A protein associated with small nuclear ribonucleoprotein particles recognizes the 3' splice site of premessenger RNA.

A HeLa cell nuclear extract active in splicing of pre-mRNA has been fractionated to identify the component that interacts with the 3' splice site. The activity that binds this region in an RNAase T1 protection assay copurifies with a 70 kd protein which is recognized by anti-Sm antibodies. Protein blots probed with labeled mRNA precursors either containing or lacking an intact 3' splice site reveal that the 70 kd polypeptide can bind pre-mRNA after immobilization on nitrocellulose and that it shows a preference for sequences located between the 3' splice junction and the site of lariat formation. Cofractionation during chromatography and immunoprecipitation by anti-2,2,7-trimethylguanosine antibodies demonstrate that the 3' splice site binding component associates with small nuclear ribonucleoprotein particles in low (1 mM) but not high (15 mM) Mg++ concentrations.