pH Gradient Drives Redox Reactions in the Nucleolar Caps to Modulate Nucleolar Integrity and Function

Enzymatic catalysis has long been thought of as the primary driver of redox processes in living cells. Biomolecular condensates, formed via phase separation, exhibit unique physicochemical properties that distinguish them from the surrounding milieu. Emerging evidence indicates that the electrochemical properties of biomolecular condensates can participate in a plethora of pathophysiological processes by potentiating chemical reactions, including redox reactions. Nevertheless, the regulatory mechanisms governing the electrochemical equilibrium of biomolecular condensates and their influence on the emergent physicochemical functions, especially in cellulo, remain poorly understood. Here, we demonstrate that specific nucleolar stressors induce the formation of highly alkaline nucleolar caps, which generate an interfacial pH gradient that drives spontaneous redox reactions. Non-enzymatic reactive oxygen species (ROS) production depends on positively charged constituents within the nucleolar caps and correlates strongly with the pH microenvironment. Excessive ROS accumulation directly modulates the partition and diffusion of proteins rich in cysteine and methionine, whereas scavenging ROS markedly accelerates the recovery of nucleolar multiphase organization and functionality upon stress removal. Collectively, our findings propose a non-enzymatic redox mechanism operating within cellular condensates and highlight the critical role of physical interfaces in orchestrating the chemical dynamics of biomolecular condensates.