The UBE2L3 ubiquitin conjugating enzyme: interplay with inflammasome signalling and bacterial ubiquitin ligases

Abstract

Inflammasome-controlled immune responses such as IL-1β release and pyroptosis play key roles in antimicrobial immunity and are heavily implicated in multiple hereditary autoimmune diseases. Despite extensive knowledge of the mechanisms regulating inflammasome activation, many downstream responses remain poorly understood or uncharacterised. The cysteine protease caspase-1 is the executor of inflammasome responses, therefore identifying and characterising its substrates is vital for better understanding of inflammasome-mediated effector mechanisms. Using unbiased proteomics, the Shenoy grouped identified the ubiquitin conjugating enzyme UBE2L3 as a target of caspase-1. In this work, I have confirmed UBE2L3 as an indirect target of caspase-1 and characterised its role in inflammasomes-mediated immune responses. I show that UBE2L3 functions in the negative regulation of cellular pro-IL-1beta via the ubiquitin- proteasome system. Following inflammatory stimuli, UBE2L3 assists in the ubiquitylation and degradation of newly produced pro-IL-1b. However, in response to caspase-1 activation, UBE2L3 is itself targeted for degradation by the proteasome in a caspase-1-dependent manner, thereby liberating an additional pool of IL-1beta which may be processed and released. UBE2L3 therefore acts a molecular rheostat, conferring caspase-1 an additional level of control over this potent cytokine, ensuring that it is efficiently secreted only in appropriate circumstances. These findings on UBE2L3 have implications for IL-1-driven pathology in hereditary fever syndromes, and autoinflammatory conditions associated with UBE2L3 polymorphisms.