Pre-mRNA splicing complex assembly is mediated by two specific pre-mRNA-snRNP interactions: Ul snRNP binds to the 5’splice site and U2 snRNP binds to the branch point. Here we show that unlike a purified Ul snRNP, which can bind to a 5’splice site, a partially purified U2 snRNP cannot interact with its target pm-mRNA sequence. We identify a previously uncharacterized activity, USAF, that is required for the U2 snRNP-branch point interaction and splicing complex formation. Using RNA substrate exclusion and competition assays, we demonstrate that USAF binds to the 3’splice site region prior to the U2 snRNP-branch point interaction. This provides an explanation for the necessity of the 3’splice site region in U2 snRNP binding and, hence, the first step of splicing..-