Successfully processed mRNAs are guided through the nuclear pore complexes (NPCs)—aqueous channels in the nuclear membrane that directly connect the nucleoplasm and cytosol (Figure 6–37). Small molecules (less than 60,000 daltons) can diffuse freely through these channels. However, most of the macromolecules in cells, including mRNAs complexed with proteins, are far too large to pass through the channels without a special process. The cell uses energy to actively transport such macromolecules in both directions through the nuclear pore complexes.
As explained in detail in Chapter 12, macromolecules are moved through nuclear pore complexes by nuclear transport receptors, which, depending on the identity of the macromolecule, escort it from the nucleus to the cytoplasm or vice versa. For mRNA export to occur, a specific nuclear transport receptor must be loaded onto the mRNA, a step that, in many organisms, takes place in concert with 3ʹ cleavage and polyadenylation. Once it helps to move an RNA molecule through the nuclear pore complex, the transport receptor dissociates from the mRNA, re-enters the nucleus, and is then used to export a new mRNA molecule.
The export of mRNA–protein complexes from the nucleus can be readily observed with the electron microscope for the unusually abundant mRNA of the insect Balbiani Ring genes. As these genes are transcribed, the newly formed RNA is seen to be packaged by proteins, including hnRNPs, SR proteins, and components of the spliceosome. This protein–RNA complex undergoes a series of structural transitions, probably reflecting RNA processing events, culminating in a curved fiber (see Figure 6–37). This curved fiber moves through the nucleoplasm and enters the nuclear pore complex (with its 5ʹ cap proceeding first), and it then undergoes another series of structural transitions as it moves through the pore. These and other observations reveal that the pre-mRNA–protein and mRNA–protein complexes are dynamic structures that gain and lose numerous specific proteins during RNA synthesis, processing, and export (Figure 6–38).
The analysis just described has been complemented by new methods that allow researchers to track the fate of more typical mRNA molecules, which can
RNA as it emerges from RNA polymerase
Figure 6–36 Structure of the core of human RNA exosome. RNA is fed into one end of the central pore and is degraded by RNAses that associate with the other end. Nine different protein subunits (each represented by a different color) make up this large ring structure. Eukaryotic cells have both a nuclear exosome and a cytoplasmic exosome; both forms include the core exosome shown here and additional subunits (including specialized RNAses) that differentiate the two forms. The nuclear exosome degrades aberrant RNAs before they are exported to the cytosol. It also processes certain types of RNA (for example, the ribosomal RNAs) to produce their final form. The cytoplasmic form of the exosome is responsible for degrading mRNAs in the cytosol, and is thus crucial in determining the lifetime of each mRNA molecule. (PDB code: 2NN6.)