that do not interact with the CP/T at this stage are variable in shape, while binding to cargo restricts their freedom of movement (14). The contour line view reveals that the narrowest constriction of the central channel is situated at the cytoplasmic side of the lumenal spoke ring in the CF class, even though it is at the nuclear side of the same ring in the LR class (Fig. 4B). These observations indicate that major rearrangements in the spokes might play a critical role in the translocation of cargo. Both classes represent major structural states of the NPC. Since the CP/T is better defined in the CF class, this state might represent the slow incorporation or release of cargo complexes into or from the FXFGframework residing in the central channel (15), which involves interaction with the cytoplasmic filaments. The more diffuse CP/ T of the LR class indicates that cargo complexes can be found in various positions once they have entered the channel Ein agreement with (16)^. Although an assignment of the classes to import, export or to predominant rate-limiting steps in both processes is not yet possible, the application of cryo-ET to transport-competent, intact nuclei holds great potential for a structural dissection of the key steps involved. The use of defined cargo and the trapping of distinct transport intermediates should ultimately enable us to arrive at a detailed mechanistic understanding of the nuclear pore complex
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