CHIP ubiquitin ligase is involved in the nucleolar stress management
The nucleolus is a dynamic nuclear biomolecular condensate involved in cellular stress response. Under proteotoxic stress, the nucleolus can store damaged proteins for refolding or degradation. HSP70 chaperone is a well-documented player in the recovery process of proteins accumulated in the nucleolus after heat shock. However, little is known about the involvement of the ubiquitin-proteasome system in the turnover of its nucleolar clients. Here we show that HSP70, independently of its ATPase activity, promotes migration of the CHIP (carboxyl terminus of HSC70-interacting protein) ubiquitin ligase into the granular component of the nucleolus, specifically after heat stress. We show that while in the nucleolus, CHIP retains mobility that depends on its ubiquitination activity. Furthermore, after prolonged exposure to heat stress, CHIP self-organizes into large, intra-nucleolar droplet-like structures whose size is determined by CHIP ubiquitination capacity. Using a heat-sensitive nucleolar protein luciferase, we show that excess CHIP impairs its regeneration, probably through deregulation of HSP70. Our results demonstrate a novel role for CHIP in managing nucleolar proteostasis in response to stress.