Interaction of Mammalian Splicing Factor SF3a with U2 snRNP and Relation of Its 60-kD Subunit to Yeast PRP9

In the assembly of a prespliceosome, U2 small nuclear ribonucleoprotein (snRNP) functions in pre-messenger RNA (mRNA) splicing together with splicing factors (SFs) 3a, SF3b, and several other proteins. The 17 S but not the 12 S form of U2 snRNP is active in splicing-complex formation. Here it is shown that the SF3a subunits correspond to three of the 17 S U2 snRNP-specific polypeptides. SF3a interacts with U2 snRNP in the presence of SF3b to generate a structure similar to 17 S U2 snRNP, which suggests a function for SF3a and SF3b in the incorporation of U2 snRNP into the spliceosome. Furthermore, the 60-kilodalton subunit of SF3a is related to the yeast splicing protein PRP9.