Keeping G proteins at bay: a complex between G protein-coupled receptor kinase 2 and Gbetagamma

The phosphorylation of heptahelical receptors by heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor kinases (GRKs) is a universal regulatory mechanism that leads to desensitization of G protein signaling and to the activation of alternative signaling pathways. We determined the crystallographic structure of bovine GRK2 in complex with G protein beta1gamma2 subunits. Our results show how the three domains of GRK2-the RGS (regulator of G protein signaling) homology, protein kinase, and pleckstrin homology domains-integrate their respective activities and recruit the enzyme to the cell membrane in an orientation that not only facilitates receptor phosphorylation, but also allows for the simultaneous inhibition of signaling by Galpha and Gbetagamma subunits.     

棉铃虫不同发育阶段围食膜蛋白的变化分析

以5龄棉铃虫幼虫围食膜蛋白为抗原制备抗体,采用Western blot方法检测了围食膜蛋白在不同发育阶段的存在及变化.结果表明,棉铃虫围食膜蛋白在胚胎期从产卵后1h已经开始表达;在幼虫蜕皮过程中亦有表达,但是蛋白表达的种类和表达量具有明显差异;在蛹及成虫中只少量表达.利用RT-PCR技术,检测了肠粘蛋白HaIIM86在棉铃虫不同发育阶段和5龄幼虫不同组织中的表达情况.结果显示,HaIIM86的分泌表达贯穿于棉铃虫的胚胎期、初孵幼虫期、蛹及成虫期的整个生活周期,并且HaIIM86的表达具有中肠组织特异性.     

G proteins Go green: a plant G protein signaling FAQ sheet

Plants, like animals, use signal transduction pathways based on heterotrimeric guanine nucleotide-binding proteins (G proteins) to regulate many aspects of development and cell signaling. Some components of G protein signaling are highly conserved between plants and animals and some are not. This Viewpoint compares key aspects of G protein signal transduction in plants and animals and describes the current knowledge of this system in plants, the questions that still await exploration, and the value of research on plant G proteins to scientists who do not study plants. Pathways in Science's Signal Transduction Knowledge Environment Connections Maps database provide details about the emerging roles of G proteins in several cellular processes of plants.     

Outer membrane active transport: structure of the BtuB:TonB complex

In Gram-negative bacteria, the import of essential micronutrients across the outer membrane requires a transporter, an electrochemical gradient of protons across the inner membrane, and an inner membrane protein complex (ExbB, ExbD, TonB) that couples the proton-motive force to the outer membrane transporter. The inner membrane protein TonB binds directly to a conserved region, called the Ton-box, of the transporter. We solved the structure of the cobalamin transporter BtuB in complex with the C-terminal domain of TonB. In contrast to its conformations in the absence of TonB, the Ton-box forms a beta strand that is recruited to the existing beta sheet of TonB, which is consistent with a mechanical pulling model of transport.     

Hydrophobic organization of membrane proteins

Membrane-exposed residues are more hydrophobic than buried interior residues in the transmembrane regions of the photosynthetic reaction center from Rhodobacter sphaeroides. This hydrophobic organization is opposite to that of water-soluble proteins. The relative polarities of interior and surface residues of membrane and water soluble proteins are not simply reversed, however. The hydrophobicities of interior residues of both membrane and water-soluble proteins are comparable, whereas the bilayer-exposed residues of membrane proteins are more hydrophobic than the interior residues, and the aqueous-exposed residues of water-soluble proteins are more hydrophilic than the interior residues. A method of sequence analysis is described, based on the periodicity of residue replacement in homologous sequences, that extends conclusions derived from the known atomic structure of the reaction center to the more extensive database of putative transmembrane helical sequences.     

Cytoprotective role of Ca2+- activated K+ channels in the cardiac inner mitochondrial membrane

Ion channels on the mitochondrial inner membrane influence cell function in specific ways that can be detrimental or beneficial to cell survival. At least one type of potassium (K+) channel, the mitochondrial adenosine triphosphate-sensitive K+ channel (mitoKATP), is an important effector of protection against necrotic and apoptotic cell injury after ischemia. Here another channel with properties similar to the surface membrane calcium-activated K+ channel was found on the mitochondrial inner membrane (mitoKCa) of guinea pig ventricular cells. MitoKCa significantly contributed to mitochondrial K+ uptake of the myocyte, and an opener of mitoKCa protected hearts against infarction.     

