Diabetic hyperglycemia: link to impaired glucose transport in pancreatic beta cells

Glucose uptake into pancreatic beta cells by means of the glucose transporter GLUT-2, which has a high Michaelis constant, is essential for the normal insulin secretory response to hyperglycemia. In both autoimmune and nonautoimmune diabetes, this glucose transport is reduced as a consequence of down-regulation of the normal beta-cell transporter. In autoimmune diabetes, circulating immunoglobulins can further impair this glucose transport by inhibiting functionally intact transporters. Insights into mechanisms of the unresponsiveness of beta cells to hyperglycemia may improve the management and prevention of diabetes.     

Localization of iron in Arabidopsis seed requires the vacuolar membrane transporter VIT1

Iron deficiency is a major human nutritional problem wherever plant-based diets are common. Using synchrotron x-ray fluorescence microtomography to directly visualize iron in Arabidopsis seeds, we show that iron is localized primarily to the provascular strands of the embryo. This localization is completely abolished when the vacuolar iron uptake transporter VIT1 is disrupted. Vacuolar iron storage is also critical for seedling development because vit1-1 seedlings grow poorly when iron is limiting. We have uncovered a fundamental aspect of seed biology that will ultimately aid the development of nutrient-rich seed, benefiting both human health and agricultural productivity.     

Underexpression of beta cell high Km glucose transporters in noninsulin-dependent diabetes

The role of defective glucose transport in the pathogenesis of noninsulin-dependent diabetes (NIDDM) was examined in Zucker diabetic fatty rats, a model of NIDDM. As in human NIDDM, insulin secretion was unresponsive to 20 mM glucose. Uptake of 3-O-methylglucose by islet cells was less than 19% of controls. The beta cell glucose transporter (GLUT-2) immunoreactivity and amount of GLUT-2 messenger RNA were profoundly reduced. Whenever fewer than 60% of beta cells were GLUT-2-positive, the response to glucose was absent and hyperglycemia exceeded 11 mM plasma glucose. We conclude that in NIDDM underexpression of GLUT-2 messenger RNA lowers high Km glucose transport in beta cells, and thereby impairs glucose-stimulated insulin secretion and prevents correction of hyperglycemia.     

Voltage- and tension-dependent lipid mobility in the outer hair cell plasma membrane

The mechanism responsible for electromotility of outer hair cells in the ear is unknown but is thought to reside within the plasma membrane. Lipid lateral diffusion in the outer hair cell plasma membrane is a sigmoidal function of transmembrane potential and bathing media osmolality. Cell depolarization or hyposmotic challenge shorten the cell and reduce membrane fluidity by half. Changing the membrane tension with amphipathic drugs results in similar reductions. These dynamic changes in membrane fluidity represent the modulation of membrane tension by lipid-protein interactions. The voltage dependence may be associated with the force-generating motors that contribute to the exquisite sensitivity of mammalian hearing.     

Sequence and structure of a human glucose transporter

The amino acid sequence of the glucose transport protein from human HepG2 hepatoma cells was deduced from analysis of a complementary DNA clone. Structural analysis of the purified human erythrocyte glucose transporter by fast atom bombardment mapping and gas phase Edman degradation confirmed the identity of the clone and demonstrated that the HepG2 and erythrocyte transporters are highly homologous and may be identical. The protein lacks a cleavable amino-terminal signal sequence. Analysis of the primary structure suggests the presence of 12 membrane-spanning domains. Several of these may form amphipathic alpha helices and contain abundant hydroxyl and amide side chains that could participate in glucose binding or line a transmembrane pore through which the sugar moves. The amino terminus, carboxyl terminus, and a highly hydrophilic domain in the center of the protein are all predicted to lie on the cytoplasmic face. Messenger RNA species homologous to HepG2 glucose transporter messenger RNA were detected in K562 leukemic cells, HT29 colon adenocarcinoma cells, and human kidney tissue.     

