Genome-wide siRNA screening reveals that DCAF4-mediated ubiquitination of optineurin stimulates autophagic degradation of Cu,Zn-superoxide dismutase.

Cu, Zn superoxide dismutase (SOD1) is one of the genes implicated in the devastating neurodegenerative disorder amyotrophic lateral sclerosis (ALS). Although the precise mechanisms of SOD1 mutant (SOD1^mut)-induced motoneuron toxicity are still unclear, defects in SOD1 proteostasis are known to have a critical role in ALS pathogenesis. We previously reported that the SOD1^mut adopts a conformation that exposes a Derlin-1-binding region (DBR) and that DBR-exposed SOD1 interacts with Derlin-1, leading to motoneuron death. We also found that an environmental change, i.e. zinc depletion, induces a conformational change in WT SOD1 (SOD1^WT) to the DBR-exposed conformation, suggesting the presence of an equilibrium state between the DBR-masked and DBR-exposed states even with SOD1^WT Here, we conducted a high-throughput screening based on time-resolved FRET to further investigate the SOD1^WT conformational change, and we used a genome-wide siRNA screen to search for regulators of SOD1 proteostasis. This screen yielded 30 candidate genes that maintained an absence of the DBR-exposed SOD1^WT conformation. Among these genes was one encoding DDB1- and CUL4-associated factor 4 (DCAF4), a substrate receptor of the E3 ubiquitin-protein ligase complex. Of note, we found that DCAF4 mediates the ubiquitination of an ALS-associated protein and autophagy receptor, optineurin (OPTN), and facilitates autophagic degradation of DBR-exposed SOD1. In summary, our screen identifies DCAF4 as being required for proper proteostasis of DBR-exposed SOD1, which may have potential relevance for the development of therapies for managing ALS.

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