Diversity of G proteins in signal transduction

The heterotrimeric guanine nucleotide-binding proteins (G proteins) act as switches that regulate information processing circuits connecting cell surface receptors to a variety of effectors. The G proteins are present in all eukaryotic cells, and they control metabolic, humoral, neural, and developmental functions. More than a hundred different kinds of receptors and many different effectors have been described. The G proteins that coordinate receptor-effector activity are derived from a large gene family. At present, the family is known to contain at least sixteen different genes that encode the alpha subunit of the heterotrimer, four that encode beta subunits, and multiple genes encoding gamma subunits. Specific transient interactions between these components generate the pathways that modulate cellular responses to complex chemical signals.     

Phosphatidylinositol-glycan anchors of membrane proteins: potential precursors of insulin mediators

BC3H1 myocytes release membrane-bound alkaline phosphatase to the incubation medium upon stimulation with insulin, following a time course that is consistent with the generation of dimyristoylglycerol and the appearance of a putative insulin mediator in the extracellular medium. The use of specific blocking agents shows, however, that alkaline phosphatase release and dimyristoylglycerol production are independent processes and that the blockade of either event inhibits the production of insulin mediator. These experiments suggest a new model of insulin action.     

Myxovirus envelope proteins: a directing influence on the fatty acids of membrane lipids

Acyl chain compositions of the lipids of three strains of influenza virus show differences not anticipated from current theories of myxovirus assembly. Fatty acids of viruses with antigenically related envelope proteins show greater resemblance than those of an unrelated strain, which suggests that these proteins influence the composition of membrane lipids at the site of viral release.     

G protein signaling from activated rat frizzled-1 to the beta-catenin-Lef-Tcf pathway

The frizzled receptors, which mediate development and display seven hydrophobic, membrane-spanning segments, are cell membrane-localized. We constructed a chimeric receptor with the ligand-binding and transmembrane segments from the beta2-adrenergic receptor (beta2AR) and the cytoplasmic domains from rat Frizzled-1 (Rfz1). Stimulation of mouse F9 clones expressing the chimera (beta2AR-Rfz1) with the beta-adrenergic agonist isoproterenol stimulated stabilization of beta-catenin, activation of a beta-catenin-sensitive promoter, and formation of primitive endoderm. The response was blocked by inactivation of pertussis toxin-sensitive, heterotrimeric guanine nucleotide-binding proteins (G proteins) and by depletion of Galphaq and Galphao. Thus, G proteins are elements of Wnt/Frizzled-1 signaling to the beta-catenin-lymphoid-enhancer factor (LEF)-T cell factor (Tcf) pathway.     

嗜水气单胞菌322A的外膜蛋白及其抗原性

用十二烷基肌氨酸钠抽提结合超速离心的方法提取一株日本鳗鲡致病性嗜水气单胞菌322A和其他4株气单胞菌标准菌株的外膜蛋白(Omp);通过SDS-PAGE分析比较这5株气单胞菌Omp的组成.结果表明,5株气单胞菌的Omp电泳可得到5-13条条带,其分子质量主要集中在20-50 ku之间,且28 ku的Omp为5株气单胞菌所共有.用罗非鱼感染322A全菌后的恢复期血清进行蛋白质印迹试验的结果显示,322A的Omp条带中有4条发生阳性反应,其分子质量分别为28、37、43和45 ku.用兔抗322A Omp的血清进行蛋白质印迹试验的结果显示,322A的Omp条带中有5条发生阳性反应,其分子质量分别为28、33、37、43和45 ku.不同抗血清的印迹结果提示了322A数条主要Omp具有免疫原性,有望成为疫苗研发的候选材料.     

Structure of Arp2/3 complex in its activated state and in actin filament branch junctions

The seven-subunit Arp2/3 complex choreographs the formation of branched actin networks at the leading edge of migrating cells. When activated by Wiskott-Aldrich Syndrome protein (WASp), the Arp2/3 complex initiates actin filament branches from the sides of existing filaments. Electron cryomicroscopy and three-dimensional reconstruction of Acanthamoeba castellanii and Saccharomyces cerevisiae Arp2/3 complexes bound to the WASp carboxy-terminal domain reveal asymmetric, oblate ellipsoids. Image analysis of actin branches indicates that the complex binds the side of the mother filament, and Arp2 and Arp3 (for actin-related protein) are the first two subunits of the daughter filament. Comparison to the actin-free, WASp-activated complexes suggests that branch initiation involves large-scale structural rearrangements within Arp2/3.     

cdc2 gene expression at the G1 to S transition in human T lymphocytes

The product of the cdc2 gene, designated p34cdc2, is a serine-threonine protein kinase that controls entry of eukaryotic cells into mitosis. Freshly isolated human T lymphocytes (G0 phase) were found to have very low amounts of p34cdc2 and cdc2 messenger RNA. Expression of cdc2 increased 18 to 24 hours after exposure of T cells to phytohemagglutinin, coincident with the G1 to S transition. Antisense oligodeoxynucleotides could reduce the increase in cdc2 expression and inhibited DNA synthesis, but had no effect on several early and mid-G1 events, including blastogenesis and expression of interleukin-2 receptors, transferrin receptors, c-myb, and c-myc. Induction of cdc2 required prior induction of c-myb and c-myc. These results suggest that cdc2 induction is part of an orderly sequence of events that occurs at the G1 to S transition in T cells.     

