蓖麻毒素A链突变体(MRTA)的分子设计

利用同源模建的方法,借助分子力学优化、分子动力学模拟退火设计构建了删除部分氨基酸序列的蓖麻毒素Ａ链突变体（ＭＲＴＡ）。采用泊松—玻尔兹曼方程对比分析了蓖麻毒素Ａ链（ＲＴＡ）与ＭＲＴＡ表观静电势分布,研究ＲＴＡ与ＭＲＴＡ蛋白表面静电性质;通过半经验量子化学ＡＭ１与分子力学结合方法探讨ＲＴＡ与ＭＲＴＡ功能域氨基酸前线分子轨道性质、能级分布,从理论上预测ＭＲＴＡ功能活性     

Cibacron Blue亲和层析及应用

以Ｆ３ＧＡ（Ｃｉｂａｃｒｏｎ ＢｌｕｅＦ３ＧＡ）为配基建立了一种可用于免疫毒素（ＩＴ）分离纯化的亲和层析方法,实验中用三种不同来源的核糖体灭活蛋白（ＲＩＰ）,即蓖麻毒素Ａ链（ＲＴＡ）,苦撤毒素（Ｍｏｍｏｒｄｉｎ,ＭＴ）和Ｓａｐｏｒｉｎ,以探讨ＲＩＰ与Ｆ３ＧＡ的相互作用。分析三种ＲＩＰ均能引起Ｆ３ＧＡ吸收光谱明显红移,提示ＲＩＰ均可与Ｆ３ＧＡ发生特异结合,将Ｆ３ＧＡ与Ｓｅｐｈａｄｅｘ交联可获得Ｂｌｕ     

蓖麻毒素A链突变体的设计和结构模建

蓖麻毒素Ａ链（ＲＴＡ）有抑制蛋白质合成的功能,可用作“生物导弹”的弹头,但其免疫原性较强。我们根据国外所做的ＲＴＡ连续突变的实验结果,以及ＰＤＢ库中ＲＴＡ及其同源蛋白的结构信息,设计了一个ＲＴＡ突变体,缺失的５个片段与功能及结构保守性关系较小。然后,我们用同源模建的方法对设计出的ＲＴＡ突变体进行三维结构模建,初步验证表明模型基本合理     

蓖麻毒素A链突变体的设计和结构模建

蓖麻毒素Ａ链（ＲＴＡ）有抑制蛋白质合成的功能,可用作“生物导弹”的弹头,但其免疫原性较强。我们根据国外所做的ＲＴＡ连续突变的实验结果,以及ＰＤＢ库中ＲＴＡ及其同源蛋白的结构信息,设计了一个ＲＴＡ突变体,缺失的５个片段与功能及结构保守性关系较小。然后,我们用同源模建的方法对设计出的ＲＴＡ突变体进行三维结构模建,初步验证表明模型基本合理     

Cibacron Blue亲和层析及应用

以Ｆ３ＧＡ（Ｃｉｂａｃｒｏｎ　Ｂｌｕｅ　Ｆ３ＧＡ）为配基建立了一种可用于免疫毒素（ＩＴ）分离纯化的亲和层析方法。实验中用三种不同来源的核糖体灭活蛋白（ＲＩＰ）,即蓖麻毒素Ａ链（ＲＴＡ）,苦瓜毒素（ｍｏｍｏｒｄｉｎ,ＭＴ）和Ｓａｐｏｒｉｎ,以探讨ＲＩＰ与Ｆ３ＧＡ的相互作用。分析显示三种ＲＩＰ均能引起Ｆ３ＧＡ吸收光诸明显红移,提示ＲＩＰ均可与Ｆ３ＧＡ发生特异结合。将Ｆ３ＧＡ与Ｓｅｐｈａｄｅｘ交联可获得Ｂｌｕｅｄｅｘ。Ｂｌｕｅｄｅｘ亲和层析是一种经济有效,简单易行,便于在各类实验室中使用的蛋白质亲和层析技术。结果表明：在低盐溶液中ＲＴＡ和ＭＴ均可迅速地与Ｂｌｕｅｄｅｘ结合,而在高盐溶液中（０．６５ｍｏｌ／ＬＮａＣｌ）又极易被洗脱回收。这一技术用于免疫毒素的研究可有效地去除游离抗体,而不影响其杀伤活性。     

