Molecular mechanisms of receptor desensitization using the beta-adrenergic receptor-coupled adenylate cyclase system as a model

Desensitization, the tendency of biological responses to wane over time despite the continuous presence of a stimulus of constant intensity, is observed in organisms as diverse as bacteria and mammals. Recently, new insights into the molecular mechanisms underlying these phenomena have emerged from the study of the receptors coupled to the ubiquitous second messenger-generating system adenylate cyclase. These mechanisms involve sequestration or down-regulation of the receptors from the cell surface as well as functionally significant covalent modifications of the receptors and/or guanine nucleotide regulatory proteins.     

Tumour promoter uncouples beta-adrenergic receptor from adenyl cyclase in mouse epidermis

Alterations in beta-adrenergic receptor number and function and in the hormonal responsiveness of adenylate cyclase have been observed in transformed cells, and tumours. Phorbol myristate acetate (PMA), a potent tumour promoter in mouse skin, induces a dramatic loss of epidermal responsiveness to catecholamines in vivo, although basal levels of cyclic AMP are not affected. In other work we have shown that PMA treatment does not alter the number or affinity of epidermal beta-receptors, although accumulation of cyclic AMP in response to isoprenaline injection is sharply inhibited. Evidence is presented here that PMA exerts this effect by uncoupling epidermal beta-receptors from adenylate cyclase.     

Ligand binding to the beta-adrenergic receptor involves its rhodopsin-like core

Recently the genes for several hormone receptors that interact with guanine nucleotide binding proteins (G proteins) have been cloned, including the hamster beta 2-adrenergic receptor (beta 2AR), a human beta AR, the turkey erythrocyte beta AR and the porcine muscarinic acetylcholine receptor (MAR). All these receptors share some amino-acid homology with rhodopsin, particularly in 7 hydrophobic stretches of residues that are believed to represent transmembrane helices. To determine whether differences in ligand specificity result from the divergence in the sequences of the hydrophilic regions of these receptors, we have expressed in mammalian cells genes for the wild-type hamster and human beta AR proteins, and a series of deletion mutant genes of the hamster beta 2AR. The pharmacology of the expressed receptors indicates that most of the hydrophilic residues are not directly involved in the binding of agonists or antagonists to the receptor. In addition, we have identified a mutant receptor that has high agonist affinity but does not couple to adenylate cyclase.     

Membrane guanylate cyclase is a cell-surface receptor with homology to protein kinases

Guanylate cyclase has been strongly implicated as a cell-surface receptor on spermatozoa for a chemotactic peptide, and on various other cells as a receptor for atrial natriuretic peptides. Resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-Leu-NH2), the chemotactic peptide released by sea urchin Arbacia punctulata eggs, is specifically crosslinked to A. punctulata spermatozoan guanylate cyclase. After the binding of the peptide the state of guanylate cyclase phosphorylation modulates enzyme activity. We report here that the deduced amino-acid sequence of the spermatozoan membrane form of guanylate cyclase predicts an intrinsic membrane protein of 986 amino acids with an amino-terminal signal sequence. A single transmembrane domain separates the protein into putative extracellular and cytoplasmic-catalytic domains. The cytoplasmic carboxyl-terminal 95 amino acids contain 20% serine, the likely regulatory sites for phosphorylation. Unexpectedly, the enzyme is homologous to the protein kinase family.     

The genomic clone G-21 which resembles a beta-adrenergic receptor sequence encodes the 5-HT1A receptor

The recent cloning of the complementary DNAs and/or genes for several receptors linked to guanine nucleotide regulatory proteins including the adrenergic receptors (alpha 1, alpha 2A, alpha 2B, beta 1, beta 2), several subtypes of the muscarinic cholinergic receptors, and the visual 'receptor' rhodopsin has revealed considerable similarity in the primary structure of these proteins. In addition, all of these proteins contain seven putative transmembrane alpha-helices. We have previously described a genomic clone, G-21, isolated by cross-hybridization at reduced stringency with a full length beta 2-adrenergic receptor probe. This clone contains an intronless gene which, because of its striking sequence resemblance to the adrenergic receptors, is presumed to encode a G-protein-coupled receptor. Previous attempts to identify this putative receptor by expression studies have failed. We now report that the protein product of the genomic clone, G21, transiently expressed in monkey kidney cells has all the typical ligand-binding characteristics of the 5-hydroxytryptamine (5-HT1A) receptor.     

Fundamental difference between the molecular interactions of agonists and antagonists with the beta-adrenergic receptor.

