Haixin Sui

Structural basis of water-specific transport through the AQP1 water channel.

Water channels facilitate the rapid transport of water across cell membranes in response to osmotic gradients. These channels are believed to be involved in many physiological processes that include renal water conservation, neuro-homeostasis, digestion, regulation of body temperature and reproduction. Members of the water channel superfamily have been found in a range of cell types from bacteria to human. In mammals, there are currently 10 families of water channels, referred to as aquaporins (AQP): AQP0–AQP9. Here we report the structure of the aquaporin 1 (AQP1) water channel to 2.2 Å resolution. The channel consists of three topological elements, an extracellular and a cytoplasmic vestibule connected by an extended narrow pore or selectivity filter. Within the selectivity filter, four bound waters are localized along three hydrophilic nodes, which punctuate an otherwise extremely hydrophobic pore segment. This unusual combination of a long hydrophobic pore and a minimal number of solute binding sites facilitates rapid water transport. Residues of the constriction region, in particular histidine 182, which is conserved among all known water-specific channels, are critical in establishing water specificity. Our analysis of the AQP1 pore also indicates that the transport of protons through this channel is highly energetically unfavourable.

Molecular architecture of axonemal microtubule doublets revealed by cryo-electron tomography

The axoneme, which forms the core of eukaryotic flagella and cilia, is one of the largest macromolecular machines, with a structure that is largely conserved from protists to mammals. Microtubule doublets are structural components of axonemes that contain a number of proteins besides tubulin, and are usually found in arrays of nine doublets arranged around two singlet microtubules. Coordinated sliding of adjacent doublets, which involves a host of other proteins in the axoneme, produces periodic beating movements of the axoneme. We have obtained a three-dimensional density map of intact microtubule doublets using cryo-electron tomography and image averaging. Our map, with a resolution of about 3 nm, provides insights into locations of particular proteins within the doublets and the structural features of the doublets that define their mechanical properties. We identify likely candidates for several of these non-tubulin components of the doublets. This work offers insight on how tubulin protofilaments and accessory proteins attach together to form the doublets and provides a structural basis for understanding doublet function in axonemes.

Direct kinetochore–spindle pole connections are not required for chromosome segregation

In the absence of continuous K-fiber attachment between each kinetochore and the spindle pole, one or more additional mechanisms dependent on dynein-mediated kinetochore transport exist to ensure proper chromosome segregation during mitosis.

The enhancement of solid solubility limits of AlCo intermetallic compound by high-energy ball milling

In this work the AlCo intermetallic compound has been prepared by high‐energy ball milling of Co and Al powder mixtures with the composition of Co‐50 at.% Al, Co‐71.4 at.% Al, Co‐76.5 at.% Al, and Co‐81.8 at.% Al, respectively. X‐ray diffraction and transmission electron microscopy analysis of powders milled for different times proved that the AlCo phase formed directly during milling of the powder mixture of the former three compositions. In the Co‐81.8 at.% Al powder mixture the amorphous phase was first formed and then crystallized to the AlCo phase. The result of energy dispersion x ray showed that the composition of AlCo phase formed during milling of the above four compositions is in agreement with that of the powder mixture, although their crystal structure is that of the AlCo intermetallic compound. The results indicate that the solubility range of intermetallic compound during mechanical alloying processes can be greatly enhanced by high‐energy ball milling. The enhancement of solubility is discu...

Unattached kinetochores rather than intrakinetochore tension arrest mitosis in taxol-treated cells

Taxol induces extensive structural reorganization of the mammalian kinetochore; however, this reorganization is not sufficient to maintain a long-term mitotic arrest unless some of the kinetochores completely lose their attachment to microtubules.

On the stable Mg-Zn-Y quasicrystals

By the conventional air-cooled casting method, bulk ingots with a large fraction of stable Mg-Zn-Y icosahedral quasicrystals, both simple and face-centered, were obtained. The quasicrystal structures were directly confirmed by high-resolution electron microscopy (HREM) observations. The electron diffraction patterns from the quasicrystals were studied by computer simulations. A coexisting crys-talline phase of the quasicrystals was identified as the Mg7Zn3 phase, which was proved to be the (1/1) approximant of the quasicrystals. The quasicrystals were stable during a continuous heating process. However, at high temperature, oxidation occurred, and thus, Y2O3 and MgO products were formed. Oxidation at high temperatures is characteristic for the Mg-Zn-Y alloys and differs from Al-base alloys.

