Disruption of base-paired U4.U6 small nuclear RNAs induced by mammalian heterogeneous nuclear ribonucleoprotein C protein.

Due to 3' end modifications, mammalian U6 small nuclear RNA (snRNA) is heterogeneous in size. The major form terminates with five U residues and a 2',3'-cyclic phosphate, but multiple RNAs containing up to 12 U residues have a 3'-OH end. They are labeled in the presence of [alpha-32P]UTP by the terminal uridylyl transferase activity present in HeLa cell nuclear extracts. That these forms all enter the U6 snRNA-containing particles, U4.U6, U4.U5.U6, and the spliceosome, has been demonstrated previously. Here, we report an interaction between the heterogeneous nuclear ribonucleoprotein (hnRNP) C protein, an abundant nuclear pre-mRNA binding protein, and the U6 snRNAs that have the longest uridylate stretches. This U6 snRNA subset is free of any one of the other snRNPs, since anti-Sm antibodies failed to immunoprecipitate hnRNP C protein. Furthermore, isolated U4.U6 snRNPs containing U6 snRNAs with long oligouridylate stretches are disrupted upon binding of hnRNP C protein either purified from HeLa cells or produced as recombinant protein from Escherichia coli. In view of these data and our previous proposal that the U6 snRNA active in splicing has 3'-OH end, we discuss a model where the hnRNP C protein has a decisive function in the catalytic activation of the spliceosome by allowing the release of U4 snRNP.