Structural basis of gating by the outer membrane transporter FecA

Siderophore-mediated acquisition systems facilitate iron uptake. We present the crystallographic structure of the integral outer membrane receptor FecA from Escherichia coli with and without ferric citrate at 2.5 and 2.0 angstrom resolution. FecA is composed of three distinct domains: the barrel, plug, and NH2-terminal extension. Binding of ferric citrate triggers a conformational change of the extracellular loops that close the external pocket of FecA. Ligand-induced allosteric transitions are propagated through the outer membrane by the plug domain, signaling the occupancy of the receptor in the periplasm. These data establish the structural basis of gating for receptors dependent on the cytoplasmic membrane protein TonB. By compiling available data for this family of receptors, we propose a mechanism for the energy-dependent transport of siderophores.     

高粱根质膜的纯化和膜蛋白的提取

利用高粱根作为实验材料,用不连续蔗糖密度梯度和Dextran T-500/PEG-3350两相法探讨了纯化质膜的过程,以及去垢剂对K+运输蛋白的增溶效率.结果表明,由6 mL 40%(W/W),4 mL 34%(W/W)和4 mL 22%(W/W)蔗糖组成的不连续蔗糖梯度,经80 000×g离心3 h后,在34%～40%界面处富含质膜.在质膜K+运输蛋白的增溶效果上,6.5 mmol/L CHAPS最为理想.     

Protein conformations in the plasma membrane

Infrared spectroscopy and optical rotatory dispersion have been used to test theories of structure of membrane protein. No evidence has been found to support the view that adjacent to the lipid there is a monolayer of protein in the beta-conformation. The extracted protein appears to be a fairly typical globular protein with a low a-helical content.     

Integrins regulate Rac targeting by internalization of membrane domains

Translocation of the small GTP-binding protein Rac1 to the cell plasma membrane is essential for activating downstream effectors and requires integrin-mediated adhesion of cells to extracellular matrix. We report that active Rac1 binds preferentially to low-density, cholesterol-rich membranes, and specificity is determined at least in part by membrane lipids. Cell detachment triggered internalization of plasma membrane cholesterol and lipid raft markers. Preventing internalization maintained Rac1 membrane targeting and effector activation in nonadherent cells. Regulation of lipid rafts by integrin signals may regulate the location of membrane domains such as lipid rafts and thereby control domain-specific signaling events in anchorage-dependent cells.     

Selective depletion in HIV infection of T cells that bear specific T cell receptor V beta sequences

The mechanism of T cell depletion during infection with the human immunodeficiency virus (HIV) is unclear. Examination of the repertoire of T cell receptor V (variable) regions in persons infected with HIV revealed the absence of a common set of V beta regions, whereas V alpha usage was normal. The lack of these V beta segments did not appear to correlate with opportunistic infections. The selective elimination of T cells that express a defined set of V beta sequences may indicate the presence of an HIV-encoded superantigen, similar to those encoded by the long terminal repeat of the mouse mammary tumor virus.     

水牛静脉注射葡萄糖后血糖水平的调节机制研究

12头健康禁食公水牛分为3组。组Ⅰ(n=6)与组Ⅱ(n=3)分别经颈静脉快速注射50%葡萄糖和0.9%生理盐水各2 mL.kg-1;组Ⅲ(n=3)为对照,不作处理。分别于注射前60、10 m in及注射后1、5、10、30、60、120、180、240、300、360和420 m in采血,立即测定血浆葡萄糖及胰岛素和胰高血糖素,分析各指标间的相关性及血浆胰岛素/血糖值(IGR1)和胰岛素/胰高血糖素值(IGR2)。结果显示:注射前,各组水牛血浆葡萄糖、胰岛素和胰高血糖素的浓度接近。组Ⅱ和组Ⅲ水牛的上述指标在所有采样时间点上组内和组间均未表现出显著性差异(P>0.05),IGR1和IGR2也没有明显变化。组Ⅰ水牛静注葡萄糖1 m in后血糖浓度迅速升至峰值,而血浆胰岛素30 m in后才升高(约20倍)至峰值,然后两者均缓慢下降,直到420 m in仍明显高于注射前的水平(P<0.05)。而胰高血糖素仅在注射葡萄糖后1和5 m in时显示出差异(P<0.05),IGR1在注射葡萄糖(组Ⅰ)后立即下降,至注射后180 m in,组Ⅰ的IGR1和IGR2与组Ⅱ和组Ⅲ的基本持平。相关性分析表明,血糖浓度与血浆胰岛素水平呈极显著正相关(r=0.73,P<0.01),与胰高血糖素浓度呈显著负相关(r=-0.58,P<0.05),但胰岛素水平与胰高血糖素浓度相关性不显著(r=0.06,P>0.05)。提示:水牛胰岛素代谢迟缓,对静注的葡萄糖清除缓慢,表现出持续性高血糖症和高胰岛素血症;水牛对于静注葡萄糖的反应不同于奶牛、犬、马等其他动物。     

