Three-dimensional structure and flexibility of a membrane-coating module of the nuclear pore complex

The nuclear pore complex mediates nucleocytoplasmic transport and consists of an assembly of multiple copies of ∼30 different proteins called nucleoporins. Kampmann and Blobel describe the structure and flexibility of the heptameric Nup84 complex by single-particle, negative-stain EM. They find that the arrangement of β-propeller and α-solenoid folds within the heptamer resembles that of the clathrin triskelion, which has been proposed to share a common evolutionary origin with the heptameric complex. The nuclear pore complex mediates nucleocytoplasmic transport in all eukaryotes and is among the largest cellular assemblies of proteins, collectively known as nucleoporins. Nucleoporins are organized into distinct subcomplexes. We optimized the isolation of a putative membrane-coating subcomplex of the nuclear pore complex, the heptameric Nup84 complex, and analyzed its structure by EM. Our data confirmed the previously reported 'Y' shape. We discerned additional structural details, including specific hinge regions at which the particle shows great flexibility. We determined the three-dimensional structures of two conformers, mapped the localization of two nucleoporins within the subcomplex and docked known crystal structures into the EM maps. The free ends of the Y-shaped particle are formed by β-propellers; the connecting segments consist of α-solenoids. Notably, the same organizational principle is found in the clathrin triskelion, which may share a common evolutionary origin with the heptameric complex.