RING E3 ligase TRIM8 is structurally selective for UBE2N-mediated ubiquitination.

RING domains catalyse ubiquitination by stabilising a closed conformation of E2-ubiquitin (Ub) conjugates, with TRIpartite Motif (TRIM) proteins representing the largest subfamily of RING-type E3 ligases. TRIM proteins are implicated in many physiological processes and diseases. For example, TRIM8 perturbation has been variously linked to neurodevelopmental defects and several cancer types, attributed to its ubiquitination of a myriad of putative substrates. However, the molecular mechanisms behind these modifications are not yet clear. Here we show that TRIM8 has preferential functionality with the E2 enzyme UBE2N to generate K63-linked poly-ubiquitin chains. By solving the crystal structure of TRIM8 RING bound to a UBE2N-Ub conjugate, we discover that its non-canonical asparagine 'linchpin' residue belies this selectivity by failing to form key contacts required to stabilise the closed conformation of other E2 enzymes. These findings not only open the door to exploring the impact of TRIM8-mediated K63 chain formation in health and disease, but also add to a growing appreciation of the specificities of RING domain catalytic activity.