Co-transcriptional mRNP formation is coordinated within a molecular mRNP packaging station in S. cerevisiae

Abstract

In eukaryotes, the messenger RNA (mRNA), the blueprint of a protein-coding gene, is processed and packaged into a messenger ribonucleoprotein particle (mRNP) by mRNA-binding proteins in the nucleus. The steps of mRNP formation - transcription, processing, packaging, and the orchestrated release of the export-competent mRNP from the site of transcription for nuclear mRNA export - are tightly coupled to ensure a highly efficient and regulated process. The importance of highly accurate nuclear mRNP formation is illustrated by the fact that mutations in components of this pathway lead to cellular inviability or to severe diseases in metazoans. We hypothesize that efficient mRNP formation is realized by a molecular mRNP packaging station, which is built by several recruitment platforms and coordinates the individual steps of mRNP formation.

Keywords: TREX; gene expression; mRNA processing; mRNP formation; molecular mRNP packaging station; nuclear mRNA export; transcription.

Figure 1

Steps of nuclear mRNP formation. The mRNA is synthesized from the protein coding gene by RNAPII as a nascent transcript. The nascent transcript is processed while transcription elongation still takes place. It is capped, spliced, cleaved, and polyadenylated. In addition, mRNA‐binding proteins bind to the mRNA and package it into an mRNP. These mRNA‐binding proteins influence mRNA stability and are necessary for nuclear export. After a likely remodeling of the mRNP, it is exported to the cytoplasm through the nuclear pore complex (NPC). In the cytoplasm the mRNA is translated by the ribosomes, which synthesize the encoded protein. Eventually, the mRNA is degraded.

Figure 2

The mRNP packaging station. As the mRNA emerges from RNAPII during transcription elongation, it is capped, spliced, cleaved, and polyadenylated (processing). In addition, a multitude of RNA‐binding proteins binds to the mRNA, packaging it in an mRNP. We hypothesize that these single steps of mRNP formation are coordinated and controlled within an mRNP packaging station. This mRNP packaging station is assembled by three recruitment platforms and possibly additional coordinating proteins such as the TREX complex, which ensure spatial proximity of all processes. The three recruitment platforms for mRNA processing and mRNA‐binding proteins are indicated by different colors (red: RNA, black: Rpb1‐CTD, and green: Spt5‐CTR). The different mRNA processing and mRNA‐binding proteins are defined in the legend below the figure. The capping enzyme is exchanged for the cap binding complex consisting of Cbc20 and Cbc80 after capping is complete. The differential phosphorylation of the CTD (C‐terminal domain of Rpb1, the largest subunit of RNAPII) is indicated at the top as well as below the chromatin (blue: S5P, orange: Y1P, green: S2P). The phosphorylation status is indicated by a gradient from 0–100% as indicated by the bars in the bottom left hand corner. Zero per cent reflects no detectable phosphorylation while 100% reflects the maximum signal observed at an average gene in a ChIP experiment.