Ordered Regions of Channel Nucleoporins Nup62, Nup54 and Nup58 Form Dynamic Complexes in Solution [Molecular Biophysics]

May 29th, 2015 by Sharma, A., Solmaz, S. R., Blobel, G., Melčak, I.

Three out of ~30 nucleoporins, Nup62, Nup54 and Nup58, line the nuclear pore channel. These ′channel′ nucleoporins each contain an ordered region of ~150-200 residues, which is predicted to be segmented into 3-4 α-helical regions of ~40-80 residues. Notably, these segmentations are evolutionarily conserved between uni- and multicellular eukaryotes. Strikingly, the boundaries of these segments match our previously reported mapping and crystal data, which collectively identified two ′cognate′ segments of Nup54, each interacting with cognate segments, one in Nup58, the other one in Nup62. Because Nup54 and Nup58 cognate segments form crystallographic hetero- or homo-oligomers, we proposed that these oligomers associate into inter-convertible ′mid-plane′ rings: a single large ring (40-50 nm diameter, consisting of eight hetero-dodecamers) or three small rings (10-20 nm diameter, each comprising eight homo-tetramers). Each ′ring cycle′ would recapitulate ′dilation′ and ′constriction′ of NPC′s central transport channel. As for the Nup54·Nup62 interactome, it forms a 1:2 triple helix (′finger′), multiples of which project alternately up and down from mid-plane ring(s). Collectively, our previous crystal data suggested a copy number of 128, 64, and 32 for Nup62, Nup54 and Nup58 respectively, that is, a 4:2:1 stoichiometry. Here, we carried out solution analysis utilizing the entire ordered regions of Nup62, Nup54 and Nup58, and demonstrate that they form a dynamic ′triple complex′ that is heterogeneously formed from our previously characterized Nup54·Nup58 and Nup54·Nup62 interactomes. These data are consistent both with our crystal structure-deduced copy numbers and stoichiometries, and also with our ring cycle model for structure and dynamics of the nuclear pore channel.