Eisenbach M, Constantinou C, Aloni H, Shinitzky M
Department of Membrane Research, Weizmann Institute of Science, Rehovot, Israel.
J Bacteriol. 1990 Sep;172(9):5218-24. doi: 10.1128/jb.172.9.5218-5224.1990.
A long-standing question in bacterial chemotaxis is whether repellents are sensed by receptors or whether they change a general membrane property such as the membrane fluidity and this change, in turn, is sensed by the chemotaxis system. This study addressed this question. The effects of common repellents on the membrane fluidity of Escherichia coli were measured by the fluorescence polarization of the probe 1,6-diphenyl-1,3,5-hexatriene in liposomes made of lipids extracted from the bacteria and in membrane vesicles. Glycerol, indole, and L-leucine had no significant effect on the membrane fluidity. NiSO4 decreased the membrane fluidity but only at concentrations much higher than those which elicit a repellent response in intact bacteria. This indicated that these repellents are not sensed by modulating the membrane fluidity. Aliphatic alcohols, on the other hand, fluidized the membrane, but the concentrations that elicited a repellent response were not equally effective in fluidizing the membrane. The response of intact bacteria to alcohols was monitored in various chemotaxis mutants and found to be missing in mutants lacking all the four methyl-accepting chemotaxis proteins (MCPs) or the cytoplasmic che gene products. The presence of any single MCP was sufficient for the expression of a repellent response. It is concluded (i) that the repellent response to aliphatic alcohols can be mediated by any MCP and (ii) that although an increase in membrane fluidity may take part in a repellent response, it is not the only mechanism by which aliphatic alcohols, or at least some of them, are effective as repellents. To determine whether any of the E. coli repellents are sensed by periplasmic receptors, the effects of repellents from various classes on periplasm-void cells were examined. The responses to all the repellents tested (sodium benzoate, indole, L-leucine, and NiSO4) were retained in these cells. In a control experiment, the response of the attractant maltose, whose receptor is periplasmic, was lost. This indicates that these repellents are not sensed by periplasmic receptors. In view of this finding and the involvement of the MCPs in repellent sensing, it is proposed that the MCPs themselves are low-affinity receptors for the repellents.
细菌趋化性中一个长期存在的问题是，驱避剂是由受体感知，还是它们改变了一般的膜特性（如膜流动性），而这种变化反过来又被趋化系统感知。本研究探讨了这个问题。通过探针1,6-二苯基-1,3,5-己三烯在由从细菌中提取的脂质制成的脂质体和膜囊泡中的荧光偏振，测量了常见驱避剂对大肠杆菌膜流动性的影响。甘油、吲哚和L-亮氨酸对膜流动性没有显著影响。硫酸镍降低了膜流动性，但只有在浓度远高于在完整细菌中引发驱避反应的浓度时才会如此。这表明这些驱避剂不是通过调节膜流动性来被感知的。另一方面，脂肪醇使膜流动性增加，但引发驱避反应的浓度在使膜流动性增加方面并不同样有效。在各种趋化性突变体中监测了完整细菌对醇类的反应，发现缺乏所有四种甲基接受趋化蛋白（MCPs）或细胞质che基因产物的突变体中没有这种反应。任何单一MCP的存在都足以表达驱避反应。得出的结论是：（i）对脂肪醇的驱避反应可由任何MCP介导；（ii）尽管膜流动性增加可能参与驱避反应，但这不是脂肪醇（或至少其中一些）作为驱避剂有效的唯一机制。为了确定大肠杆菌的任何驱避剂是否由周质受体感知，研究了不同类别的驱避剂对周质缺失细胞的影响。对所有测试的驱避剂（苯甲酸钠、吲哚、L-亮氨酸和硫酸镍）的反应在这些细胞中得以保留。在一个对照实验中，其受体位于周质的引诱剂麦芽糖的反应消失了。这表明这些驱避剂不是由周质受体感知的。鉴于这一发现以及MCPs参与驱避剂感知，有人提出MCPs本身就是驱避剂的低亲和力受体。
