Structure of phosphorylated SF1 bound to U2AF⁶⁵ in an essential splicing factor complex.

The essential splicing factors U2AF⁶⁵ and SF1 cooperatively bind consensus sequences at the 3' end of introns. Phosphorylation of SF1 on a highly conserved "SPSP" motif enhances its interaction with U2AF⁶⁵ and the pre-mRNA. Here, we reveal that phosphorylation induces essential conformational changes in SF1 and in the SF1/U2AF⁶⁵/3' splice site complex. Crystal structures of the phosphorylated (P)SF1 domain bound to the C-terminal domain of U2AF⁶⁵ at 2.29 Å resolution and of the unphosphorylated SF1 domain at 2.48 Å resolution demonstrate that phosphorylation induces a disorder-to-order transition within a previously unknown SF1/U2AF⁶⁵ interface. We find by small-angle X-ray scattering that the local folding of the SPSP motif transduces into global conformational changes in the nearly full-length (P)SF1/U2AF⁶⁵/3' splice site assembly. We further determine that SPSP phosphorylation and the SF1/U2AF⁶⁵ interface are essential in vivo. These results offer a structural prototype for phosphorylation-dependent control of pre-mRNA splicing factors.