Tripartite Motif containing protein 21 (TRIM21) is a ubiquitously expressed cytosolic antibody receptor that protects host cells from viral infection by binding to antibodies deposited on viral capsids. Upon detection of antibody coated virus in the cytosol, TRIM21 triggers proteasome dependent degradation of the invading virus and activates pro-inflammatory signalling pathways. Importantly, TRIM21 is capable of targeting any antibody-bound protein for degradation, an activity that forms the basis of the rapid protein-depletion technique called Trim-Away.
Like other members of the TRIM protein family, TRIM21 has a highly conserved tripartite motif consisting of RING-type E3 ubiquitin ligase, B-box and coiled-coil domains. The C-terminal PRYSPRY domain of TRIM21 mediates high affinity interaction with antibody Fc while the RING domain is responsible for mediating the effector functions of TRIM21 by catalysing the formation of ubiquitin chains. Therefore,TRIM21 occupies a distinct niche at the interface between adaptive and innate immunity.
Previous studies have shown that the E2 ubiquitin conjugating enzymes Ube2w and Ube2N/Ube2v2 are required for the effector functions of TRIM21. However, the mechanism by which the TRIM21 RING domain catalyses the formation of ubiquitin chains is unknown. Using a combination of biochemical and cellular assays we have identified key residues in the TRIM21 RING domain that are responsible for stabilising the interaction with E2-ubiquitin complexes as well as residues and interactions that are required for efficient catalysis of ubiquitin transfer.
Understanding the molecular mechanism of TRIM21 catalysis provides insight into the regulation and activation of TRIM21 and may help further our understanding of other TRIM proteins involved in innate immunity. Furthermore, this mechanistic understanding may allow us to manipulate TRIM21 activity as desired for biotechnology applications such as Trim-away.