Antibodies have been extensively studied since the 1890s, revealing their importance in protective immunity and allowing their development in numerous research and therapeutic applications. However, these studies have focused almost exclusively on the extracellular effector mechanisms of antibodies such as opsonisation, complement activation, antibody-dependent cytotoxicity and receptor blocking. The discovery of TRIM21 as a cytosolic antibody receptor has revolutionised our understanding of humoral immunity and revealed that antibodies can mediate protective immunity inside cells. Recently, this TRIM21 mediated antiviral mechanism has been repurposed to degrade endogenous cellular proteins in a technique known as “Trim-Away”.

TRIM21 has been shown to be a critical component of humoral immunity in mouse infection models but there is no known human disease associated with the loss of TRIM21 antiviral activity. To address this question, I investigated the impact of naturally occurring polymorphisms on the antiviral functions of TRIM21. Interestingly, coding variants in TRIM21 are individually rare and all missense variants tested empirically retained at least some antiviral activity. This suggest that the antiviral functions of TRIM21 are under positive selection and that mutations leading to a complete loss of function are likely to be incredibly rare.

How TRIM21 is activated and regulated is not completely understood. My data has revealed that detection of antibody bound viruses initiates TRIM21-dependent anchored K63-chain polyubiquitination. This is significant as TRIM21 is known to functionally require the E2 enzyme Ube2W, which monoubiquitinates TRIM21 in vitro, and Ube2N, which extends monoubiquitinated TRIM21 to form anchored K63-linked chains. These ubiquitin chains are thought to drive proteasome recruitment and pro-inflammatory NF-B signalling. Using a combination of biochemical and cellular assays I have identified key residues in the TRIM21 RING domain that are responsible for efficient catalysis of ubiquitin transfer. In addition, I have discovered that the catalytic activity of TRIM21 is regulated through a combination of B-box autoinhibition and substrate-induced RING dimerization. Knowledge of the molecular mechanisms of TRIM21 regulation, activation and catalysis is enabling the further development of TRIM21 based research technologies such as Trim-Away. In one example, I show how TRIM21 can be exploited to allow light-induced protein degradation in living cells.