Snapshots of nuclear pore complexes in action captured by cryo-electron tomography

Abstract

Nuclear pore complexes reside in the nuclear envelope of eukaryotic cells and mediate the nucleocytoplasmic exchange of macromolecules¹. Traffic is regulated by mobile transport receptors that target their cargo to the central translocation channel, where phenylalanine-glycine-rich repeats serve as binding sites². The structural analysis of the nuclear pore is a formidable challenge given its size, its location in a membranous environment and its dynamic nature. Here we have used cryo-electron tomography³ to study the structure of nuclear pore complexes in their functional environment, that is, in intact nuclei of Dictyostelium discoideum. A new image-processing strategy compensating for deviations of the asymmetric units (protomers) from a perfect eight-fold symmetry enabled us to refine the structure and to identify new features. Furthermore, the superposition of a large number of tomograms taken in the presence of cargo, which was rendered visible by gold nanoparticles, has yielded a map outlining the trajectories of import cargo. Finally, we have performed single-molecule Monte Carlo simulations of nuclear import to interpret the experimentally observed cargo distribution in the light of existing models for nuclear import.