SRSF1 modulates the dark nucleolar cap pH to restore nucleolar integrity and function.

The nucleolus is a multiphase condensate with three subcompartments organized by an internal pH gradient. Although pH regulates condensate dynamics and function, how it is controlled under various physiological and pathological conditions remains poorly understood. Here we show that splicing factor SRSF1 is shuttled from nuclear speckles to the dark nucleolar caps (DNCs) under stress that often interrogates nucleolar rRNA synthesis. SRSF1 DNC localization is partially reliant on a molecular interaction with DDX18. Loss of SRSF1 impedes the pH homeostasis of the DNCs and subsequently obstructs the restoration of the nucleolar multiphase and functional recovery from stress. The arginine/serine-rich (RS) domain of SRSF1, due to its high positive charge, is accountable for alkalizing the DNC microenvironment. Interestingly, synthetic arginine-rich dipeptide derivatives of the SRSF1 RS domain safeguard the nucleolus from pH and functional disturbance. Our findings uncover unprecedented mechanistic insights into pH regulation of the nucleolar caps.