Controlled by disorder: phosphorylation modulates SRSF1 domain availability for spliceosome maturation

Serine/arginine-rich splicing factor 1 (SRSF1) is key in the mRNA lifecycle including transcription, splicing, nonsense-mediated decay, and nuclear export. Consequently, its dysfunction is linked to cancers, viral evasion, and developmental disorders. The functionality of SRSF1 relies on its interactions with other proteins and RNA molecules. These processes are regulated by phosphorylation of its unstructured arginine/serine-rich tail (RS). Here, we characterize how phosphorylation affects SRSF1s protein and RNA interaction and phase separation. Using NMR paramagnetic relaxation enhancement and chemical shift perturbation, we find that when unphosphorylated, SRSF1s RS interacts with its first RNA-recognition motif (RRM1). Phosphorylation of RS decreases its interactions with RRM1 and increases its interactions with the RNA-binding site. This change in SRSF1s intramolecular interactions increases the availability of protein-interacting sites on RRM1 and weakens RNA binding of SRSF1. Phosphorylation alters the phase separation of SRSF1 by diminishing the role of arginine in intermolecular interactions. These findings provide an unprecedented view of how SRSF1 influences the early-stage spliceosome assembly. SUMMARYPhosphorylation of SRSF1 is pivotal in pre-mRNA processing and is dysregulated in various pathologies. Modeling of SRSF1 based on NMR restraints reveals phosphorylation alters the accessibility of protein-protein and protein-RNA interaction sites on SRSF1s RRM1 domain, altering its binding preferences