The Spliceosome Assembly Pathway in Mammalian Extracts

A mammalian splicing commitment complex was functionally defined by using a template commitment assay. This complex was partially purified and shown to be a required intermediate for complex A formation. The productive formation of this commitment complex required both splice sites and the polypyrimidine tract. U1 small nuclear ribonucleoprotein (snRNP) was the only spliceosomal U snRNP required for this formation. A protein factor, very likely U2AF, is probably involved in the formation of the splicing commitment complex. From the kinetics of appearance of complex A and complex B, it was previously postulated that complex A represents a functional intermediate in spliceosome assembly. Complex A was partially purified and shown to be a required intermediate for complex B (splicesome) formation. Thus, a spliceosome pathway is for the first time supported by direct biochemical evidence: RNA + U1 snRNP + ?U2 auxiliary factor + ?Y → CC + U2 snRNP + Z → A + U4/6,5 snRNPs + β → B.