Search

Nuclear export of microRNAs (miRNAs) by exportin-5 (Exp-5) is an essential step in miRNA biogenesis. Here, we present the 2.9 angstrom structure of the pre-miRNA nuclear export machinery formed by pre-miRNA complexed with Exp-5 and a guanine triphosphate (GTP)--bound form of the small nuclear guanine triphosphatase (GTPase) Ran (RanGTP). The x-ray structure shows that Exp-5:RanGTP recognizes the 2-nucleotide 3' overhang structure and the double-stranded stem of the pre-miRNA. Exp-5:RanGTP shields the pre-miRNA stem from degradation in a baseball mitt-like structure where it is held by broadly distributed weak interactions, whereas a tunnel-like structure of Exp-5 interacts strongly with the 2-nucleotide 3' overhang through hydrogen bonds and ionic interactions. RNA recognition by Exp-5:RanGTP does not depend on RNA sequence, implying that Exp-5:RanGTP can recognize a variety of pre-miRNAs. 10.1126/science.1178705

Gap junctions consist of arrays of intercellular channels between adjacent cells that permit the exchange of ions and small molecules. Here we report the crystal structure of the gap junction channel formed by human connexin 26 (Cx26, also known as GJB2) at 3.5Å resolution, and discuss structural determinants of solute transport through the channel. The density map showed the two membrane-spanning hemichannels and the arrangement of the four transmembrane helices of the six protomers forming each hemichannel. The hemichannels feature a positively charged cytoplasmic entrance, a funnel, a negatively charged transmembrane pathway, and an extracellular cavity. The pore is narrowed at the funnel, which is formed by the six amino-terminal helices lining the wall of the channel, which thus determines the molecular size restriction at the channel entrance. The structure of the Cx26 gap junction channel also has implications for the gating of the channel by the transjunctional voltage., Intercellular signalling is one of the most essential properties of multicellular organisms. Gap junctions are specialized membrane regions containing hundreds of intercellular communication channels that allow the passage of molecules [...]

Vaults are among the largest cytoplasmic ribonucleoprotein particles and are found in numerous eukaryotic species. Roles in multidrug resistance and innate immunity have been suggested, but the cellular function remains unclear. We have determined the x-ray structure of rat liver vault at 3.5 angstrom resolution and show that the cage structure consists of a dimer of half-vaults, with each half-vault comprising 39 identical major vault protein (MVP) chains. Each MVP monomer folds into 12 domains: nine structural repeat domains, a shoulder domain, a cap-helix domain, and a cap-ring domain. Interactions between the 42-turn-long cap-helix domains are key to stabilizing the particle. The shoulder domain is structurally similar to a core domain of stomatin, a lipid-raft component in erythrocytes and epithelial cells.

The [epsilon] subunit of bacterial and chloroplast [F.sub.0][F.sub.1]-ATP synthases modulates their ATP hydrolysis activity. Here, we report the crystal structure of the ATP-bound [epsilon] subunit from a thermophilic Bacillus PS3 at 1.9-[Angstrom] resolution. The C-terminal two a-helices were folded into a hairpin, sitting on the [beta] sandwich structure, as reported for Escherichia coli. A previously undescribed ATP binding motif, I(L)DXXRA, recognizes ATP together with three arginine and one glutamate residues. The E. coli [epsilon] subunit binds ATP in a similar manner, as judged on NMR. We also determined solution structures of the C-terminal domain of the PS3 [epsilon] subunit and relaxation parameters of the whole molecule by NMR. The two helices fold into a hairpin in the presence of ATP but extend in the absence of ATP. The latter structure has more helical regions and is much more flexible than the former. These results suggest that the [epsilon] C-terminal domain can undergo an arm-like motion in response to an ATP concentration change and thereby contribute to regulation of [F.sub.0][F.sub.1]-ATP synthase. ATP hydrolysis | ATP-binding motif | ATPase regulation | ATP synthase | [F.sub.1] rotation

The sterol regulatory element-binding protein Z (SREBP-2), a nuclear transcription factor that is essential for cholesterol metabolism, enters the nucleus through a direct interaction of its helix-loop-helix leucine zipper domain with importin-β. We show the crystal structure of importin-β complexed with the active form of SREBP-2. Importin-β uses characteristic tong helices like a pair of chopsticks to interact with an SREBP-2 dimer. Importin-β changes its conformation to reveal a pseudo-twofold symmetry on its surface structure so that it can accommodate a symmetric dimer molecule. Importin-β may use a similar strategy to recognize other dimeric cargoes., One of the most important steps in the control of cellular gene expression in eukaryotic cells is the nucleocytoplasmic transport of functional molecules through the nuclear envelope. In response to [...]

The crystal structure of bovine heart cytochrome c oxidase at 2.8 A resolution with an R value of 19.9 percent reveals 13 subunits, each different from the other, five phosphatidyl ethanolamines, three phosphatidyl glycerols and two cholates, two hemes A, and three copper, one magnesium, and one zinc. Of 3606 amino acid residues in the dimer, 3560 have been converged to a reasonable structure by refinement. A hydrogen-bonded system, including a propionate of a heme A (heme a), part of peptide backbone, and an imidazole ligand of [CU.sub.A], could provide an electron transfer pathway between [CU.sub.A] and heme a. Two possible proton pathways for pumping, each spanning from the matrix to the cytosolic surfaces, were identified, including hydrogen bonds, internal cavities likely to contain water molecules, and structures that could form hydrogen bonds with small possible conformational change of amino acid side chains. Possible channels for chemical protons to produce [H.sub.2]O, for removing the produced water, and for [O.sub.2] respectively, were identified., Cytochrome c oxidase is the terminal oxidase of cell respiration, a process that reduces molecular oxygen to water with the electrons from cytochrome c, coupled to pumping protons from the [...]

The high resolution three-dimensional x-ray structure of the metal sites of bovine heart cytochrome c oxidase is reported. Cytochrome c oxidase is the largest membrane protein yet crystallized and analyzed at atomic resolution. Electron density distribution of the oxidized bovine cytochrome c oxidase at 2.8 A resolution indicates a dinuclear copper center with an unexpected structure similar to a [2Fe-2S]-type iron-sulfur center. Previously predicted zinc and magnesium sites have been located, the former bound by a nuclear encoded subunit on the matrix side of the membrane, and the latter situated between heme [a.sub.3] and [CU.sub.A], at the interface of subunits 1 and 11. The [O.sub.2] binding site contains heme [a.sub.3] iron and copper atoms (CUB) with an interatomic distance of 4.5 A; there is no detectable bridging ligand between iron and copper atoms in spite of a strong antiferromagnetic coupling between them. A hydrogen bond is present between a hydroxyl group of the hydroxyfarnesylethyl side chain of heme [a.sub.3] and an OH of a tyrosine. The tyrosine phenol plane is immediately adjacent and perpendicular to an imidazole group bonded to [CU.sub.B], suggesting a possible role in intramolecular electron transfer or conformational control, the latter of which could induce the redox-coupled proton pumping. A phenyl group located halfway between a pyrrole plane of the heme [a.sub.3] and an imidazole plane liganded to the other heme (heme a) could also influence electron transfer or conformational control., Bovine heart cytochrome c oxidase is a large multicomponent membrane protein complex with molecular size of 200 kilodaltons comprising 13 different polypeptide subunits. Located in the subunits are two heme [...]