重组蓖麻毒素A链与几种天然单链核糖体失活蛋白的活性研究

采用ｐＥｘＳｅｃⅠ载体系统进行了蓖麻毒素Ａ链的原核表达,经ＣＭ－Ｓｅｐｈａｒｏｓｅ一步纯化后,获得了纯度约８０％的重组蓖麻毒素Ａ链．将其与几种天然单链核糖体失活蛋白进行了超螺旋ＤＮＡ裂解研究和无细胞体系中蛋白合成抑制试验,结果表明,重组蓖麻毒素Ａ链具有类似于天然单链核糖体失活蛋白的活性,但两种测活方法之间没有明显的相关性     

苦荞麦营养器官的解剖学研究

苦荞麦营养器官的解剖学研究慕勤国（长庆油田第二中学,甘肃省庆阳地区马岭７４５１１３）关键词苦荞麦,营养器官,解剖ＡＮＡＴＯＭＩＣＡＬＳＴＵＤＩＥＳＯＮＴＨＥＶＥＧＥＴＡＴＩＶＥＯＲＧＡＮＳＯＦＦＡＧＯＰＹＲＵＭＴＡＴＡＲＩＣＵＭ￥ＭｕＱｉｎｇｕｏ（Ｔ...     

蓖麻毒素A链突变体的设计,表达与活性研究

利用蛋白质结构同源模建并结合表观静电势分析,设计了拟具有生物学活性的蓖麻毒素Ａ链的突变体。将ＰＣＲ扩增的突变体基因,导入ｐＫＫ２２３－３载体中,于大肠杆菌（Ｅ．ｃｏｌｉ）中获得高效、可溶性表达,而且,确证了表达产物具有预期的生物学活性。     

首届周尧昆虫分类学奖励基金隆重颁奖

首届周尧昆虫分类学奖励基金隆重颁奖首届周尧昆虫分类学奖励基金隆重颁奖ＣＨＯＵＩＯＲＥＷＡＲＤＦＯＲＥＮＴＯＭＯＬＯＧＩＣＡＬＴＡＸＯＮＯＭＩＳＴＡＷＡＲＤＥＤＦＯＲＴＨＥＦＩＲＳＴＴＩＭＥＩＮＴＨＥ６ＴＨＮＡＴＩＯＮＡＬＥＮＴＯＭＯＬＯＧＹＭＥＥＴＩ...     

蓖麻毒素A链的基因克隆

作为导向药物蓖麻毒素Ａ链在大肠杆菌中表达时不含糖基侧链,在体内半衰期长,可提高共作为导向药物的疗效。我们根据Ｒｉｃｉｎ基因核苷酸序列,设计Ｒｉｃｉｎ－Ａ的上,下游引物,通过ＰＣＲ（多聚酶链式反应）方法,扩增出Ｒｉｃｉｎ－Ａ链基因。与ｐＵＣ１９载体连接,转化到ＪＭ１０３大肠杆菌中,得到重组克隆。对其进行几种酶切鉴定,证明酶切位点正确,又经序列分析,读出与文献发表的Ｒｉｃｉｎ－Ａ序列只有两个碱基不同,     

A single point mutation in ricin A-chain increases toxin degradation and inhibits EDEM1-dependent ER retrotranslocation

Ricin is a potent plant cytotoxin composed of an A-chain [RTA (ricin A-chain)] connected by a disulfide bond to a cell binding lectin B-chain [RTB (ricin B-chain)]. After endocytic uptake, the toxin is transported retrogradely to the ER (endoplasmic reticulum) from where enzymatically active RTA is translocated to the cytosol. This transport is promoted by the EDEM1 (ER degradation-enhancing α-mannosidase I-like protein 1), which is also responsible for directing aberrant proteins for ERAD (ER-associated protein degradation). RTA contains a 12-residue hydrophobic C-terminal region that becomes exposed after reduction of ricin in the ER. This region, especially Pro250, plays a crucial role in ricin cytotoxicity. In the present study, we introduced a point mutation [P250A (substitution of Pro250 with alanine)] in the hydrophobic region of RTA to study the intracellular transport of the modified toxin. The introduced mutation alters the secondary structure of RTA into a more helical structure. Mutation P250A increases endosomal-lysosomal degradation of the toxin, as well as reducing its transport from the ER to the cytosol. Transport of modified RTA to the cytosol, in contrast to wild-type RTA, appears to be EDEM1-independent. Importantly, the interaction between EDEM1 and RTA(P250A) is reduced. This is the first reported evidence that EDEM1 protein recognition might be determined by the structure of the ERAD substrate.     

