Splicing factor SRSF1 is a pH-stat to restore nucleolar integrity and function

Abstract

Nucleolus is a multiphase biomolecular assembly containing three distinct subcompartments. Evidence is just emerging that nucleolar proteins with characteristic electrochemical properties condense to generate a pH gradient, which serves as a key driving force to organize the nucleolar sub-phases. Given the indispensable functionality of nucleolus in most, if not all, of the cellular activities, it is vital for a cell to sense potential fluctuation in the nucleolar pH and subsequently maintain the pH homeostasis. The mechanisms how a cell achieve this task remains poorly decoded. Here, we show that splicing factor SRSF1 is shuttled from nuclear speckles (NSs) to the nucleolus in times of stress that interrogates the nucleolar pH. SRSF1 nucleolar localization is reliant on an acidic patch and molecular interactions with a nucleolar-resident protein, DDX18. Loss of SRSF1 impedes pH homeostasis in the nucleolus, and subsequently obstructs the restoration of the nucleolar multiphase and function. The arginine residues in the RS (arginine/serine rich) domain of SRSF1, endowed by high isoelectric point (pI), directly alkalize the nucleolar microenvironment. Interestingly, synthetic arginine-rich dipeptides derivative of SRSF1 RS domain safeguard nucleolus from pH and functional disturbance. Our findings uncover unprecedented mechanistic insights into nucleolar pH-sensing and regulation.