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of nta ate W Nup153 is flexible within the NPC, although this nucleoporin is anchored to the nuclear side of the NPC. By using domain-specific antibodies, we N b el ed proteins collectively termed nucleoporins (Nups). the central framework is the central pore, w90 nm their cargo. The transport receptor, in turn, is able structure. Immuno-EM studies of the vertebrate nucleo-porin Nup153 demonstrated a high degree of doi:10.1016/j.jmb.2005.06.034long and 40–50 nm in diameter in its narrowestExtensive electron microscopy (EM) studies mainly in Xenopus oocytes have led to a draft of the 3-D NPC architecture.5–8 Accordingly, the NPC is composed of an 8-fold symmetric central frame-work that is continuous with a cytoplasmic and a nuclear ring moiety. Eight cytoplasmic filaments decorate the cytoplasmic ring, whereas the nuclear ring is topped by the nuclear basket. Enclosed by to mediate the interaction of the cargo–receptor complex with the NPC. It is thought that the main, albeit not the exclusive,13 interaction between the transport receptors and the NPC is mediated by a subset of nucleoporins that harbor FG (phenyl-alanine-glycine)-repeat domains. Based on X-ray crystallography and biophysical analysis, FG-repeat domains exhibit little secondary 14–20*Corresponding autho