Antagonist binding to the beta-adrenergic receptor is largely entropy driven, with only a small enthalpy component. The binding of agonists, on the other hand, is associated with a large decrease in enthalpy which permits a highly unfavourable decrease in entropy. The thermodynamic differences between the binding of agonists and antagonists may provide new insights into the molecular basis for hormone stimulation of adenylate cyclase activity.     

Cloning of the gene and cDNA for mammalian beta-adrenergic receptor and homology with rhodopsin

The adenylate cyclase system, which consists of a catalytic moiety and regulatory guanine nucleotide-binding proteins, provides the effector mechanism for the intracellular actions of many hormones and drugs. The tissue specificity of the system is determined by the particular receptors that a cell expresses. Of the many receptors known to modulate adenylate cyclase activity, the best characterized and one of the most pharmacologically important is the beta-adrenergic receptor (beta AR). The pharmacologically distinguishable subtypes of the beta-adrenergic receptor, beta 1 and beta 2 receptors, stimulate adenylate cyclase on binding specific catecholamines. Recently, the avian erythrocyte beta 1, the amphibian erythrocyte beta 2 and the mammalian lung beta 2 receptors have been purified to homogeneity and demonstrated to retain binding activity in detergent-solubilized form. Moreover, the beta-adrenergic receptor has been reconstituted with the other components of the adenylate cyclase system in vitro, thus making this hormone receptor particularly attractive for studies of the mechanism of receptor action. This situation is in contrast to that for the receptors for growth factors and insulin, where the primary biochemical effectors of receptor action are unknown. Here, we report the cloning of the gene and cDNA for the mammalian beta 2AR. Analysis of the amino-acid sequence predicted for the beta AR indicates significant amino-acid homology with bovine rhodopsin and suggests that, like rhodopsin, beta AR possesses multiple membrane-spanning regions.     

纯文学:一个值得质疑的策略上的概念

纯文学的概念在逻辑上是不成立的.它只是文学家捍卫自己独立性的策略.一是以个人主题来抵制共名状态的时代主题,一是以文本回避政治权力的侵蚀.它对于文学价值的认知和创造意义是明显的,却不是文学的本质和出路.     

制备纯Fock态的一种方法

采用二能级原子与单模腔场的双光子共振作用和单光子共振作用,并通过对原子进行选择性测量,在一定条件下,制备了纯Ｆｏｃｋ│２ｎ〉和│２ｎ＋１〉态。     

采用纯代码方式编写多级下拉式菜单

为实现个性化多级式下拉式菜单的设计和解决普通BASIC语言下已开发的软件程序中无多级式下拉式菜单功能的问题,提出利用纯代码编写多级下拉式菜程序的设计思想,并通过设计实例介绍了手工编程设计多级下拉式菜单的思路、问题的解决方案和程序代码设计的基本方法。     

A single micromanipulated stem cell gives rise to multiple T-cell receptor gene rearrangements in the thymus in vitro

The extensive range of specificities of T-cell receptors is generated, as for immunoglobulins, by rearrangement of genetic information. Much valuable information about rearrangement processes has been inferred by comparing DNA from (monoclonal) lymphoid lines with germ-line DNA and, for B cells, from rearrangements in some Abelson murine leukaemia virus-transformed cell lines. However, because it is difficult to isolate and grow precursor populations, it has not proved possible to study rearrangements occurring in normal untransformed cells in vitro. Here we show that a single T-cell precursor colonizing an alymphoid thymus lobe in organ culture can generate multiple receptor beta-chain gene rearrangements. These observations provide unequivocal evidence for the intra-thymic diversification of the T-cell repertoire. They also offer the possibility of investigating rearrangement and its control in the clonal progeny of a single normal T-cell precursor without the perturbations involved in the use of viral transformation or the production of T-cell hybridomas.     

填充方钴矿热电材料:从单填到多填

以传统窄带半导体材料为主要对象的高性能热电材料研究近年来发展迅速并取得了明显进展.本文以含本征晶格孔洞的笼状结构CoSb3基方钴矿化合物的相关研究为主线,综述近年来热电材料的主要研究进展,并分析了杂质原子在孔洞中部分填充特性为基础的填充方钴矿化合物的结构调控、电-热输运性能协同调控、以及热电性能优化的内在物理机制及其实验实现.在本征孔洞结构的方钴矿化合物中引入部分填充的杂质原子,通过局域声子散射而显著降低晶格热导率,同时可以优化电输运性能.研究还发现这样一类特殊结构化合物的电热输运性能可以通过选择不同价态与不同局域振动频率的多种不同填充原子的组合填充而实现可以近乎独立地调控与优化,热电优值达到1.7@850K,实现了明显具有声子玻璃-电子晶体特征的一类高性能热电材料.研究工作一方面明显提高了填充方钴矿材料的热电性能,另一方面加深了相关物理机制的理解,对进一步的新热电材料体系的设计具有指导意义.     

