Component of the 17S U2 SnRNP complex of the spliceosome, a large ribonucleoprotein complex that removes introns from transcribed pre-mRNAs.The 17S U2 SnRNP complex 1 directly participates in early spliceosome assembly and 2 mediates recognition of the intron branch site during pre-mRNA splicing by promoting the selection of the pre-mRNA branch-site adenosine, the nucleophile for the first step of splicing.Within the 17S U2 SnRNP complex, SF3A1 is part of the SF3A subcomplex that contributes to the assembly of the 17S U2 snRNP, and the subsequent assembly of the pre-spliceosome 'E' complex and the pre-catalytic spliceosome 'A' complex.Involved in pre-mRNA splicing as a component of pre-catalytic spliceosome 'B' complexes.