Trivalent RING Assembly on Retroviral Capsids Activates TRIM5 Ubiquitination and Innate Immune Signaling

Abstract

SummaryTRIM5 is a RING domain E3 ubiquitin ligase with potent antiretroviral function. TRIM5 assembles into a hexagonal lattice on retroviral capsids, causing envelopment of the infectious core. Concomitantly, TRIM5 initiates innate immune signaling and orchestrates disassembly of the viral particle, yet how these antiviral responses are regulated by capsid recognition is unclear. We show that hexagonal assembly triggers N-terminal polyubiquitination of TRIM5 that collectively drives antiviral responses. In uninfected cells, N-terminal monoubiquitination triggers non-productive TRIM5 turnover. Upon TRIM5 assembly on virus, a trivalent RING arrangement allows elongation of N-terminally anchored K63-linked ubiquitin chains (N-K63-Ub). N-K63-Ub drives TRIM5 innate immune stimulation and proteasomal degradation. Inducing ubiquitination before TRIM5 assembly triggers premature degradation and ablates antiviral restriction. Conversely, driving N-K63 ubiquitination after TRIM5 assembly enhances innate immune signaling. Thus, the hexagonal geometry of TRIM5’s antiviral lattice converts a capsid-binding protein into a multifunctional antiviral platform.

Structural basis of generic versus specific E2–RING E3 interactions in protein ubiquitination

“…Identification of the minimal set of interactions that are required for all E2–RING E3 interactions, which may be referred to as canonical E2–RING E3 interactions, can be achieved by comparison of the structures of the most versatile members of the E2 family. These are the closely related members of the UBE2D family, which build K48‐type Ub chains and target many regulatory proteins for destruction, and their distant relative UBE2N, which dimerise with catalytically‐dead E2 variants UBE2V1 or UBE2V2 to build K63‐type Ub chains and trigger immune responses . Remarkably, in vitro studies have shown that UBE2D1, UBE2D2, UBE2D3, and UBE2N can individually interact with over 30 RING E3s, with approximately 60% of these RING E3 interaction patterns overlapping .…”

Retrovirus Restriction by TRIM5α: RINGside at a Cage Fight

“…(A) The basal oligomeric state of TRIM5α is an antiparallel dimer. Under steady-state conditions, B-box dependent association of the TRIM5α dimers can lead to Ube2W recruitment, resulting in the formation of N-Ub-TRIM5α, which is turned over by the ubiquitin fusion degradation (UFD) machinery (Fletcher et al, 2018). This process ordinarily maintains TRIM5α at a low steady-state level, but if TRIM5α levels increase sufficiently then the protein can form larger cytoplasmic bodies.…”

“…(B) Each vertex in a TRIM5α hexagonal lattice is mediated by a B-box domain trimer (Wagner et al, 2016), which also clusters three RING domains. (C) Hexagonal lattice formation brings three TRIM5α RING domains together at each vertex (step 1) and drives a dynamic ‘two-plus-one’ K63-linked Ub chain elongation mechanism (Fletcher et al, 2018). Two RING subunits initially form a catalytic dimer and cooperate with Ube2N/V2 to attach 2–3 more Ub moieties in trans to the anchoring N-monoUb on the third RING (step 2).…”

“…The substrate RING then exchanges into the original RING dimer (step 3) and can now collaborate with Ube2N/V2 to further extend the K63-linked chains in cis (step 4). The creation of N-anchored, K63-linked Ub chains on TRIM5α then leads to proteasome recruitment and NF-κB activation (Fletcher et al, 2018). …”

“…‘Caging’ of viral capsids by the TRIM5α lattice inhibits viral infectivity, but this is only half the story. A new report (Fletcher et al, 2018) now explains the Ub-dependent reactions that induce: 1) rapid TRIM5α turnover, which likely prevents unwanted immune activation; 2) capsid dissociation, which blocks reverse transcription and augments restriction; and 3) production of anti-viral interferons and cytokines, which alerts the immune system to invasion.…”