Nuclear transport is the mechanism used by eukaryotic cells to transport macromolecules across the nuclear membrane. It plays an important role in the regulation of gene expression, since it governs both the export of messenger RNA (that carries the genetic information in the cytoplasm before protein synthesis) and the import of promoters that initiate the transcription of DNA into RNA. The exact mechanism of nuclear transport has not yet been solved, although the different proteins playing a part in it have been identified.
The nuclear import of proteins involves the recognition of the cargo by two helper proteins, leading to the formation of a complex, which is subsequently translocated to the nucleus through a protein assembly called the nuclear pore complex. The directionality of transport is due at least in part to the small protein Ran, present in the Ran-GTP form in the nucleus and in the hydrolyzed Ran-GDP form in the cytoplasm, and able to dissociate the cargo only in the first case. Using fluorescence correlation spectroscopy, we tested the efficiency of this molecular switch by measuring the diffusion coefficient of a fluorescent cargo in cytoplasmic conditions (in presence of Ran-GDP) or in nucleoplasmic conditions (in presence of Ran-GDP).
We find that whereas in the nucleus the observed mobility corresponds to the expected slightly hindered diffusion of the cargo, in the cytoplasm it is too small to correspond to the diffusion of the complex. This could be explained either by the presence of a typical mesh size within the cytoplasm, critically slowing down the complex compared to the simple cargo, or by specific interaction of the complex with cellular structures such as the microtubule network, in which case this could be a way for the cell to direct the cargo towards the nuclear pore complex at the surface of the nucleus.