The basic NPC structure is arranged in three rings, the nucleoplasmic and cytoplasmic (green) rings and an inner ring (purple). The central part is occupied by a protein meshwork (blue) important for the transport and exclusion function of the pore. Cytoplasmic and nucleoplasmic extensions (brown) emanate from the respective ring structures. A prototypical Y-complex is shown with its core components, Nup85, Nup160, Sec13, Nup96, Nup107, and Nup133 (following the human nomenclature and lacking the nonconserved complex members). Schizosaccharomyces pombe Nup131 and Nup132 Y-complex derivatives are shown. For simplicity, the nonconserved components, including Nup37 and Ely5 in S. pombe, are omitted. Sec13, so far regarded as a conserved Y-complex component with a second function as a COP-II coat subunit, is present in S. pombe but does not localize to NPCs [13]. COP-II, coat protein complex II; NPC, nuclear pore complex; Nup, nucleoporin; Sec, secretory.