The nuclear pore complex (NPC) is essential for the transit of molecules between the cytoplasm and nucleoplasm of a cell and until recently was thought to allow intermediate-sized molecules (relative molecular mass of ~10,000) to diffuse freely across the nuclear envelope. However, the depletion of calcium from the nuclear envelope of Xenopus laevis oocytes was shown to regulate the passage of intermediate-sized molecules. Two distinct conformational states of the NPC were observed by field emission scanning electron microscopy and atomic force microscopy. A central plug occluded the NPC channel after nuclear calcium stores had been depleted and free diffusion of intermediate-sized molecules had been blocked. Thus, the NPC conformation appears to gate molecular movement across the nuclear envelope.
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