Hexagonal assembly of a restricting TRIM5α protein

Abstract

TRIM5α proteins are restriction factors that protect mammalian cells from retroviral infections by binding incoming viral capsids, accelerating their dissociation, and preventing reverse transcription of the viral genome. Individual TRIM5 isoforms can often protect cells against a broad range of retroviruses, as exemplified by rhesus monkey TRIM5α and its variant, TRIM5-21R, which recognize HIV-1 as well as several distantly related retroviruses. Although capsid recognition is not yet fully understood, previous work has shown that the C-terminal SPRY/B30.2 domain of dimeric TRIM5α binds directly to viral capsids, and that higher-order TRIM5α oligomerization appears to contribute to the efficiency of capsid recognition. Here, we report that recombinant TRIM5-21R spontaneously assembled into two-dimensional paracrystalline hexagonal lattices comprising open, six-sided rings. TRIM5-21R assembly did not require the C-terminal SPRY domain, but did require both protein dimerization and a B-box 2 residue (Arg121) previously implicated in TRIM5α restriction and higher-order assembly. Furthermore, TRIM5-21R assembly was promoted by binding to hexagonal arrays of the HIV-1 CA protein that mimic the surface of the viral capsid. We therefore propose that TRIM5α proteins have evolved to restrict a range of different retroviruses by assembling a deformable hexagonal scaffold that positions the capsid-binding domains to match the symmetry and spacing of the capsid surface lattice. Capsid recognition therefore involves a synergistic combination of direct binding interactions, avidity effects, templated assembly, and lattice complementarity.

Structural insight into HIV-1 capsid recognition by rhesus TRIM5α

“…On the other hand, TRIM5α possesses an intriguing curvature-independent pattern-recognition property. Given that TRIM5α proteins bind to the contiguous surface of the HIV capsid that comprises varying interhexamer interfaces, the flat CA lattice formed in vitro, as well as the spherical surface of the N-MLV capsid (4,12), it must be able to recognize CA assemblies with different curvatures.…”

“…1A), similar to many tripartite/ RBCC motif (TRIM) family members (7). The RING domain functions as an E3 ubiquitin ligase (8); the B-box 2 domain mediates formation of higher-order structure and self-association (9,10); and the coiled-coil domain mediates dimerization (11) and facilitates the formation of the hexagonal lattice (12). The PRY/SPRY domain is essential for recognition of retroviral capsids and determines the specificity of restriction (13,14).…”

The tripartite motif coiled-coil is an elongated antiparallel hairpin dimer

“…The 17-nm length of the TRIM25 189-379 dimer corresponds almost exactly to the length of each edge of the assembled TRIM5α hexagon (18), suggesting that the TRIM25 structure can also inform our understanding of the TRIM5α hexagonal lattice. Intermolecular disulfide bond formation was used to probe and compare the structures of the TRIM25 and TRIM5α dimers in solution.…”

“…We also present evidence that this dimer architecture is conserved across other TRIM family members, including TRIM5α. Finally, our studies allow us to assign the domain organization in the low-resolution EM reconstruction of the TRIM5α lattice (18) and thereby gain new insights into the mechanism of retroviral capsid pattern recognition.…”

“…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.…”