Poolman Bert, Spitzer Jan J, Wood Janet M
Department of Biochemistry, Groningen Biomolecular Sciences and Biotechnology and Materials Science Center(plus), University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Biochim Biophys Acta. 2004 Nov 3;1666(1-2):88-104. doi: 10.1016/j.bbamem.2004.06.013.
Bacteria act to maintain their hydration when the osmotic pressure of their environment changes. When the external osmolality decreases (osmotic downshift), mechanosensitive channels are activated to release low molecular weight osmolytes (and hence water) from the cytoplasm. Upon osmotic upshift, osmoregulatory transporters are activated to import osmolytes (and hence water). Osmoregulatory channels and transporters sense and respond to osmotic stress via different mechanisms. Mechanosensitive channel MscL senses the increasing tension in the membrane and appears to gate when the lateral pressure in the acyl chain region of the lipids drops below a threshold value. Transporters OpuA, BetP and ProP are activated when increasing external osmolality causes threshold ionic concentrations in excess of about 0.05 M to be reached in the proteoliposome lumen. The threshold activation concentrations for the OpuA transporter are strongly dependent on the fraction of anionic lipids that surround the cytoplasmic face of the protein. The higher the fraction of anionic lipids, the higher the threshold ionic concentrations. A similar trend is observed for the BetP transporter. The lipid dependence of osmotic activation of OpuA and BetP suggests that osmotic signals are transmitted to the protein via interactions between charged osmosensor domains and the ionic headgroups of the lipids in the membrane. The charged, C-terminal domains of BetP and ProP are important for osmosensing. The C-terminal domain of ProP participates in homodimeric coiled-coil formation and it may interact with the membrane lipids and soluble protein ProQ. The activation of ProP by lumenal, macromolecular solutes at constant ionic strength indicates that its structure and activity may also respond to macromolecular crowding. This excluded volume effect may restrict the range over which the osmosensing domain can electrostatically interact. A simplified version of the dissociative double layer theory is used to explain the activation of the transporters by showing how changes in ion concentration could modulate interactions between charged osmosensor domains and charged lipid or protein surfaces. Importantly, the relatively high ionic concentrations at which osmosensors become activated at different surface charge densities compare well with the predicted dependence of 'critical' ion concentrations on surface charge density. The critical ion concentrations represent transitions in Maxwellian ionic distributions at which the surface potential reaches 25.7 mV for monovalent ions. The osmosensing mechanism is qualitatively described as an "ON/OFF switch" representing thermally relaxed and electrostatically locked protein conformations.
当环境渗透压发生变化时，细菌会采取行动维持自身的水合作用。当外部渗透压降低（渗透减压）时，机械敏感通道被激活，以从细胞质中释放低分子量渗透溶质（从而释放水）。在渗透升压时，渗透调节转运蛋白被激活以导入渗透溶质（从而导入水）。渗透调节通道和转运蛋白通过不同机制感知并响应渗透胁迫。机械敏感通道MscL感知膜中不断增加的张力，并且当脂质酰基链区域的侧向压力降至阈值以下时似乎会打开。当外部渗透压增加导致在蛋白脂质体腔中达到超过约0.05 M的阈值离子浓度时，转运蛋白OpuA、BetP和ProP被激活。OpuA转运蛋白的阈值激活浓度强烈依赖于围绕蛋白质细胞质面的阴离子脂质的比例。阴离子脂质的比例越高，阈值离子浓度越高。BetP转运蛋白也观察到类似趋势。OpuA和BetP渗透激活的脂质依赖性表明，渗透信号通过带电渗透传感结构域与膜中脂质的离子头部基团之间的相互作用传递到蛋白质。BetP和ProP的带电荷的C末端结构域对于渗透传感很重要。ProP的C末端结构域参与同二聚体卷曲螺旋的形成，并且它可能与膜脂质和可溶性蛋白质ProQ相互作用。在恒定离子强度下，腔内大分子溶质对ProP的激活表明其结构和活性也可能对大分子拥挤做出反应。这种排除体积效应可能会限制渗透传感结构域能够进行静电相互作用的范围。解离双层理论的简化版本用于解释转运蛋白的激活，展示了离子浓度的变化如何调节带电渗透传感结构域与带电脂质或蛋白质表面之间的相互作用。重要的是，在不同表面电荷密度下渗透传感器被激活时相对较高的离子浓度与“临界”离子浓度对表面电荷密度的预测依赖性很好地相符。临界离子浓度代表麦克斯韦离子分布中的转变，对于单价离子，此时表面电位达到25.7 mV。渗透传感机制定性地描述为一个“开/关开关”，代表热松弛和静电锁定的蛋白质构象。
