Spliceosomal E Complex U2 snRNP Spliceosomal A Complex hPrp5 hTra2 Formation of the Spliceosomal A Complex Pubmed12176931 Pubmed12477934 Reactome Database ID Release 4372124 Reactome, http://www.reactome.org ReactomeREACT_788 The A complex is the first ATP-dependent complex in spliceosome assembly. U2AF recruits the U2 snRNP to bind to the branch site in the E complex in an ATP-dependent fashion, to form the A complex. The U2 snRNA base-pairs with the branch site, causing the branch-site adenosine to bulge out, which later positions it for nucleophilic attack at the 5' splice site. The A complex serves as a substrate for formation of the B complex.