SF3A3 is one of the components of the SF3a splicing factor complex of the mature U2 snRNP (small nuclear ribonucleoprotein particle). In yeast, SF3a shows a bifurcated assembly structure of three subunits, Prp9 (subunit 3), Prp11 (subunit 2) and Prp21 (subunit 1). Prp9 and Prp21 were not thought to interact with each other but the alpha1 helix of Prp9 does make important contacts with the SURP2 domain of Prp21, thus the two do interact via a bidentate-binding mode. Prp9 harbours a major binding site for stem-loop IIa of U2 snRNA [1].

1: Structure and assembly of the SF3a splicing factor complex of U2 snRNP. Lin PC, Xu RM; EMBO J. 2012;31:1579-1590. PMID:22314233