Macrocortin: a polypeptide causing the anti-phospholipase effect of glucocorticoids

Anti-inflammatory glucocorticoids inhibit prostaglandin (PG) biosynthesis by preventing arachidonic acid release from phospholipids rather than inhibiting the cyclooxygenase. As in other cells, this steroid action depends on receptor occupation and de novo protein/RNA biosynthesis. We have previously shown in guinea pig perfused lungs and rat peritoneal leukocytes that the effect of steroids in PG generation is mediated by an uncharacterized 'second messenger'. Now, we report that this factor (which we have named 'macrocortin') is an intracellular polypeptide whose release and synthesis are stimulated by steroids. Macrocortin derived from rat peritoneal leukocytes is very similar to that released from guinea pig lungs.     

市民小说:沟通通俗文学与新文学的桥梁

中国现当代文学中的市民小说是一种反映市民特别是中下层市民的生活、文化价值观念和审美情趣的,是以广大市民为接受主体的小说.它跨越了通俗文学和新文学的界限,实现了雅俗之间卓有成效的整合,成为沟通二者之间的桥梁,这在张恨水、老舍、张爱玲、苏青、池莉等作家的作品中有明显的体现.     

Isolation of cDNA clones encoding the 20K non-glycosylated polypeptide chain of the human T-cell receptor/T3 complex

The antigen receptor on human T lymphocytes consists of two variable immunoglobulin-like glycoproteins, alpha and beta, which occur in association with three invariable T3 membrane proteins. In humans two of these proteins, T3-gamma and T3-delta, are glycoproteins of relative molecular mass (Mr) 25,000 (25K) and 20,000 (20K), respectively, while the third, T3-epsilon, is a 20K non-glycosylated protein. On the surface of murine T cells, a non-glycosylated protein dimer composed of 17K subunits (T3-zeta) is found associated with the T-cell receptor alpha and beta chains and the three T3-like polypeptide chains. It is generally accepted that major histocompatibility complex-restricted antigen recognition is a function of the alpha-beta heterodimer. This has led to the postulation that the proteins of the T3 complex are involved in the signal transduction that immediately follows antigen recognition via the antigen receptor. Events believed to be involved in early T-cell activation, such as rapid increases in phosphatidylinositol turnover and free intracellular calcium, can be triggered by antibodies directed against either the T3 complex or the clonotypic receptor. We have previously reported our findings on the cloning of the complementary DNA and genomic structure encoding both the human and murine 20K glycoprotein, T3-delta (refs 11-13). We now present our results on the cloning of the cDNA encoding the human 20K non-glycosylated chain, T3-epsilon.     

Suppressor T cells control the HLA-linked low responsiveness to streptococcal antigen in man

We have previously reported that low immune responsiveness to the streptococcal cell wall (SCW) antigen is controlled by an HLA-linked dominant gene which we designated as an immune suppression gene to the SCW antigen (Is-SCW) without knowing its function. We have extended the study of the genetic control of the immune response to the SCW antigen and confirmed both the bimodal distribution of immune responsiveness and HLA-linked dominant inheritance of low responsiveness. Here we report an analysis of the function and expression of Is-SCW at the cellular level and demonstrate that the Is-SCW controls the generation of antigen-specific suppressor T cell in low responders.     

Different conformations for the same polypeptide bound to chaperones DnaK and GroEL.

The proteins DnaK (hsp70) and GroEL (cpn60) from Escherichia coli are prototypes of two classes of molecular chaperones conserved throughout evolution. The analysis of transferred nuclear Overhauser effects in two-dimensional NMR spectra is ideally suited to determine chaperone-bound conformations of peptides. The peptide vsv-C (amino-acid sequence KLIGVLSSLFRPK) stimulates the ATPase of BiP and Hsc70 (ref. 3) and the intrinsic ATPase of DnaK. The affinity of the vsv-C peptide for DnaK is greatly reduced in the presence of ATP. Here we analyse transferred nuclear Overhauser effects and show that the peptide is in an extended conformation while bound to DnaK but is helical when bound to GroEL. NMR also indicates that the mobility of the peptide backbone is reduced more by binding to DnaK than by binding to GroEL, whereas the side chains are less mobile when bound to GroEL.