In vivo mobility of proteins involved in nuclear transport studied by fluorescence correlation spectroscopy

Using fluorescence correlation spectroscopy we measured the apparent mobility of a nuclear transport cargo (a streptavidin labeled with a nuclear localization signal) both in the cytoplasm and the nucleus of living cells, and we compared it to the mobility of a streptavidin labeled with mutations of the nuclear localization signal known not to support nuclear import, and with the mobility of a set of inert molecules (dextrans) of different sizes. In the cytoplasm, the mobility of the transport cargo is found to be significantly reduced compared to its mobility in the nucleus, or to the mobility of the streptavidins labeled with a mutant nuclear localization signal. This can be partly explained by the fact that the transport cargo forms a complex with two nuclear import mediator proteins (importin α and importin β) in the cytoplasm, but could also be partly due to specific interactions of this cargo with the cell cytoskeleton.