- RNA is transported through a membrane as a ribonucleoprotein particle.
- All eukaryotic RNAs that function in the cytoplasm must be exported from the nucleus.
- tRNAs and the RNA component of a ribonuclease are imported into mitochondria.
- mRNAs can travel long distances between plant cells.
A bacterium consists of only a single compartment, so all the RNAs function in the same environment in which they are synthesized. This is most striking in the case of mRNA, where translation occurs simultaneously with transcription (see 5.7 The life cycle of bacterial messenger RNA ).
RNA is transported through membranes in the variety of instances summarized in Figure 5.28. It poses a significant thermodynamic problem to transport a highly negative RNA through a hydrophobic membrane, and the solution is to transport the RNA packaged with proteins.
In eukaryotic cells, RNAs are transcribed in the nucleus, but translation occurs in the cytoplasm. Each type of RNA must be transported into the cytoplasm to assemble the apparatus for translation. The rRNA assembles with ribosomal proteins into immature ribosome subunits that are the substrates for the transport system. tRNA is transported by a specific protein system (see 8.28 Transport receptors carry cargo proteins through the pore). mRNA is transported as a ribonucleoprotein, which forms on the RNA transcript in the nucleus (see 24 RNA splicing and processing). These processes are common to all eukaryotic cells. Many mRNAs are translated in the cytosol, but some are localized within the cell, by means of attachment to a cytoskeletal element. One situation in which localization occurs is when it is important for a protein product to be produced near to the site of its incorporation into some macromolecular structure (Jansen, 2001).
Some RNAs are made in the nucleus, exported to the cytosol, and then imported into mitochondria. The mitochondria of some organisms do not code for all of the tRNAs that are required for protein synthesis (see 3.20 Organelle genomes are circular DNAs that code for organelle proteins). In these cases, the additional tRNAs must be imported from the cytosol. The enzyme ribonuclease P, which contains both RNA and protein subunits, is coded by nuclear genes, but is found in mitochondria as well as the nucleus (Puranam and Attardi, 2001). This means that the RNA must be imported into the mitochondria.
We know of some situations in which mRNA is even transported between cells. During development of the oocyte in Drosophila, certain mRNAs are transported into the egg from the nurse cells that surround it. The nurse cells have specialized junctions with the oocyte that allow passage of material needed for early development. This material includes certain mRNAs. Once in the egg, these mRNAs take up specific locations. Some simply diffuse from the anterior end where they enter, but others are transported the full length of the egg to the posterior end by a motor attached to microtubules (see 31.7 How are mRNAs and proteins transported and localized?).
The most striking case of transport of mRNA has been found in plants. Movement of individual nucleic acids over long distances was first discovered in plants, where viral movement proteins help propagate the viral infection by transporting an RNA virus genome through the plasmodesmata (connections between cells) (see Lucas and Gilbertson, 1994; Ghoshroy et al., 1997). Plants also have a defense system, that causes cells to silence an infecting virus, and this too may involve the spread of components including RNA over long distance between cells (see Vance and Vaucheret, 2001). Now it has turned out that similar systems may transport mRNAs between plant cells. Although the existence of the systems has been known for some time, it is only recently that their functional importance has been demonstrated (Kim et al., 2001). This was shown by grafting wild-type tomato plants onto plants that had the dominant mutation Me (which causes a change in the shape of the leaf). mRNA from the mutant stock was transported into the leaves of the wild-type graft, where it changed their shape.