Evidence for Ca2+- and ATP-sensitive peripheral channels in nuclear pore complexes

In eukaryotic cells the nuclear envelope (NE) serves as a functional barrie r between cytosol and nucleoplasm perforated by nuclear pore complexes (NPC s). Both active and passive transport of ions and macromolecules are though t to be mediated by the centrally located large NPC channel. However, 3-dim ensional imaging of NPCs based on electron microscopy indicates the existen ce of additional small channels of unknown function located in the NPC peri phery. By means of the recently developed nuclear hourglass technique that measures NE electrical conductance, we evaluated passive electrically drive n transport through NPCs. In isolated Xenopus laevis oocyte nuclei, we vari ed ambient Ca2+ and ATP in the cytosolic solution and/or chelated Ca2+ in t he perinuclear stores in order to assess the role of Ca2+ in regulating pas sive ion transport. We noticed that NE electrical conductance is large unde r conditions where macromolecule permeability is known to be low. In additi on, atomic force microscopy applied to native NPCs detects multiple small p ores in the NPC periphery consistent with channel openings. Peripheral pore s were detectable only in the presence of ATP. We conclude that NPC transpo rt of ions and macromolecules occurs through different routes. We present a model in which NE ion flux does not occur through the central NPC channel but rather through Ca2+ - and ATP-activated peripheral channels of individu al NPCs.