Structural Basis for the Functional Coupling of the Alternative Splicing Factors Smu1 and RED

The proteins Smu1 and RED have been jointly implicated in the regulation of alternative splicing, mitosis, and influenza virus infection, but how they interact and whether their diverse cellular functions are coupled is unknown. We identified an N-terminal region of Smu1 and a central region of RED that stably interact. Structural analyses revealed that the RED-binding region of Smu1 contains an N-terminal LisH motif linked to a core domain and a C-terminal alpha helix that folds back onto the LisH motif. Smu1 dimerizes via its LisH motif and C-terminal alpha helix and undergoes global conformational changes upon RED binding. In the ensuing hetero-tetrameric Smu1-RED complex, two molecules of RED use short alpha helices to bind hydrophobic grooves of two Smu1 core domains. Our results show how Smu1 and RED form a functional module that exhibits intriguing similarities to transcriptional co-repressor complexes, arranging multiple additional protein-protein interaction sites for contacting splicing and/or chromatin factors.