Functional role of TRIM E3 ligase oligomerization and regulation of catalytic activity

TRIM E3 ubiquitin ligases regulate a wide variety of cellular processes and are particularly important during innate immune signalling events. They are characterized by a conserved tripartite motif in their N‐terminal portion which comprises a canonical RING domain, one or two B‐box domains and a coiled‐coil region that mediates ligase dimerization. Self‐association via the coiled‐coil has been suggested to be crucial for catalytic activity of TRIMs; however, the precise molecular mechanism underlying this observation remains elusive. Here, we provide a detailed characterization of the TRIM ligases TRIM25 and TRIM32 and show how their oligomeric state is linked to catalytic activity. The crystal structure of a complex between the TRIM25 RING domain and an ubiquitin‐loaded E2 identifies the structural and mechanistic features that promote a closed E2~Ub conformation to activate the thioester for ubiquitin transfer allowing us to propose a model for the regulation of activity in the full‐length protein. Our data reveal an unexpected diversity in the self‐association mechanism of TRIMs that might be crucial for their biological function.