Architectural nucleoporins Nup157/170 and Nup133 are structurally related and descend from a second ancestral element.

The nuclear pore complex (NPC) constitutes one of the largest protein assemblies in the eukaryotic cell and forms the exclusive gateway to the nucleus. The stable, approximately 15-20-MDa scaffold ring of the NPC is built from two multiprotein complexes arranged around a central 8-fold axis. Here we present crystal structures of two large architectural units, yNup170(979-1502) and hNup107(658-925) x hNup133(517-1156), each a constituent of one of the two multiprotein complexes. Conservation of domain arrangement and of tertiary structure suggests that Nup157/170 and Nup133 derived from a common ancestor. Together with the previously established ancestral coatomer element (ACE1), these two elements constitute the major alpha-helical building blocks of the NPC scaffold and define its branched, lattice-like architecture, similar to vesicle coats like COPII. We hypothesize that the extant NPC evolved early during eukaryotic evolution from a rudimentary structure composed of several identical copies of a few ancestral elements, later diversified and specified by gene duplication.