title: Arrested yeast splicing complexes indicate stepwise snRNP recruitment during in vivo spliceosome assembly.
abstract: Pre-mRNA splicing is catalyzed by the spliceosome, a macromolecular machine dedicated to intron removal and exon ligation. Despite an abundance of in vitro information and a small number of in vivo studies, the pathway of yeast (Saccharomyces cerevisiae) in vivo spliceosome assembly remains uncertain. To address this situation, we combined in vivo depletions of U1, U2, or U5 snRNAs with chromatin immunoprecipitation (ChIP) analysis of other splicing snRNPs along an intron-containing gene. The data indicate that snRNP recruitment to nascent pre-mRNA predominantly proceeds via the canonical three-step assembly pathway: first U1, then U2, and finally the U4/U6*U5 tri-snRNP. Tandem affinity purification (TAP) using a U2 snRNP-tagged protein allowed the characterization of in vivo assembled higher-order splicing complexes. Consistent with an independent snRNP assembly pathway, we observed high levels of U1-U2 prespliceosomes under U5-depletion conditions, and we observed significant levels of a U2/U5/U6/Prp19-complex mature splicing complex under wild-type conditions. These complexes have implications for the steady-state distribution of snRNPs within nuclei and also reinforce the stepwise recruitment of U1, U2, and the tri-snRNP during in vivo spliceosome assembly.
authors.affiliation: Howard Hughes Medical Institute, Biology Department, Brandeis University, Waltham, Massachusetts 02454, USA.
authors.forename: Daniel F
authors.initials: DF
authors.lastname: Tardiff
authors.forename: Michael
authors.initials: M
authors.lastname: Rosbash
mesh_terms: D000199:Actins / Q000378:metabolism; D047369:Chromatin Immunoprecipitation; D008954:Models, Biological; D012326:RNA Splicing / Q000235:genetics / Q000502:physiology; D017412:Ribonucleoprotein, U1 Small Nuclear / Q000378:metabolism; D017413:Ribonucleoprotein, U2 Small Nuclear / Q000378:metabolism; D017411:Ribonucleoproteins, Small Nuclear / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D017381:Spliceosomes / Q000378:metabolism*
chemical_list: C465210:Act1 protein, S cerevisiae; D000199:Actins; D017412:Ribonucleoprotein, U1 Small Nuclear; D017413:Ribonucleoprotein, U2 Small Nuclear; D017411:Ribonucleoproteins, Small Nuclear; D029701:Saccharomyces cerevisiae Proteins