BAR domains as sensors of membrane curvature: the amphiphysin BAR structure

The BAR (Bin/amphiphysin/Rvs) domain is the most conserved feature in amphiphysins from yeast to human and is also found in endophilins and nadrins. We solved the structure of the Drosophila amphiphysin BAR domain. It is a crescent-shaped dimer that binds preferentially to highly curved negatively charged membranes. With its N-terminal amphipathic helix and BAR domain (N-BAR), amphiphysin can drive membrane curvature in vitro and in vivo. The structure is similar to that of arfaptin2, which we find also binds and tubulates membranes. From this, we predict that BAR domains are in many protein families, including sorting nexins, centaurins, and oligophrenins. The universal and minimal BAR domain is a dimerization, membrane-binding, and curvature-sensing module.     

Hurler's syndrome: deficiency of a specific beta galactosidase isoenzyme

A marked deficiency of a specific thermolabile beta-galactosidase isoenzyme (pH optimum 3 to 5) was found in liver and kidney tissues of five patients with the Hurler's syndrome (types 1 to 3).     

Structure of the membrane protein FhaC: a member of the Omp85-TpsB transporter superfamily

In Gram-negative bacteria and eukaryotic organelles, beta-barrel proteins of the outer membrane protein 85-two-partner secretion B (Omp85-TpsB) superfamily are essential components of protein transport machineries. The TpsB transporter FhaC mediates the secretion of Bordetella pertussis filamentous hemagglutinin (FHA). We report the 3.15 A crystal structure of FhaC. The transporter comprises a 16-stranded beta barrel that is occluded by an N-terminal alpha helix and an extracellular loop and a periplasmic module composed of two aligned polypeptide-transport-associated (POTRA) domains. Functional data reveal that FHA binds to the POTRA 1 domain via its N-terminal domain and likely translocates the adhesin-repeated motifs in an extended hairpin conformation, with folding occurring at the cell surface. General features of the mechanism obtained here are likely to apply throughout the superfamily.     

A mechanosensitive ion channel in the yeast plasma membrane

Mechanosensitive ion channels use mechanical energy to gate the dissipation of electrochemical gradients across cell membranes. This function is fundamental to physiological processes such as hearing and touch. In electrophysiological studies of ion channels in the plasma membrane of the yeast Saccharomyces cerevisiae, channels were observed that were activated by, and adapted to, stretching of the membrane. Adaptation of channel activity to mechanical stimuli was voltage-dependent. Because these mechanosensitive channels pass both cations and anions, they may play a role in turgor regulation in this walled organism.     

Hormone-calcium interactions with the plasma membrane of rat liver cells

The binding constants and the number of binding sites for insulin, glucagon, epinephrine, cyclic adenosine monophosphate, and calcium ions for the plasma membrane of rat liver were determined by Scatchard plots. The plots are biphasic or multiphasic, an indication of at least two types of binding sites for each ligand. At least three types of binding sites were found for insulin. In the concentration range of 10(-6) to 10(-8) molar, glucagon, epinephrine, and hydro-cortisone increased calcium ion binding to the plasma membrane, whereas insulin decreased this binding. At hormone concentrations of 10(-6) to 10(-7) molar, glucagon was the most effective in increasing calcium binding, but at a hormone concentration of 10(-8) molar, hydrocortisone was the most effective in stimulating calcium binding. Adenosine triphosphate reversed the effect of insulin and inhibited the effect of the other hormones. These studies suggest a relation between hormones and calcium with respect to membrane structure and function.