SRSF1 modulates the pH microenvironment of the dark nucleolar caps to restorenucleolar integrity and function

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 for organizing the nucleolar sub-phases. Given the indispensable functionality of the nucleolus in most, if not all, cellular activities, it is vital for a cell to sense potential fluctuations in the nucleolar pH and subsequently maintain pH homeostasis. The mechanisms by which a cell achieves this task remain poorly decoded. Here, we show that splicing factor SRSF1 is shuttled from nuclear speckle (NS) to the nucleolus during times of stress, which interrogates the nucleolar pH. SRSF1 nucleolar localization is reliant on an acidic patch. Loss of SRSF1 impedes pH homeostasis in the dark nucleolar cap and subsequently obstructs the restoration of the nucleolar multiphase and function. The RS (arginine/serine rich) domain of SRSF1, due to its high isoelectric point (pI), directly alkalizes the nucleolar microenvironment. Interestingly, synthetic arginine-rich dipeptide derivatives of SRSF1 RS domain safeguard the nucleolus from pH and functional disturbance. Our findings uncover unprecedented mechanistic insights into nucleolar pH-sensing and regulation.