Conformational changes of the in situ nuclear pore complex

By bridging the double membrane separating the cell nucleus and cytoplasm, nuclear pore complexes (NPCs) are crucial pathways for the exchange of ions , proteins, and RNA between these two cellular compartments. A structure in the central lumen of the NPC, called the nuclear transport protein, centra l granule, or nuclear plug, appeared to gate diffusion of intermediate-size d molecules (10-40 kDa) across the nuclear membranes. Visualization of the NPC required drying and fixation of the specimen for electron and atomic fo rce microscopy (AFM), a requirement that has raised doubts about the physio logical relevance of the observation. Here we present AFM images of the out er nuclear membranes and NPCs of Xenopus laevis oocytes under more physiolo gical conditions. Measured under a variety of Ca2+ depletion conditions, th e central granule appeared to occupy and occlude the lumen of the pore in > 80% of NPCs compared to <10% in controls. In a few instances images were ob tained of the same NPCs as the solution was changed from control saline to store depletion conditions, and finally to store repletion conditions. We c onclude that the central lumen of the nuclear pore complex undergoes a conf ormational change in response to depletion of nuclear cisternal Ca2+ levels .