Permeability of single nuclear pores

In this first application of optical single transporter recording (OSTR), a recently established technique for optically monitoring the activity of si ngle transporters in membrane patches (Tschodrich-Rotter and Peters. 1998. J. Microsc. 192:114-125), the passive permeability of the nuclear pore comp lex (NPC) was measured for a homologous series of hydrophilic probe molecul es. Nuclei were isolated from Xenopus oocytes and firmly attached to filter s containing small cylindrical pores. Transport through membrane patches sp anning filter pores was measured by scanning microphotolysis. Thus the perm eability coefficients of single NPCs were determined for fluorescently labe led dextrans of similar to 4, 10, and 20 kDa. Dextrans of greater than or e qual to 40 kDa could not permeate the NPC. The data were consistent with a model in which the NPC contains a single diffusion channel. By application of established theories for the restricted diffusion through small pores, t he diffusion channel was approximated as a cylinder with a radius of 4.4-6. 1 nm (mean 5.35 nm). Because the transport rate constant of the single NPC was known, the equivalent length of the channel could be also determined an d was found to be 40-50 nm (mean 44.5 nm). The symmetry of the NPC implies that a singular component such as the diffusion channel is located at the c enter of the NPC. Therefore a common transport pathway apparently mediates both passive and signal-dependent transport. To test this hypothesis, measu rements of signal-dependent transport and of the mutual effects signal-depe ndent and passive transport may exert on each other are in progress.