Mechanism of B-box 2 domain-mediated higher-order assembly of the retroviral restriction factor TRIM5α

Restriction factors and pattern recognition receptors are important components of intrinsic cellular defenses against viral infection. Mammalian TRIM5 alpha proteins are restriction factors and receptors that target the capsid cores of retroviruses and activate ubiquitin-dependent antiviral responses upon capsid recognition. Here, we report crystallographic and functional studies of the TRIM5 alpha B-box 2 domain, which mediates higher-order assembly of TRI M5 proteins. The B-box can form both dimers and trimers, and the trimers can link multiple TRIM5 alpha proteins into a hexagonal net that matches the lattice arrangement of capsid subunits and enables avid capsid binding. Two modes of conformational flexibility allow TRIM5 alpha to accommodate the variable curvature of retroviral capsids. B-box mediated interactions also modulate TRIM5 alpha's E3 ubiquitin ligase activity, by stereochemically restricting how the N-terminal RING domain can dimerize. Overall, these studies define important molecular details of cellular recognition of retroviruses, and how recognition links to downstream processes to disable the virus.