The snRNP-free U1A (SF-A) complex(es): Identification of the largest subunit as PSF, the polypyrimidine-tract binding protein-associated splicing factor

We have previously shown that a specific monoclonal antibody prepared again st the U1A protein, MAb 12E12, is unique in its ability to recognize a form of U1A which is not associated with the U1snRNP. This unique form of U1A, termed snRNP-free U1A or SF-A, was found to be complexed with a novel set o f non-snRNP proteins (O'Connor et al., 1997, RNA 3:1444-1455). Here we demo nstrate that the largest protein in these SF-A complex(es), p105, is the po lypyrimidine-tract binding protein-associated factor (PSF), an auxiliary sp licing factor. We show that PSF copurifies and co-immunoprecipitates with S F-A from 293T cell nucleoplasm and that it interacts with SF-A in vitro. In addition, we show that MAb 12E12 inhibits both splicing and polyadenylatio n in an in vitro coupled splicing and polyadenylation reaction. This sugges ts that SF-A and/or the SF-A complex(es) perform an important function in b oth processing reactions and possibly in last exon definition.