The Small GTPase RAB18 Recruits CUL3 Ubiquitin E3 Ligase to Lipid Droplets During Adipogenesis

Abstract

In Mammalia, the COP9 signalosome (CSN) CSN^CSN7A and CSN^CSN7B variants are organized in latent complexes containing cullin-RING ubiquitin ligases (CRLs). Here we show that CSN^CSN7A variant preferentially binds to CRL3, whereas CSN^CSN7B mostly interacts with CRL4A or CRL4B. Disrupting deneddylation using the chemical compound CSN5i-3 or neddylation by MLN4924, do not impede the formation of latent CSN^CSN7A-CRL3 or CSN^CSN7B-CRL4A complexes. Ubiquitin-specific protease 15 exclusively binds to CSN^CSN7A-CRL3, while p27 KIP attaches to CSN^CSN7B-CRL4, whereas RAB18 as well as caveolin 1 co-precipitate with both particles. Curcumin-induced cell death requires latent CSN^CSN7B-CRL4A. Knockout of CSN7B in HeLa cells leads to resistance against curcumin. Remarkably, the small GTPase RAB18 recruits latent CSN^CSN7A-CRL3 complex to lipid droplets (LDs), where CRL3 is activated by neddylation, an essential event for LD formation and adipogenesis. Both knockdown of CSN7A and RAB18 or destabilization of latent complexes by cutting off CSN7A C-terminal 201-275 amino acids block adipogenic differentiation.

Keywords: CSNCSN7A, CSNCSN7B, CRLs, USP15, p27KIP, RAB18, CAV1, curcumin, adipogenesis, lipid droplets.