (L2). The coiled-coil region mediates oligomerization, and both homooligomeric and heterooligomeric TRIMs have been described (7-13). Furthermore, many TRIM proteins form higher-order assemblies in vitro and form punctate or fibrous structures in cells (14-16). For example, TRIM5α assembly allows the protein to function as a cytosolic patternrecognition receptor that can intercept the incoming capsids of diverse retroviruses, including HIV-1 (6). This results in species-specific "restriction" of viral replication (5), capsid dissociation (5, 6), and induction of innate immune responses (17). Retroviral capsids are recognized through a remarkable mechanism of multivalent pattern recognition. TRIM5α forms a homodimer (10, 11, 18), which can further assemble into a 2D lattice of linked hexagons (18). The hexagonal TRIM5α net matches the symmetry and spacing of the retroviral capsid surface lattice, thereby positioning multiple C-terminal B30.2/SPRY domains to interact with their repeating binding epitopes on the capsid.Struct...

SUMMARY Antiviral response pathways induce interferon by higher-order assembly of signaling complexes called signalosomes. Assembly of the RIG-I signalosome is regulated by K63-linked polyubiquitin chains, which are synthesized by the E3 ubiquitin ligase, TRIM25. We have previously shown that the TRIM25 coiled-coil domain is a stable, antiparallel dimer that positions two catalytic RING domains on opposite ends of an elongated rod. We now show that the RING domain is a separate self-association motif that engages ubiquitin-conjugated E2 enzymes as a dimer. RING dimerization is required for catalysis, TRIM25-mediated RIG-I ubiquitination, interferon induction, and antiviral activity. We also provide evidence that RING dimerization and E3 ligase activity are promoted by binding of the TRIM25 SPRY domain to the RIG-I effector domain. These results indicate that TRIM25 actively participates in higher-order assembly of the RIG-I signalosome and helps to fine-tune the efficiency of the RIG-I-mediated antiviral response.

TRIM25 is a multi-domain, RING-type E3 ubiquitin ligase of the tripartite motif family that has important roles in multiple RNA-dependent processes. In particular, TRIM25 functions as an effector of RIG-I and ZAP, which are innate immune sensors that recognize viral RNA and induce ubiquitin-dependent anti-viral response mechanisms. TRIM25 is reported to also bind RNA, but the molecular details of this interaction or its relevance to anti-viral defense have not been elucidated. Here, we characterize the RNA-binding activity of TRIM25 and find that the protein binds both single-stranded and double-stranded RNA. Multiple regions of TRIM25 contribute to this functionality, including the C-terminal SPRY domain and a lysine-rich motif in the linker segment connecting the SPRY and coiled-coil domains. RNA binding modulates TRIM25’s ubiquitination activity in vitro, its localization in cells, and its anti-viral activity. Taken together with other studies, our results indicate that RNA binding by TRIM25 has at least three important functional consequences: by enhancing ubiquitination activity, either through allosteric effects or through clustering of multiple TRIM25 molecules; by modulating the multi-domain structure of the TRIM25 dimer, and thereby structural coupling of the SPRY and RBCC elements during the ubiquitination reaction; and by facilitating subcellular localization of the E3 ligase during virus infection.

Tripartite motif (TRIM) proteins comprise a large family of RING-type ubiquitin E3 ligases that regulate important biological processes. An emerging general model is that TRIMs form elongated antiparallel coiled-coil dimers that prevent interaction of the two attendant RING domains. The RING domains themselves bind E2 conjugating enzymes as dimers, implying that an active TRIM ligase requires higher-order oligomerization of the basal coiled-coil dimers. Here, we report crystal structures of the TRIM23 RING domain in isolation and in complex with an E2-ubiquitin conjugate. Our results indicate that TRIM23 enzymatic activity requires RING dimerization, consistent with the general model of TRIM activation.

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