Quantitative topographical analysis of nuclear pore complex function using scanning force microscopy.

The only avenue for macromolecular communication between nucleoplasmic and cytoplasmic compartments of the cell is through the nuclear pore complexes (NPCs), which are thus situated at a key location for the control of downstream transcriptional processes. The translocation of cargo through the NPC is mediated by transport receptors, which have the difficult task of making specific, yet transitory, interactions with the NPC in order to support efficient transport. In this report we have examined two stages in the translocation process using scanning force microscopy. We show that the initial docking of importin beta 45-462 is rapid and occurs at nanomolar concentrations. Later stages of transport involve translocation through the central lumen of the NPC and are thought to involve hydrophobic interactions between transport receptors and the NPC. Using calcium-depleted nuclear envelopes we argue that luminal regions of the NPC exhibit hydrophobic characteristics that are not observed for other regions of the NPC.