title: Novel and essential subunits in the 300-kilodalton nuclear cap binding complex of Trypanosoma brucei.
abstract: One of the unique aspects of RNA processing in trypanosomatid protozoa is the presence of a cap 4 structure (m7Gpppm2(6)AmpAmpCmpm3Um) at the 5' end of all mRNAs. The cap 4 becomes part of the mRNA through trans-splicing of a 39-nucleotide-long sequence donated by the spliced leader RNA. Although the cap 4 modifications are required for trans-splicing to occur, the underlying mechanism remains to be determined. We now describe an unconventional nuclear cap binding complex (CBC) in Trypanosoma brucei with an apparent molecular mass of 300 kDa and consisting of five protein components: the known CBC subunits CBP20 and importin-alpha and three novel proteins that are only present in organisms featuring a cap 4 structure and trans-splicing. Competitive binding studies are consistent with a specific interaction between the CBC and the cap 4 structure. Downregulation of several individual components of the T. brucei CBC by RNA interference demonstrated an essential function at an early step in trans-splicing. Thus, our studies are consistent with the CBC providing a mechanistic link between cap 4 modifications and trans-splicing.
authors.affiliation: Department of Epidemiology and Public Health, Yale University Medical School, BCMM 136C, 295 Congress Ave., New Haven, CT 06536-0812, USA.
authors.forename: Hongjie
authors.initials: H
authors.lastname: Li
authors.forename: Christian
authors.initials: C
authors.lastname: Tschudi
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive / Q000235:genetics / Q000502:physiology; D008969:Molecular Sequence Data; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D039381:RNA Cap-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D034622:RNA Interference; D012333:RNA, Messenger / Q000378:metabolism; D016053:RNA, Protozoan / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics; D020038:RNA, Spliced Leader / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D012261:Ribonucleoproteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D020040:Trans-Splicing / Q000235:genetics / Q000502:physiology*; D014346:Trypanosoma brucei brucei / Q000235:genetics* / Q000378:metabolism; D028901:alpha Karyopherins / Q000235:genetics / Q000378:metabolism / Q000502:physiology
chemical_list: D021122:Protein Subunits; D015800:Protozoan Proteins; D039381:RNA Cap-Binding Proteins; D012333:RNA, Messenger; D016053:RNA, Protozoan; D034741:RNA, Small Interfering; D020038:RNA, Spliced Leader; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; D028901:alpha Karyopherins
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