A NON-FIBONACCI TYPE OF ORTHORHOMBIC DECAGONAL APPROXIMANT

Abstract An orthorhombic phase with a = 1.23 nm, b = 1.24 nm and c = 3.07nm has been found in the Al12Fe2Cr alloy. The distribution of strong diffraction spots in the [010], [100] and [001] electron diffraction patterns (EDPs) corresponds to the tenfold, the 2D twofold, and the 2P twofold EDPs respectively of a decagonal quasicrystal. Therefore this orthorhombic phase is called a decagonal approximant. However, unlike other orthorhombic approximants, its parameter c = 3.07 nm does not agree with any of the values obtained by substituting a rational ratio of two consecutive Fibonacci numbers, such as 1/1, 2/1, 3/2, 5/3, etc., for the irrational τ in one of the quasiperiodic directions in a decagonal quasicrystal. Its [010] high-resolution electron microscopy image consists of a network of 72° and 36° rhombi whose vertices are surrounded by a decagon of image points. The ratio of the thick to thin rhombi is 4 to 1 rather than a Fibonacci ratio. This is a new type of decagonal approximant. On ordering, a C-c...

Crystal structure of human calmodulin-like protein: insights into its functional role

A calmodulin (CaM)‐like protein (hCLP) is expressed in human mammary epithelial cells but appears to be limited to certain epithelial cells such as those found in skin, prostate, breast and cervical tissues. A decrease in the expression of this protein is associated with the occurrence of tumors in breast epithelium. The structure of hCLP determined to 1.5 Å resolution by X‐ray crystallography shows a distinct 30° displacement along the interconnecting central helix, when compared to the highly conserved structure of vertebrate CaM, resulting in a difference in the relative orientation of its two globular domains. Additionally, the electric surface potential landscape at the target protein binding regions on the two globular domains of hCLP is significantly different from those of CaM, indicating that the respective ranges of hCLP and hCaM target proteins do not fully overlap. Observations that hCLP can competitively inhibit CaM activation of target proteins also imply a role for hCLP in which it may also serve as a modulator of CaM activity in the epithelial cells where hCLP is expressed.

Crystallization and preliminary X-ray crystallographic analysis of water channel AQP1

Aquaporin-1 (AQP1), a water channel from bovine red blood cells has been deglycosylated, purified to homogeneity and crystallized in a form suitable for X-ray crystallographic study. Crystals are grown using polyethylene glycol as precipitant and belong to the tetragonal space group I422, with unit-cell parameters a = b = 93.4, c = 180.4 A. The crystals diffract beyond 2.2 A resolution.

Structural Model of the Orthorhombic Non-Fibonacci Approximant in the Al12Fe2Cr Alloy

Al5.103(Fe, Cr), orthorhombic, Imm2, a = 12.34, b = 12.41, c = 30.71 A, V = 4701 A3, atoms/cell = 305 (average), Dx \simeq 3.4 g cm−3. A structural model of the non-Fibonacci approximant O-AIFeCr with composition Al5.103(Fe, Cr) found in an Al12Fe2Cr alloy has been derived from its high-resolution electron microscopy images and the structure of the hexagonal \mu-Al4Mn [Shoemaker, Keszler & Shoemaker (1989). Acta Cryst. B45, 13–20], being isostructural to \mu-Al4Cr. Among the 14 unique positions of transition metal (TM) atoms, 13 are icosahedrally coordinated: five of these icosahedra interpenetrate into each other, forming icosahedral chains along [010], and the remaining eight icosahedra with their twofold axis parallel to this axis. This structural model describes a (010) layer structure with a sequence P^{m}FP(P^{m}FP)_I, where a flat layer F (a mirror plane) is sandwiched between two puckered P and Pm layers, and (P^{m}FP)I is related to P^{m}FP through a body-centred translation. The interatomic distances are 2.51–2.74 A for TM—TM, 2.38–2.99 for TM—Al and 2.41–3.18 A, for Al—Al. Simulated images and electron diffraction patterns calculated based on this structural model are comparable with the experimentally observed results.