The Golgi-associated protein ArfGAP1 has an unusual membrane-adsorbing amphipathic alpha-helix: its polar face is weakly charged, containing mainly serine and threonine residues. We show that this feature explains the specificity of ArfGAP1 for curved versus flat lipid membranes. We built an algorithm to identify other potential amphipathic alpha-helices rich in serine and threonine residues in protein databases. Among the identified sequences, we show that three act as membrane curvature sensors. In the golgin GMAP-210, the sensor may serve to trap small vesicles at the end of a long coiled coil. In Osh4p/Kes1p, which transports sterol between membranes, the sensor controls access to the sterol-binding pocket. In the nucleoporin Nup133, the sensor corresponds to an exposed loop of a beta-propeller structure. Ser/Thr-rich amphipathic helices thus define a general motif used by proteins of various functions for sensing membrane curvature.

Nucleocytoplasmic transport occurs through nuclear pore complexes (NPCs) whose complex architecture is generated from a set of only ∼30 proteins, termed nucleoporins. Here, we explore the domain structure of Nup133, a nucleoporin in a conserved NPC subcomplex that is crucial for NPC biogenesis and is believed to form part of the NPC scaffold. We show that human Nup133 contains two domains: a COOH-terminal domain responsible for its interaction with its subcomplex through Nup107; and an NH2-terminal domain whose crystal structure reveals a seven-bladed β-propeller. The surface properties and conservation of the Nup133 β-propeller suggest it may mediate multiple interactions with other proteins. Other β-propellers are predicted in a third of all nucleoporins. These and several other repeat-based motifs appear to be major elements of nucleoporins, indicating a level of structural repetition that may conceptually simplify the assembly and disassembly of this huge protein complex.

The nuclear pore complex (NPC) is a protein assembly that contains several distinct subcomplexes. The mammalian nucleoporin (Nup)-107 is part of a hetero-oligomeric complex, that also contains Nup160, Nup133, Nup96, and the mammalian homolog of yeast Sec13p. We used transfection of HeLa cells with small interfering RNAs to specifically deplete mRNA for Nup107. In a domino effect, Nup107 depletion caused codepletion of a subset of other Nups on their protein but not on their mRNA level. Among the affected Nups was a member of the Nup107 subcomplex, Nup133, whereas two other tested members of this complex, Nup96 and Sec13, were unaffected and assembled into Nup107͞Nup133-deficient NPCs. We also tested several phenylalanine-glycine repeat-containing Nups that serve as docking sites for karyopherins. Some of these, such as Nup358, Nup214 on the cytoplasmic, and Nup153 on the nucleoplasmic side of the NPC, failed to assemble into Nup107͞ Nup133-depleted NPCs, whereas p62, a Nup at the center of the NPC, was unaffected. Interestingly, the filamentous, NPC-associated protein Tpr also failed to assemble into the NPCs of Nup107-depleted cells. These data indicate that Nup107 functions as a keystone Nup that is required for the assembly of a subset of Nups into the NPC. Despite the depletion of Nup107 and the accompanying effects on other Nups, there was no significant effect on the growth rate of these cells and only a partial inhibition of mRNA export. These data indicate redundancy of Nups in the function of the mammalian NPC.nucleoporin stability ͉ cell viability ͉ mRNA export