SF3B1 Phosphorylation Prompts U2AF2 Dissociation for Widespread Control of pre-mRNA Splicing

Reversible pre-mRNA splicing factor phosphorylation is a well-documented feature of spliceosome assembly, activation, and disassembly, yet its functional and mechanistic roles are still emerging. SF3B1, a core spliceosome subunit, is extensively phosphorylated before the first catalytic step of pre-mRNA splicing. Intriguingly, most SF3B1 phosphorylation sites surround its binding sites for U2AF2, an early-stage pre-mRNA splicing factor. Here, we discovered that SF3B1 phosphorylation significantly decreases its association with U2AF2. We determined two crystal structures revealing electrostatic repulsion between an acidic U2AF2 -helix and the negatively-charged phosphoryl group of SF3B1. Variants with amino acid substitutions that prevent or mimic SF3B1 phosphorylation perturbed thousands of splice sites, primarily those marked by uridine-rich splice site signals recognized by U2AF2. Collectively, our findings demonstrate a widespread but previously unrecognized role for SF3B1 phosphorylation as a gateway for U2AF2 dissociation so that pre-mRNA splicing can proceed.