In vivo recruitment of exon junction complex proteins to transcription sites in mammalian cell nuclei.
Studies over the past years indicate that there is extensive coupling between nuclear export of mRNA and pre-mRNA processing. Here, we visualized the distribution of exon junction complex (EJC) proteins and RNA export factors relative to sites of abundant pre-mRNA synthesis in the nucleus. We analyzed both HeLa cells infected with adenovirus and murine erythroleukemia (MEL) cells stably transfected with the human beta-globin gene. Using in situ hybridization and confocal microscopy, we observe accumulation of EJC proteins (REF/Aly, Y14, SRm160, UAP56, RNPS1, and Magoh) and core spliceosome components (U snRNPs) at sites of transcription. This suggests that EJC proteins bind stably to pre-mRNA cotranscriptionally. No concentration of the export factors NXF1/TAP, p15, and Dbp5 was detected on nascent transcripts, arguing that in mammalian cells these proteins bind the mRNA shortly before or after release from the sites of transcription. These results also suggest that binding of EJC proteins to the mRNA is not sufficient to recruit TAP-p15, consistent with recent findings showing that the EJC does not play a crucial role in mRNA export. Contrasting to the results obtained in MEL cells expressing normal human beta-globin transcripts, mutant pre-mRNAs defective in splicing and 3'end processing do not colocalize with SRm160, REF, UAP56, or Sm proteins. This shows that the accumulation of EJC proteins at transcription sites requires efficient processing of the nascent pre-mRNAs, arguing that transcription per se is not sufficient for the stable assembly of the EJC.
Custódio
Noélia
N
Institute of Molecular Medicine, Faculty of Medicine, University of Lisbon, Lisbon 1649-028, Portugal.
Carvalho
Célia
C
Condado
Inês
I
Antoniou
Michael
M
Blencowe
Benjamin J
BJ
Carmo-Fonseca
Maria
M
D000256:Adenoviridae / Q000378:metabolism; D000257:Adenoviridae Infections / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism*; D053487:DEAD-box RNA Helicases; D005914:Globins / Q000235:genetics / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D046911:Macromolecular Substances; D029543:Nucleocytoplasmic Transport Proteins / Q000378:metabolism; D020365:RNA Helicases / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D022821:RNA Splice Sites; D016601:RNA-Binding Proteins / Q000378:metabolism*; D017381:Spliceosomes / Q000378:metabolism*
D002352:Carrier Proteins; D046911:Macromolecular Substances; C122111:NXF1 protein, human; C498546:NXT1 protein, human; D029543:Nucleocytoplasmic Transport Proteins; D022821:RNA Splice Sites; D016601:RNA-Binding Proteins; D005914:Globins; C121148:DDX19B protein, human; D053487:DEAD-box RNA Helicases; D020365:RNA Helicases