Insights into the gate of the nuclear pore complex

Abstract

Nuclear pore complexes (NPCs) serve as the gateway of the cell nucleus. These macromolecular assemblies form selective aqueous translocation channels permitting the free diffusion of small molecules, as well as receptor-mediated transport of large cargoes. Over the past decade, major progress has been made in both the structural determination of individual nucleoporins and subcomplexes by X-ray crystallography and in the structural analysis of the entire NPC by cryo-electron tomography (cryo-ET). The metazoan NPC structure from Xenopus laevis oocytes was recently resolved up to 20 Å by combining cryo-ET with advanced image processing techniques, revealing for the first time the architecture of the central channel. Here, we discuss the structure of the Xenopus laevis NPC and consider future perspectives that will eventually allow reconstructing the scaffold and gate of the NPC with higher resolution and identifying its transport-relevant regions. This will eventually allow us to describe the structure of the NPC 'in action'.

Keywords: Cryo-electron tomography; FG-repeat domains; Xenopus laevis; nuclear pore complex; nucleoporins; oocytes.

Figure 1.

The central channel of the NPC. A rendered view of a central x–y-section (10-nm thick) of the native NPC shows the organization of the central channel; the SR is depicted in blue, the filamentous protrusions of the central channel ring in orange, the nuclear envelope in gray and the luminal densities in yellow. A dotted ring in the center highlights the very central 17 nm that were masked in in this structural map. The figure was modified from.

Figure 2.

Structural differences of the NPC at different transport states. (A) Schematic representation of cargo being transported through the NPC in the wildtype state (wild type-NPC, left) and after actinomycin D treatment (ActD-NPC, right). Ribosomal subunits are depicted in green colors, RNA in blue colors, and proteins in red colors. (B and C) A view of the 25-nm thick central nucleocytoplasmic section of the wild type-NPC (left) and the ActD-NPC (right) demonstrates the structural differences of the 2 states of the NPC. (B) The local resolution of the structures is depicted by surface coloring. (C) The local cross-resolution values of the structures are visualized by surface coloring and reveal regions where structural changes occur due to altered transport activity (red color). (B and C) Resolution values are given by the color key. The figure was modified from.

Figure 3.

Possible transport routes across the NPC. (A) A view of the 25-nm thick central nucleocytoplasmic section of the NPC with the cytoplasmic side facing upward. The axes show the dimensions of the NPC in the x- and z-direction. (B) A surface-rendered view of the NPC with the nuclear side facing upward displays a view on the nucleoplasmic ring of the NPC and its extended porous meshwork (green). (A and B) The nuclear envelope is depicted in gray, the luminal densities in yellow, the SR in blue, the CPR is shown in gold, and the NPR in green color. The central transport route through the NPC is indicated by orange tubes. An alternative route is traced by purple tubes. (B) Notably, the peripheral channels (purple tubes) extend all the way through this meshwork. The figure was modified from.