Cell cycle-dependent coupling of the calcitonin receptor to different G proteins.

Calcitonin is a calcium regulating peptide hormone with binding sites in kidney and bone as well as in the central nervous system. The mechanisms of signal transduction by calcitonin receptors were studied in a pig kidney cell line where the hormone was found to regulate sodium pumps. Calcitonin receptors activated the cyclic adenosine monophosphate (cAMP) or the protein kinase C (PKC) pathways. The two transduction pathways required guanosine triphosphate (GTP)-binding proteins (G proteins) (the choleratoxin sensitive Gs and the pertussis toxin sensitive Gi, respectively) and led to opposite biological responses. Moreover, selective activation of one or the other pathway was cell cycle-dependent. Therefore, calcitonin may induce different biological responses in target cells depending on their positions in the cell cycle. Such a modulation of ligand-induced responses could be of importance in rapidly growing cell populations such as during embryogenesis, growth, and tumor formation.     

Localization of the G protein betagamma complex in living cells during chemotaxis

Gradients of chemoattractants elicit signaling events at the leading edge of a cell even though chemoattractant receptors are uniformly distributed on the cell surface. In highly polarized Dictyostelium discoideum amoebas, membrane-associated betagamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) were localized in a shallow anterior-posterior gradient. A uniformly applied chemoattractant generated binding sites for pleckstrin homology (PH) domains on the inner surface of the membrane in a pattern similar to that of the Gbetagamma subunits. Loss of cell polarity resulted in uniform distribution of both the Gbetagamma subunits and the sensitivity of PH domain recruitment. These observations indicate that Gbetagamma subunits are not sufficiently localized to restrict signaling events to the leading edge but that their distribution may determine the relative chemotactic sensitivity of polarized cells.     

创伤弧菌外膜蛋白的分离及其抗原性分析

分别采用Sarkosyl和PMSF法分离、提取鳗源创伤弧菌FJ03-X2的外膜蛋白,并应用SDS-PAGE和Western-blotting分析比较两种方法提取的外膜蛋白的组分和抗原性。SDS-PAGE分析结果表明,Sarkosyl法分离的主要外膜蛋白分子量集中在14～66 kDa之间;而PMSF法分离的主要外膜蛋白分子量分布在14～92 kDa之间。在两种方法中,分子量38 kDa、36 kDa的外膜蛋白均为高丰度蛋白。两种方法提取的创伤弧菌外膜蛋白经SDS-PAGE后,用兔抗创伤弧菌FJ03-X2血清进行Western-blotting,结果显示,兔抗FJ03-X2高免血清能识别绝大多数外膜蛋白组分,说明FJ03-X2菌株的外膜蛋白具有良好的抗原性。其中,分子量分别为66kDa、47 kDa、44 kDa、38 kDa、36 kDa、34 kDa的外膜蛋白呈强阳性反应,是主要的免疫原组分。同时,经比较分析认为PMSF法提取创伤弧菌外膜蛋白的效果更好。     

The trans Golgi network: sorting at the exit site of the Golgi complex

The Golgi complex is a series of membrane compartments through which proteins destined for the plasma membrane, secretory vesicles, and lysosomes move sequentially. A model is proposed whereby these three different classes of proteins are sorted into different vesicles in the last Golgi compartment, the trans Golgi network. This compartment corresponds to a tubular reticulum on the trans side of the Golgi stack, previously called Golgi endoplasmic reticulum lysosomes (GERL).     

Structure of the lambda complex at 2.5 A resolution: details of the repressor-operator interactions

The crystal structure of a complex containing the DNA-binding domain of lambda repressor and a lambda operator site was determined at 2.5 A resolution and refined to a crystallographic R factor of 24.2 percent. The complex is stabilized by an extensive network of hydrogen bonds between the protein and the sugar-phosphate backbone. Several side chains form hydrogen bonds with sites in the major groove, and hydrophobic contacts also contribute to the specificity of binding. The overall arrangement of the complex is quite similar to that predicted from earlier modeling studies, which fit the protein dimer against linear B-form DNA. However, the cocrystal structure reveals important side chain-side chain interactions that were not predicted from the modeling or from previous genetic and biochemical studies.