Nuclear speckle dynamics are controlled by polyphosphate inhibition of CLK proteins.

Nuclear speckles (NS) are membraneless nuclear organelles that act as critical hubs for pre-messenger RNA splicing. Defects in splicing are linked to several human diseases, including cancer, Alzheimer's disease, and dystrophies. While CLK kinases regulate the mobilization of splicing factors from NS, the molecular mechanisms underlying NS assembly and dissolution remain unclear. Using an adaptation of the Biotinylation by Antibody Recognition technique, we identified polyphosphate (polyP) as a novel and essential regulator of NS dynamics. Polyphosphate, a highly conserved polyanion composed of a chain of phosphate molecules, is involved in several functions in mammalian cells. Here, we show that polyP interacts with the NS core component SRRM2, and its depletion disrupts NS organization releasing splicing factors into the nucleoplasm. RNA-seq analysis reveals that polyP depletion increases exon exclusion, particularly in transcripts with multiple isoforms, highlighting its role in splicing regulation. Mechanistically, we demonstrate that polyP acts as a physiological inhibitor of CLK3 kinase, preventing the phosphorylation of SR proteins and thereby maintaining NS stability. Our findings not only expand our understanding of NS biology but also provide new insights into the polyP involvement in splicing-related diseases.