The role of binding ligand in toxic hybrid proteins: a comparison of EGF-ricin, EGF-ricin A-chain, and ricin

To analyze the influence of ricin B-chain on the toxicity of hybrid-protein conjugates, the rate of cellular uptake of conjugates, and the rate at which ricin A-chain (RTA) is delivered to the cytoplasm, we have constructed toxic hybrid proteins consisting of epidermal growth factor (EGF) coupled in disulfide linkage either to ricin or to RTA. EGF-ricin is no more toxic on A431 cells than EGF-RTA. The two conjugates demonstrate similar kinetics of cellular uptake (defined as antibody irreversible toxicity). EGF-RTA and EGF-ricin, like ricin, required a 2-2 1/2 hour period at 37 degrees before the onset of protein synthesis inhibition occurred. Our results suggest that RTA determines the processes which carry it, either in conjugate or toxin, from the plasma membrane binding site to the cytoplasm following endocytosis, and the ricin B chain is not required for these processes.     

Fragment-based identification of determinants of conformational and spectroscopic change at the ricin active site

Background  

Ricin is a potent toxin and known bioterrorism threat with no available antidote. The ricin A-chain (RTA) acts enzymatically to cleave a specific adenine base from ribosomal RNA, thereby blocking translation. To understand better the relationship between ligand binding and RTA active site conformational change, we used a fragment-based approach to find a minimal set of bonding interactions able to induce rearrangements in critical side-chain positions.     

Introduction of a disulfide bond leads to stabilization and crystallization of a ricin immunogen

RTA1-33/44-198 is a catalytically inactive, single-domain derivative of the ricin toxin A-chain (RTA) engineered to serve as a stable protein scaffold for presentation of native immunogenic epitopes (Olson et al., Protein Eng Des Sel 2004;17:391-397). To improve the stability and solubility of RTA1-33/44-198 further, we have undertaken the design challenge of introducing a disulfide (SS) bond. Nine pairs of residues were selected for placement of the SS-bond based on molecular dynamics simulation studies of the modeled single-domain chain. Disulfide formation at either of two positions (R48C/T77C or V49C/E99C) involving a specific surface loop (44-55) increased the protein melting temperature by ~5°C compared with RTA1-33/44-198 and by ~13°C compared with RTA. Prolonged stability studies of the R48C/T77C variant (> 60 days at 37°C, pH 7.4) confirmed a > 40% reduction in self-aggregation compared with RTA1-33/44-198 lacking the SS-bond. The R48C/T77C variant retained affinity for anti-RTA antibodies capable of neutralizing ricin toxin, including a monoclonal that recognizes a human B-cell epitope. Introduction of either R48C/T77C or V49C/E99C promoted crystallization of RTA1-33/44-198, and the X-ray structures of the variants were solved to 2.3 A or 2.1 A resolution, respectively. The structures confirm formation of an intramolecular SS-bond, and reveal a single-domain fold that is significantly reduced in volume compared with RTA. Loop 44 to 55 is partly disordered as predicted by simulations, and is positioned to form self-self interactions between symmetry-related molecules. We discuss the importance of RTA loop 34 to 55 as a nucleus for unfolding and aggregation, and draw conclusions for ongoing structure-based minimalist design of RTA-based immunogens.     

In vitro selection of RNA molecules that inhibit the activity of ricin A-chain

The cytotoxin ricin disables translation by depurinating a conserved site in eukaryotic rRNA. In vitro selection has been used to generate RNA ligands (aptamers) specific for the catalytic ricin A-chain (RTA). The anti-RTA aptamers bear no resemblance to the normal RTA substrate, the sarcin-ricin loop (SRL), and were not depurinated by RTA. An initial 80-nucleotide RNA ligand was minimized to a 31-nucleotide aptamer that contained all sequences and structures necessary for interacting with RTA. This minimal RNA formed high affinity complexes with RTA (K(d) = 7.3 nM) which could compete directly with the SRL for binding to RTA. The aptamer inhibited RTA depurination of the SRL and could partially protect translation from RTA inhibition. The IC(50) of the aptamer for RTA in an in vitro translation assay is 100 nM, roughly 3 orders of magnitude lower than a small molecule inhibitor of ricin, pteroic acid, and 2 orders of magnitude lower than the best known RNA inhibitor. The novel anti-RTA aptamers may find application as diagnostic reagents for a potential biological warfare agent and hold promise as scaffolds for the development of strong ricin inhibitors.     

