mRNPs
In eukaryotic cells, primary transcripts are processed and bound by proteins before export to the cytoplasm. Nuclear production of export-competent messenger ribonucleoprotein particles (mRNPs) is a complicated process, and mRNP biogenic events that function sub-optimally are rapidly attacked by surveillance leading to degradation of the mRNA.
Export of nuclear mRNAs is therefore constantly challenged by the opposing force of mRNA retention and decay. This balance ensures that only ’perfect’ transcripts persist, and that non-functional and potentially deleterious transcripts are removed early in their biogenesis.
Thus, eukaryotic systems of mRNP quality control can be viewed as simple Darwinian principles operating at the molecular level.
The spatial separation of nuclear transcription and cytoplasmic translation in eukaryotic cells implies that mRNAs have to travel.
On their journey, proteins involved in the various steps of transcript formation, processing and transport dynamically interact with mRNAs to form diverse messenger ribonucleoprotein complexes (mRNPs).
The protein complexes involved in distinct phases of manufacturing a bona fide mRNA in the nucleus are tightly coupled.
Active genes migrate into preassembled, shared nuclear sub-compartments suggesting that mRNAs are churned out in large ’transcription factories’ with distinct but interconnected divisions.
Nuclear factors have now been identified that specifically control the quality of mRNAs without affecting mRNP biogenesis or export.
Transcription is coupled with the concomitant assembly of RNA-binding proteins to the nascent mRNA to generate a stable and export-competent mRNP particle.
RNA-binding factors recruited at active transcription sites specify the processing, nuclear export, subcellular localization, translation and stability of the mRNA.
The assembly of the mRNP particle starts with the association of the cap-binding protein complex followed by the splicing-dependent assembly of the exon-junction complex in intron-containing genes and by the binding of RNA-export adaptor proteins.
mRNP assembly is a genetically controlled process that plays a key role in gene expression and other cellular processes, including the maintenance of genome integrity.
References
Aguilera A. Cotranscriptional mRNP assembly: from the DNA to the nuclear pore. Curr Opin Cell Biol. 2005 Jun;17(3):242-50. PMID: #15901492#
Sommer P, Nehrbass U. Quality control of messenger ribonucleoprotein particles in the nucleus and at the pore. Curr Opin Cell Biol. 2005 Jun;17(3):294-301. PMID: #15901500#
Saguez C, Olesen JR, Jensen TH. Formation of export-competent mRNP: escaping nuclear destruction. Curr Opin Cell Biol. 2005 Jun;17(3):287-93. PMID: #15901499#