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Molecular & Cellular Proteomics 6:798-811, 2007.
© 2007 by The American Society for Biochemistry and Molecular Biology, Inc.






,**,

From the
Department of Clinical Biochemistry, Rigshospitalet, || Institute of Molecular Biology, and
Bioinformatics Centre, University of Copenhagen, DK-2100 Copenhagen, Denmark and ¶ Danish Fundamental Metrology, Danish Technical University, DK-2800 Lyngby, Denmark
Localized mRNAs are transported to sites of local protein synthesis in large ribonucleoprotein (RNP) granules, but their molecular composition is incompletely understood. Insulin-like growth factor II mRNA-binding protein (IMP) zip code-binding proteins participate in mRNA localization, and in motile cells IMP-containing granules are dispersed around the nucleus and in cellular protrusions. We isolated the IMP1-containing RNP granules and found that they represent a unique RNP entity distinct from neuronal hStaufen and/or fragile X mental retardation protein granules, processing bodies, and stress granules. Granules were 100300 nm in diameter and consisted of IMPs, 40 S ribosomal subunits, shuttling heterologous nuclear RNPs, poly(A)-binding proteins, and mRNAs. Moreover granules contained CBP80 and factors belonging to the exon junction complex and lacked eIF4E, eIF4G, and 60 S ribosomal subunits, indicating that embodied mRNAs are not translated. Granules embodied mRNAs corresponding to about 3% of the human embryonic kidney 293 mRNA transcriptome. Messenger RNAs encoding proteins participating in the secretory pathway and endoplasmic reticulum-associated quality control, as well as ubiquitin-dependent metabolism, were enriched in the granules, reinforcing the concept of RNP granules as post-transcriptional operons.

To whom correspondence should be addressed. Tel.: 45-3545-2223; Fax: 45-3545-4640, E-mail: FCN{at}rh.dk
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