UC San Diego

In the nematode Caenorhabditis elegans, a stress/immune response pathway called the Intracellular Pathogen Response (IPR) increases resistance to proteotoxic stress. In mutants with constitutive IPR activation, a CUL-6 cullin-RING ubiquitin ligase complex was previously shown to protect the host from heat shock. This complex recently was found to target a putative substrate, the molecular chaperone HSP-90, leading to its degradation to improve resistance to heat shock (thermotolerance), as a readout for proteostasis. However, which F-box proteins might be involved in targeting HSP-90 was unclear. Here I assessed the role of an F-box protein, FBXA-11, using fbxa-11 RNAi, an fbxa-11 partial deletion mutant, and a null allele mutant fbxa-11(jy194) that I generated via CRISPR-Cas9-mediated genome editing. My results demonstrated differing effects of fbxa-11 RNAi on thermotolerance compared to fbxa-11 mutants. Preliminarily, I found that fbxa-11 RNAi does not affect thermotolerance in fbxa-11 mutants, suggesting the discrepancy is this case may not be due to off-target effects, but rather due to compensatory increased thermotolerance in fbxa-11 mutants. These findings are consistent with a model that wild-type fbxa-11 promotes thermotolerance. Also consistent with this model, I found that fbxa-11(jy194) suppresses the increased thermotolerance caused by overexpression of CUL-6. Through examining the effects on HSP-90 levels upon fbxa-11 RNAi and fbxa-11 mutations, I found mixed results indicating there may be off-target effects of fbxa-11 RNAi in this context. I also found that the CUL-6 ubiquitin ligase complex likely targets substrates in addition to HSP-90 to promote thermotolerance. In conclusion, we identified FBXA-11 as a new F-box adapter protein potentially acting in a CUL-6 ubiquitin ligase complex to improve thermotolerance as part of the IPR, furthering our understanding of how C. elegans copes with proteotoxic stress.