Spatial Proteomics Sheds Light on the Biology of Nucleolar Chaperones

Within the nucleus, the nucleolus is a dynamic compartment which is critical to maintain cellular homeostasis under normal, stress and disease conditions. During the last years, proteomics research provided new information on the complexity of nucleolar proteomes. These studies also established that many chaperones, co-chaperones and other factors involved in proteostasis associate with nucleoli in the absence of stress or disease. Moreover, quantitative proteomics demonstrated that physiological and environmental changes alter the nucleolar profile of chaperones and co-chaperones. At present, the emphasis has shifted towards sophisticated in-depth analyses of the nucleolar proteome. As such, turnover and posttranslational modifications are now quantified for individual proteins that associate with nucleoli. This large body of work generated new insights into the sumoylation, phosphorylation and acetylation of the nucleolar proteome. At the same time, we have gained a better understanding of the nucleolar organization, as novel subcompartments were identified within the nucleolus that are induced by physiological and other forms of stress. Notably, some of these subcompartments are also enriched for chaperones. To review these results, we will focus on recent studies that analyzed the nucleolar proteome, and particular emphasis will be given to nucleolar chaperones. Despite remarkable progress in the field, crucial questions regarding the physiological relevance of nucleolar chaperones remain to be answered in the years ahead. We conclude our update by discussing these future directions in the context of the latest developments in the nucleolar and chaperone fields.