Upon discovery of ACE1, we proposed a model of the NPC scaffold based on and analogous to the lattice-like assembly of COPII vesicle coats (). COPII vesicle coats are composed of two layers. The membrane-proximal or inner layer is composed of the three proteins Sar1, Sec23 and Sec24, while the membrane-distal or outer layer can be exclusively constructed from the ACE1-protein Sec31 and the ²-propeller protein Sec13.15
In the COPII outer coat, one assembly unit is composed of two Sec31·Sec13 heterodimers that interact in a homotypic fashion via the crowns of Sec31's ACE1 domain, positioning the N-terminal ²-propeller of Sec31 and the adjacent ²-propeller of Sec13 as a tandem pair at either end of this extended rod-shaped structure ().14
Thus, ACE1 domains form the symmetrical edge elements of the lattice, while four Sec31-²-propellers from adjacent edges form the vertex elements in the assembled COPII coat. Beyond spacing the N-terminal Sec31 ²-propeller and ACE1-domain, the role of the Sec13 ²-propeller is still largely unclear. In our lattice model of the NPC, we predicted that the ACE1 domains of Nup84 and Nup145C form a heterotypic edge element analogous to the homotypic edge element of the COPII coat, indicating that COPII and the NPC share not only building blocks, but also assemble at least in part by similar mechanisms.10
We were able to provide the evidence for this edge element in the Nup84·Nup145C·Sec13 crystal structure.16
While Sec31·Sec13 is the principal building block of the COPII outer coat, the Y-complex is equally essential for the construction of the NPC coat. Biochemical and structural studies have characterized the interactions between nucleoporins within the Y-complex.4
Interactions between the stacked helical domains of the components Nup133, Nup84, Nup145C, Nup120 and Nup85 appear sufficient to assemble its branched Y-shaped structure. The ²-propeller domains, of which there are four (N-terminal domains of Nup133 and Nup120, Seh1 and Sec13), are not central to these interactions, leaving them as candidates for mediating interactions outside of the Y.
Figure 2 Membrane-coating scaffolds of the NPC and vesicle coats. Schematics of the lattice like assemblies of the NPC and COPII scaffolds are shown, with ACE1 modules colored as in . Enlarged views of the ACE1 edge elements are shown for both the NPC (more ...)
Interestingly, structural analogs to the inner COPII coat components Sec23, Sec24 and Sar1 have not been observed in the NPC. Yet, we speculate that the NPC lattice built from Y- and Nic96 complexes is connected to the pore membrane via adaptors, consistent with the ~8 nm gap between the scaffold and the pore membrane observed in cryo-tomography of the NPC.17
We speculate, that this gap could serve as a conduit for the transport of inner nuclear membrane proteins to their destination after insertion into the endoplasmic reticulum upon synthesis. In contrast to COPII vesicles, which need to be assembled and disassembled quickly and repeatedly, the NPC is a very stable entity, virtually without turnover in the non-dividing cell.18
Thus the adaptor components may be very different, with Ndc1 and its direct interactors being strong candidates.19
Besides the difference in membrane-anchorage, the NPC-COPII analogy also differs in another way. While both systems coat highly curved membranes, the pore membrane has a decidedly different topology from a spherical vesicle. The COPII coat only needs to stabilize a contiguous, positively curved (convex) membrane. The nuclear pore membrane, like the hole in a doughnut, is convex in the longitudinal direction, but negatively curved (concave) in the latitudinal direction.20
While the vesicle membrane has neither end nor beginning, the pore membrane ends at the perimeter of the opening. Thus, the NPC likely has elements that are specific to the distinct pore membrane topology. In the lattice model, the Y-complex is oriented relative to the pore membrane such that the Nup145C·Nup84 edge element follows the positive curvature, positioning Nup133 at the periphery of the NPC.16
However, how the Y complexes are connected is unclear. While the interactions may be direct, they may also be mediated by Nic96 complex components, or other factors.