Mediates the splicing of pre-mRNA by binding to the loop I region of U1-snRNA. Component of the spliceosome. Interacts with CYP63, U2AF35A, U2AF35B, SRZ21, RSZ22, SR34, SR45, SR45A and SCL33. The exchange between speckles and nucleoplasm is sensitive to ongoing transcription and phosphorylation. Ubiquitous. Phosphorylated. The association and dissociation with SR45 is not affected by the phosphorylation status (PubMed:18414657).