MRNPs

Andrei, M. A., Ingelfmger, R., Heintzmann, R., Aschel, T., Rivera-Pomar, R., and Luhrmann, R. (2005). A role for eIF4E and eIF4E-transporter in targeting mRNPs to mammalian processing bodies. RNA 11, 717-727. 

Kedersha, N., Stoecklin, G., Ayodele, M., Yacono, P., Lykke-Andersen, J., Fritzler, M. J., Scheuner, D., Kaufman, R. J., Golan, D. E., and Anderson, P. (2005). Stress granules and processing bodies are dynamically linked sites of mRNP remodeling. J. Cell Biol. 169, 871-884. 

Zalfa, F., Achsel, T., and Bagni, C. (2006). mRNPs, polysomes, or granules FMRP in neuronal protein synthesis. Curr. Opin. Neurobiol. 16, 265-269. 

Jensen, T.H., Dower, K., Libri, D., Rosbash, M. (2003) Early formation of mRNP license for export or quality control Mol. Cell 11,1129-1138. 

Assembly with mRNP and translation factors to form functional polysomes... 

Because the export machinery selectively binds transport-competent mRNPs, i.e., mRNAs associated with the appropriate nuclear proteins, the diffusion-based model for intranuclear RNA movement provides an additional checkpoint for gene expression at the level of mRNA export. This type of control has been demonstrated for tRNA export only mature tRNAs are transported to the cytoplasm because incompletely processed tRNAs do not efficiently bind to exportin-t, the tRNA-specific export receptor. 

Le Hir, H., Moore, M.J. and Maquat, L.E. (2000b) Pre-mRNA splicing alters mRNP composition evidence for stable association of proteins at exon-exon junctions. Genes Dev., 14, 1098-1108. 

Feng Y, Absher D, Eberhart DE, Brown V, Malter HE, Warren ST. FMRP associates with polyribosomes as an mRNP and the I304N mutation of severe fragile X syndrome abolishes this association. Molec CeU 1997 1 109-18. 

Aguilera, A. 2005, Cotranscriptional mRNP assembly From the IVX.A to the nuclear pore. Curr. Opin. (iell Biol. 17 242-].50,... 

Fig. 4.5. Mg2+-dependence of the identity of UV cross-linked proteins. A randomly labelled 5 untranslated region from a growth factor mRNA was mixed with a cytoplasmic lysate at increasing Mg2+ concentrations in the binding buffer, and UV cross-linking was carried out as described in the text. PTB is the polypyrimidine tract binding protein, p50 is the major core protein of mRNPs, and p69 is a leader-specific binding protein.

Once the processing of an mRNA is completed in the nucleus, it remains associated with specific hnRNP proteins in a messenger ribonuclear protein complex, or mRNP. Before it can be translated into the encoded protein, it must be exported out of the nucleus into the cytoplasm. The nucleus is separated from the cytoplasm by two membranes, which form the nuclear envelope (see Figure 5-19). Like the plasma membrane surrounding cells, each nuclear membrane consists of a water-impermeable phospholipid bllayer and vari-... 

Most mRNPs Are Exported from the Nucleus with the Aid of an mRNA-Exporter... 

Other Proteins That Assist in mRNP Export In addition to the mRNA-exporter and FG-nucleoporins, several other types of proteins are involved in the transport of mRNPs by this mechanism. As mentioned earlier, the mRNA-exporter is thought to bind to mRNAs cooperatively with specific mRNP proteins. For example, SR proteins associated with exons appear to stimulate the binding of the mRNA-exporter to processed mRNAs in mRNPs. Thus SR proteins not only... 

Another participant in mRNP transport to the cytoplasm Is the nuclear cap-bIndIng complex, mentioned earlier as protection against exonuclease attack on the 5 end of nascent transcripts and pre-mRNAs. Electron microscopy experiments discussed below have demonstrated that the 5 end of mRNAs lead the way through the nuclear pore complex. Recent experiments in yeast indicate that the 3 poly(A) tail plays an Important role In mRNP transport, suggesting that a poly(A)-binding protein participates. Nucleoporins associated with the NPC cytoplasmic filaments In addition to FG-nucleoporins are required for mRNA export and may function to dissociate the mRNA-exporter and other mRNP proteins that accompany the mRNP through the pore. 

Once the mRNP reaches the cytoplasm, most of the mRNP proteins that associated with the mRNA in the nucleus, the nuclear cap-binding complex, and the nuclear poly (A)-binding protein (PABPII) dissociate and are shuttled back to the nucleus. In the cytoplasm, the 5 cap of an exported mRNA is bound by the eIF4E translation initiation factor, the poly(A) tail is bound by multiple copies of the cytoplasmic poly(A)-binding protein (PABPI), and other RNA-binding proteins associate with the body of the mRNA, forming a cytoplasmic mRNP that has a lower ratio of protein to RNA than nuclear mRNPs. 

Most mRNPs are exported from the nucleus by a het-erodlmerlc mRNA-exporter that interacts with FG-repeats (see Figure 12-24). The direction of transport (nucleus cytoplasm) may result from dissociation of the exporter-mRNP complex in the cytoplasm by an as yet uncharacterized mechanism that does not depend on Ran. 

What Is the evidence that an mRNA exporter directs mRNPs through nuclear pores What is the evidence that most vertebrates also utilize such an exporter ... 

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