Nuclear pore complexes (NPCs) reside in the nuclear membrane and mediate macromolecules exchange between the nucleus and cytoplasm. Although the protein components of NPCs, termed collectively nucleoporin, have been identified, how nucleoporins arrange in NPC, however, is still an enigma. NPC is a highly symmetric protein complex, which contains an eight-fold rotational symmetry across the center of the pore and a two-fold symmetry in the plane of the nuclear envelope. In addition, according to electron microscopic reconstruction model, NPC can also be considered schematically as a series of concentric cylinders. A peripheral cylinder coating the nuclear pore membrane contains a heptameric Nup84 complex, which harbors Nup145C/Sec13 complex in the center. Hence, X-ray crystallographic approach was employed to determine the Nup145C/Sec13 structure. Notably, in the two crystal forms I obtained, Nup145C/Sec13 oligmerized to form a slightly bent hetero-octamer rod. In the meantime, the oligomerization state could be observed in solution as well. Furthermore, as indicated by structural analyses, the interacting interfaces not only are evolutionarily conserved but also cover huge area. Taken together, the Nup145C/Sec13 hetero-octamer rod could be physiologically relevant without much doubt. In concordance with the stoichiometric considerations of NPC, then we confidently propose a model with respect to a coat for the nuclear pore membrane. In the model, the coat is constructed of four rings and each ring is composed of eight heptamers. Moreover, due to the three axes on the Nup145C/Sec13 rod, the hetero-octamer is also places vertically in order to connect these four anti-parallel rings. Remarkably, the Nup85/Seh1p crystal structures that were determined recently indicated that the assembly of this pair bears reseblance in its shape and dimension to that of another pair, Nup145C/Sec13. Thus, Nup85/Seh1 does form another hetero-octamer pole as we predicted in the model. Surprisingly, in contrast to the rigidity of the Nup145C/Sec13 hetero-octamer, the Nup85/Seh1 structures reveal a hinge motion that may facilitate the conformational change in the NPC for importing the integral membrane proteins
