Attractant regulation of the aspartate receptor-kinase complex: Limited cooperative interactions between receptors and effects of the receptor modification state
Attractant regulation of the aspartate receptor-kinase complex: Limited cooperative interactions between receptors and effects of the receptor modification state
复制标题
DOI：
10.1021/bi0002737
复制
发表时间：
2000-08-08
期刊：
影响因子：
2.9
通讯作者：
Falke, JJ
中科院分区：
文献类型：
--
作者：
Bornhorst, JA;Falke, JJ
细菌趋化系统对各种引诱剂浓度的反应方式仍不完全清楚。原则上，趋化性受体之间的正协同性可以解释细菌对极低引诱剂浓度作出反应的能力。通过利用体外受体偶联激酶试验，已测定的引诱依赖性反应曲线的鼠伤寒沙门氏菌天冬氨酸化学受体。所选的引诱剂α-甲基天冬氨酸最初由Segall，Block和贝格[（1986）Proc. Natl. Chem. Acad. Sci. U.S.A.83，8987-8991]。引诱剂响应曲线显示出有限的正协同性，产生1.7-2.4的Hill系数，并且该Hill系数相对独立于受体修饰状态和CheA与受体的摩尔比。这些结果不利于模型，其中有大量的受体二聚体在广泛的受体阵列之间的强合作的相互作用。相反，结果与少量偶联受体二聚体之间的合作相互作用一致。因为体外受体偶联激酶测定利用高于由过表达产生的天然受体密度，所以观察到的正协同性可能高估了天然受体群体中存在的正协同性。二聚体之间的这种正协同性与先前在单个二聚体内的两个对称配体结合位点之间观察到的负协同性完全相容。天冬氨酸受体的引诱剂亲和力被发现依赖于其共价适应位点的修饰状态。在体外受体偶联激酶测定中，增加修饰水平使表观引诱剂亲和力降低至少10倍。这一观察有助于解释趋化性途径在体内对广泛的引诱剂浓度作出反应的能力。
The manner hv which the bacterial chemotaxis system responds to a wide range of attractant concentrations remains incompletely understood. In principle, positive cooperativity between chemotaxis receptors could explain the ability of bacteria to respond to extremely low attractant concentrations. By utilizing an in vitro receptor-coupled kinase assay, the attractant-dependent response curve has been measured for the Salmonella typhimurium aspartate chemoreceptor. The attractant chosen, ct-methyl aspartate, was originally used to quantitate high receptor sensitivity at low attractant concentrations by Segall, Block, and Berg [(1986) Proc. Natl. Acad. Sci. U.S.A. 83, 8987-8991]. The attractant response curve exhibits limited positive cooperativity, yielding a Hill coefficient of 1.7-2.4, and this Hill coefficient is relatively independent of both the receptor modification state and the mole ratio of CheA to receptor. These results disfavor models in which there are strong cooperative interactions between large numbers of receptor dimers in an extensive receptor array. Instead, the results are consistent with cooperative interactions between a small number of coupled receptor dimers. Because the in vitro receptor-coupled kinase assay utilizes higher than native receptor densities arising from overexpression, the observed positive cooperativity may overestimate that present in native receptor populations. Such positive cooperativity between dimers is fully compatible with the negative cooperativity previously observed between the two symmetric ligand binding sites within a single dimer. The attractant affinity of the aspartate receptor is found to depend on the modification state of its covalent adaptation sites. Increasing the the level of modification decreases the apparent attractant affinity at least 10-fold in the in vitro receptor-coupled kinase assay. This observation helps explain the ability of the chemotaxis pathway to respond to a broad range of attractant concentrations in vivo.