Binding Affinity and Cooperativity Control U2B″/snRNA/U2A′ RNP Formation

[Image: see text] The U1A and U2B″ proteins are components of the U1 and U2 snRNPs, respectively, where they bind to snRNA stemloops. While localization of U1A and U2B″ to their respective snRNP is a well-known phenomenon, binding of U2B″ to U2 snRNA is typically thought to be accompanied by the U2A′ protein. The molecular mechanisms that lead to formation of the RNA/U2B″/U2A′ complex and its localization to the U2 snRNP are investigated here, using a combination of in vitro RNA–protein and protein–protein fluorescence and isothermal titration calorimetry binding experiments. We find that U2A′ protein binds to U2B″ with nanomolar affinity but binds to U1A with only micromolar affinity. In addition, there is RNA-dependent cooperativity (linkage) between protein–protein and protein–RNA binding. The unique combination of tight binding and cooperativity ensures that the U2A′/U2B″ complex is partitioned only to the U2 snRNP.