title: A doughnut-shaped heteromer of human Sm-like proteins binds to the 3'-end of U6 snRNA, thereby facilitating U4/U6 duplex formation in vitro.
abstract: We describe the isolation and molecular characterization of seven distinct proteins present in human [U4/U6.U5] tri-snRNPs. These proteins exhibit clear homology to the Sm proteins and are thus denoted LSm (like Sm) proteins. Purified LSm proteins form a heteromer that is stable even in the absence of RNA and exhibits a doughnut shape under the electron microscope, with striking similarity to the Sm core RNP structure. The purified LSm heteromer binds specifically to U6 snRNA, requiring the 3'-terminal U-tract for complex formation. The 3'-end of U6 snRNA was also co-precipitated with LSm proteins after digestion of isolated tri-snRNPs with RNaseT(1). Importantly, the LSm proteins did not bind to the U-rich Sm sites of intact U1, U2, U4 or U5 snRNAs, indicating that they can only interact with a 3'-terminal U-tract. Finally, we show that the LSm proteins facilitate the formation of U4/U6 RNA duplices in vitro, suggesting that the LSm proteins may play a role in U4/U6 snRNP formation.
authors.affiliation: Institut für Molekularbiologie und Tumorforschung, Universität Marburg, Emil Mannkopff-Strasse 2, 35037 Marburg, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Achsel
authors.forename: H
authors.initials: H
authors.lastname: Brahms
authors.forename: B
authors.initials: B
authors.lastname: Kastner
authors.forename: A
authors.initials: A
authors.lastname: Bachi
authors.forename: M
authors.initials: M
authors.lastname: Wilm
authors.forename: R
authors.initials: R
authors.lastname: Lührmann
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D066298:In Vitro Techniques; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D012326:RNA Splicing; D012342:RNA, Small Nuclear / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017414:Ribonucleoprotein, U4-U6 Small Nuclear / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D017381:Spliceosomes / Q000378:metabolism
chemical_list: D012342:RNA, Small Nuclear; D017414:Ribonucleoprotein, U4-U6 Small Nuclear