Nuclear transport of single molecules: dwell times at the nuclear pore complex

The mechanism by which macromolecules are selectively translocated through the nuclear pore complex (NPC) is still essentially unresolved. Single molecule methods can provide unique information on topographic properties and kinetic processes of asynchronous supramolecular assemblies with excellent spatial and time resolution. Here, single-molecule far-field fluorescence microscopy was applied to the NPC of permeabilized cells. The nucleoporin Nup358 could be localized at a distance of 70 nm from POM121-GFP along the NPC axis. Binding sites of NTF2, the transport receptor of RanGDP, were observed in cytoplasmic filaments and central framework, but not nucleoplasmic filaments of the NPC. The dwell times of NTF2 and transportin 1 at their NPC binding sites were 5.8 ± 0.2 and 7.1 ± 0.2 ms, respectively. Notably, the dwell times of these receptors were reduced upon binding to a specific transport substrate, suggesting that translocation is accelerated for loaded receptor molecules. Together with the known transport rates, our data suggest that nucleocytoplasmic transport occurs via multiple parallel pathways within single NPCs

Light Sheet Microscopy for Single Molecule Tracking in Living Tissue

Single molecule observation in cells and tissue allows the analysis of physiological processes with molecular detail, but it still represents a major methodological challenge. Here we introduce a microscopic technique that combines light sheet optical sectioning microscopy and ultra sensitive high-speed imaging. By this approach it is possible to observe single fluorescent biomolecules in solution, living cells and even tissue with an unprecedented speed and signal-to-noise ratio deep within the sample. Thereby we could directly observe and track small and large tracer molecules in aqueous solution. Furthermore, we demonstrated the feasibility to visualize the dynamics of single tracer molecules and native messenger ribonucleoprotein particles (mRNPs) in salivary gland cell nuclei of Chironomus tentans larvae up to 200 µm within the specimen with an excellent signal quality. Thus single molecule light sheet based fluorescence microscopy allows analyzing molecular diffusion and interactions in complex biological systems

Rapid translocation of NTF2 through the nuclear pore of isolated nuclei and nuclear envelopes

The mechanism by which macromolecules are translocated through the nuclear pore complex (NPC) is little understood. However, recent measurements of nuclear transport in permeabilized cells showed that molecules binding to phenylalanine-glycine-rich repeats (FG repeats) in NPC proteins were translocated much faster through the NPC than molecules not interacting with FG repeats. We have studied that substrate preference of the NPC in isolated oocyte nuclei and purified nuclear envelopes by optical single transporter recording. NTF2, the transport receptor of RanGDP, was exported ∼30 times faster than green fluorescent protein, an inert molecule of approximately the same size. The data confirm that restricted diffusion of inert molecules and facilitated transport of FG-repeat binding proteins are basic types of translocation through the NPC, demonstrating that the functional integrity of the NPC can be conserved in isolated nuclei and nuclear envelopes and thus opening new avenues to the analysis of nucleocytoplasmic transport