Intracellular antibody signalling is regulated by phosphorylation of the Fc receptor TRIM21

Abstract

Cell surface Fc receptors activate inflammation and are tightly controlled to prevent autoimmunity. Antibodies also simulate potent immune signalling from inside the cell via the cytosolic antibody receptor TRIM21, but how this is regulated is unknown. Here we show that TRIM21 signalling is constitutively repressed by its B-Box domain and activated by phosphorylation. The B-Box occupies an E2 binding site on the catalytic RING domain by mimicking E2-E3 interactions, inhibiting TRIM21 ubiquitination and preventing immune activation. TRIM21 is derepressed by IKKβ and TBK1 phosphorylation of an LxxIS motif in the RING domain, at the interface with the B-Box. Incorporation of phosphoserine or a phosphomimetic within this motif relieves B-Box inhibition, promoting E2 binding, RING catalysis, NF-κB activation and cytokine transcription upon infection with DNA or RNA viruses. These data explain how intracellular antibody signalling is regulated and reveal that the B-Box is a critical regulator of RING E3 ligase activity.

TRIM21—From Intracellular Immunity to Therapy

“…Structurally, this is explained by the fact that the B-box mimics and occupies the E2 ubiquitin ligase binding site on the TRIM21 RING domain, thereby preventing its auto-ubiquitination and activation of immune signaling. This was revealed by solving a crystal structure of a truncated form of TRIM21 containing only the RING and B-Box domains (59). These structural observations were confirmed by nuclear magnetic resonance (NMR) where E2 enzyme Ube2N bound to RING but not RING-B-box variants of TRIM21.…”

“…The auto-inhibitory mechanism has been shown to be released by phosphorylation of a non-conserved serine residue at position 80 in the human TRIM21 RING domain by IKKβ or TBK1 kinases, driving B-box displacement from the RING (59). In line with this, stimulation of cells with poly(I:C) or infection with antibody coated Ad5 resulted in TRIM21 phosphorylation.…”

“…When TRIM21 is overexpressed in cells, no NF-κB activation or proteasomal degradation occurs in the absence of virus and antibody. Moreover, substitutions in TRIM21 corresponding to residues crucial for TRIM5α dimerization do not render TRIM21 catalytically inactive (59). These differences are observed despite the fact that the isolated RING domains of both TRIM5α and TRIM21 are highly catalytically active in vitro.…”

“…Recent results have revealed that TRIM21 is auto-regulated via the B-Box domain, as deletion of the B-box increases the catalytic activity of its RING domain (59). This represents a novel function for the B-box domain.…”

“…This in turn drives autoubiquitination, resulting in more rapid protein turnover by proteasomal targeting. However, such an activation strategy does not seem to be used by TRIM21 since its B-Box is auto-inhibitory (59). When TRIM21 is overexpressed in cells, no NF-κB activation or proteasomal degradation occurs in the absence of virus and antibody.…”