DISRUPTION OF BASE-PAIRED U4-CENTER-DOT-U6 SMALL NUCLEAR RNAS INDUCEDBY 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 -P-32]UTP by the terminal uridylyl transferase activity present in HeL a cell nuclear extracts. That these forms all enter the U6 snRNA-conta ining particles, U4 . U6, U4 . U5 . U6, and the spliceosome, has been demonstrated previously. Here, we report an interaction between the he terogeneous nuclear ribonucleoprotein (hnRNP) C protein, an abundant n uclear pre-mRNA binding protein, and the U6 snRNAs that have the longe st uridylate stretches. This U6 snRNA subset is free of any one of the other snRNPs, since anti-Sm antibodies failed to immunoprecipitate hn RNP C protein. Furthermore, isolated U4 . U6 snRNPs containing U6 snRN As with long oligouridylate stretches are disrupted upon binding of hn RNP C protein either purified from HeLa cells or produced as recombina nt protein from Escherichia coli. In view of these data and our previo us proposal that the U6 snRNA active in splicing has 3'-OH end, we dis cuss 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.