U2AF assemblies drive sequence-specific splice site recognition.

The essential splicing factor U2AF is known to help anchoring U2 snRNP at the branch site. Its C-terminal UHM domain interacts with ULM motifs of SF3b155, an U2 snRNP protein. Here, we report a cooperative binding of U2AF and the related protein CAPERα to the multi-ULM domain of SF3b155.