title: The [U4/U6.U5] tri-snRNP-specific 27K protein is a novel SR protein that can be phosphorylated by the snRNP-associated protein kinase.
abstract: SR proteins play important roles in the recognition and selection of the 3' and 5' splice site of a given intron and contribute to the phosphorylation/dephosphorylation-mediated regulation of pre-mRNA splicing. Recent studies have demonstrated that the U1 snRNP is recruited to the 5' splice site by protein/protein interactions involving the SR domains of the U1-70K protein and SF2/ASF. Recently, it was suggested that SR proteins might also contribute to the binding of the [U4/U6.U5] tri-snRNP to the pre-spliceosome (Roscigno RF, Garcia-Blanco MA, 1995, RNA 1:692-706), although it remains unclear whether these SR proteins interact with proteins of the tri-snRNP complex. As a first step toward the identification of proteins that could potentially mediate the integration of the [U4/U6.U5] tri-snRNP complex into the spliceosome, we investigated whether purified [U4/U6.U5] tri-snRNP complexes contain SR proteins. Three proteins in the tri-snRNP complex with approximate molecular weights of 27, 60, and 100 kDa were phosphorylated by purified snRNP-associated protein kinase, which has been shown previously to phosphorylate the serine/ arginine-rich domains of U1-70K and SF2/ASF (Woppmann A et al., 1993, Nucleic Acids Res 21:2815-2822). These proteins are thus prime candidates for novel tri-snRNP SR proteins. Here, we describe the biochemical and molecular characterization of the 27K protein. Analysis of a cDNA encoding the 27K protein revealed an N-terminal SR domain strongly homologous (54% identity) to the SR domain of the U1 snRNP-specific 70K protein. In contrast to many other SR proteins, the 27K protein does not contain an RNA-binding domain. The 27K protein can be phosphorylated in vitro by the snRNP-associated protein kinase and exhibits several isoelectric variants upon 2D gel electrophoresis. Thus, the tri-snRNP-specific 27K protein could potentially be involved in SR protein-mediated protein/protein interactions and, additionally, its phosphorylation state could modulate pre-mRNA splicing.
authors.affiliation: Institut für Molekularbiologie und Tumorforschung, Marburg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Fetzer
authors.forename: J
authors.initials: J
authors.lastname: Lauber
authors.forename: C L
authors.initials: CL
authors.lastname: Will
authors.forename: R
authors.initials: R
authors.lastname: Lührmann
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D018076:DNA, Complementary / Q000235:genetics; D015180:Electrophoresis, Gel, Two-Dimensional; D005656:Fungal Proteins / Q000235:genetics*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011485:Protein Binding; D011494:Protein Kinases / Q000378:metabolism*; D012326:RNA Splicing; D012260:Ribonucleases; D017414:Ribonucleoprotein, U4-U6 Small Nuclear / Q000378:metabolism*; D012261:Ribonucleoproteins; D017411:Ribonucleoproteins, Small Nuclear / Q000235:genetics* / Q000276:immunology / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D017381:Spliceosomes / Q000378:metabolism*
chemical_list: D018076:DNA, Complementary; D005656:Fungal Proteins; D017414:Ribonucleoprotein, U4-U6 Small Nuclear; D012261:Ribonucleoproteins; D017411:Ribonucleoproteins, Small Nuclear; C105252:tri-snRNP-specific protein, 27-kDa, human; D011494:Protein Kinases; D012260:Ribonucleases; C105253:POP4 protein, human