Kirby J R, Niewold T B, Maloy S, Ordal G W
Department of Biochemistry, University of Illinois, Urbana 61801, USA.
Mol Microbiol. 2000 Jan;35(1):44-57. doi: 10.1046/j.1365-2958.2000.01676.x.
The methyl-accepting chemotaxis protein, McpB, is the sole receptor mediating asparagine chemotaxis in Bacillus subtilis. In this study, we show that wild-type B. subtilis cells contain approximately 2,000 copies of McpB per cell, that these receptors are localized polarly, and that titration of only a few receptors is sufficient to generate a detectable behavioural response. In contrast to the wild type, a cheB mutant was incapable of tumbling in response to decreasing concentrations of asparagine, but the cheB mutant was able to accumulate to low concentrations of asparagine in the capillary assay, as observed previously in response to azetidine-2-carboxylate. Furthermore, net demethylation of McpB is logarithmically dependent on asparagine concentration, with half-maximal demethylation of McpB occurring when only 3% of the receptors are titrated. Because the corresponding methanol production is exponentially dependent on attractant concentration, net methylation changes and increased turnover of methyl groups must occur on McpB at high concentrations of asparagine. Together, the data support the hypothesis that methylation changes occur on asparagine-bound McpB to enhance the dynamic range of the receptor complex and to enable the cell to respond to a negative stimulus, such as removal of asparagine.
甲基接受趋化蛋白McpB是介导枯草芽孢杆菌中天冬酰胺趋化作用的唯一受体。在本研究中，我们发现野生型枯草芽孢杆菌细胞每细胞含有约2000个McpB拷贝，这些受体定位于细胞极性部位，并且仅滴定少数受体就足以产生可检测到的行为反应。与野生型相比，cheB突变体不能因天冬酰胺浓度降低而翻滚，但在毛细管试验中，cheB突变体能够积累到低浓度的天冬酰胺，这与之前对氮杂环丁烷-2-羧酸的反应观察结果一致。此外，McpB的净去甲基化对数依赖于天冬酰胺浓度，当仅滴定3%的受体时，McpB发生半最大去甲基化。由于相应的甲醇产生指数依赖于引诱剂浓度，在高浓度天冬酰胺下，McpB上必定发生甲基化变化和甲基周转增加。总之，这些数据支持这样的假设，即天冬酰胺结合的McpB上发生甲基化变化以增强受体复合物的动态范围，并使细胞能够对负面刺激（如天冬酰胺的去除）做出反应。