Inhibition of ricin A-chain (RTA) catalytic activity by a viral genome-linked protein (VPg)

Ricin is a plant derived protein toxin produced by the castor bean plant (Ricinus communis). The Centers for Disease Control (CDC) classifies ricin as a Category B biological agent. Currently, there is neither an effective vaccine that can be used to protect against ricin exposure nor a therapeutic to reverse the effects once exposed. Here we quantitatively characterize interactions between catalytic ricin A-chain (RTA) and a viral genome-linked protein (VPg) from turnip mosaic virus (TuMV). VPg and its N-terminal truncated variant, VPg_1–110, bind to RTA and abolish ricin's catalytic depurination of 28S rRNA in vitro and in a cell-free rabbit reticulocyte translational system. RTA and VPg bind in a 1 to 1 stoichiometric ratio, and their binding affinity increases ten-fold as temperature elevates (5 °C to 37 °C). RTA-VPg binary complex formation is enthalpically driven and favored by entropy, resulting in an overall favorable energy, ΔG = −136.8 kJ/mol. Molecular modeling supports our experimental observations and predicts a major contribution of electrostatic interactions, suggesting an allosteric mechanism of downregulation of RTA activity through conformational changes in RTA structure, and/or disruption of binding with the ribosomal stalk. Fluorescence anisotropy studies show that heat affects the rate constant and the activation energy for the RTA-VPg complex, Ea = −62.1 kJ/mol. The thermodynamic and kinetic findings presented here are an initial lead study with promising results and provides a rational approach for synthesis of therapeutic peptides that successfully eliminate toxicity of ricin, and other cytotoxic RIPs.     

The tRNA N2,N2-dimethylguanosine-26 methyltransferase encoded by gene trm1 increases efficiency of suppression of an ochre codon in Schizosaccharomyces pombe

The plant toxin ricin has proven valuable as a membrane marker in studies of endocytosis as well as studies of different intracellular transport steps. The toxin, which consists of two polypeptide chains, binds by one chain (the B-chain) to both glycolipids and glycoproteins with terminal galactose at the cell surface. The other chain (the A-chain) enters the cytosol and inhibits protein synthesis enzymatically. After binding the toxin is endocytosed by different mechanisms, and it is transported via endosomes to the Golgi apparatus and the endoplasmic reticulum before translocation of the A-chain to the cytosol. The different transport steps have been analyzed by studying trafficking of ricin as well as modified ricin molecules.     

Binding of ricin A-chain to negatively charged phospholipid vesicles leads to protein structural changes and destabilizes the lipid bilayer

Ricin is a heterodimeric protein toxin in which a catalytic polypeptide (the A-chain or RTA) is linked by a disulfide bond to a cell-binding polypeptide (the B-chain or RTB). During cell entry, ricin undergoes retrograde vesicular transport to reach the endoplasmic reticulum (ER) lumen, from where RTA translocates into the cytosol, probably by masquerading as a substrate for the ER-associated protein degradation (ERAD) pathway. In partitioning studies in Triton X-114 solution, RTA is predominantly found in the detergent phase, whereas ricin holotoxin, native RTB, and several single-chain ribosome-inactivating proteins (RIPs) are in the aqueous phase. Fluorescence spectroscopy and far-UV circular dichroism (CD) demonstrated significant structural changes in RTA as a result of its interaction with liposomes containing negatively charged phospholipid (POPG). These lipid-induced structural changes markedly increased the trypsin sensitivity of RTA and, on the basis of the protein fluorescence determinations, abolished its ability to bind to adenine, the product resulting from RTA-catalyzed depurination of 28S ribosomal RNA. RTA also released trapped calcein from POPG vesicles, indicating that it destabilized the lipid bilayer. We speculate that membrane-induced partial unfolding of RTA during cell entry may facilitate its recognition as an ERAD substrate.     

The mechanism of action of barley toxin: a type 1 ribosome-inactivating protein with RNA N-glycosidase activity

In a previous report (Endo, Y. and Tsurugi, K. (1987) J. Biol. Chem. 262, 8128-8130) it was shown that the RNA N-glycosidase activity of ricin A-chain was responsible for the ability of this protein to inactivate eukaryotic ribosomes. The objective of the present study was to determine whether a similar mechanism was used by a ribosome-inactivating protein from pearled barley (barley toxin). Rat liver ribosomes were incubated either with ricin A-chain or barley toxin, and the rRNA was extracted and treated with acidic aniline to hydrolyze phosphodiester bonds rendered susceptible by removal of a purine or pyrimidine base. Evaluation of the rRNA by polyacrylamide/agarose electrophoresis disclosed two 28 S rRNA-derived fragments which differed in size from those generated by untreated (control) ribosomes. Sequencing of the smaller of these fragments confirmed that - as is the case for ricin A-chain - the aniline-sensitive site in barley toxin-treated ribosomes was between A and G in 28 S rRNA. We conclude that barley toxin inactivates ribosomes via a mechanism identical to that of ricin A-chain: enzymatic hydrolysis of the N-glycosidic bond at A of 28 S rRNA.