Functional association of U2 snRNP with the ATP-independent spliceosomal complex E

In the current model for spliceosome assembly, U1 snRNP binds to the 5' spl ice site in the E complex followed by ATP-dependent binding of U2 snRNP to the branchpoint sequence (BPS) in the A complex. Here we report the charact erization of highly purified, functional E complex. We provide evidence tha t this complex contains functional U2 snRNP and that this snRNP is required for E complex assembly. The BPS is not required for U2 snRNP binding in th e E complex. These data suggest a model for spliceosome assembly in which U 1 and U2 snRNPs first associate with the spliceosome in the E complex and t hen an ATP-dependent step results in highly stable U2 snRNP binding to the BPS in the A complex.