Diamant S, Eliahu N, Rosenthal D, Goloubinoff P
Department of Plant Sciences, Institute of Life Sciences, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
J Biol Chem. 2001 Oct 26;276(43):39586-91. doi: 10.1074/jbc.M103081200. Epub 2001 Aug 21.
Salt and heat stresses, which are often combined in nature, induce complementing defense mechanisms. Organisms adapt to high external salinity by accumulating small organic compounds known as osmolytes, which equilibrate cellular osmotic pressure. Osmolytes can also act as "chemical chaperones" by increasing the stability of native proteins and assisting refolding of unfolded polypeptides. Adaptation to heat stress depends on the expression of heat-shock proteins, many of which are molecular chaperones, that prevent protein aggregation, disassemble protein aggregates, and assist protein refolding. We show here that Escherichia coli cells preadapted to high salinity contain increased levels of glycine betaine that prevent protein aggregation under thermal stress. After heat shock, the aggregated proteins, which escaped protection, were disaggregated in salt-adapted cells as efficiently as in low salt. Here we address the effects of four common osmolytes on chaperone activity in vitro. Systematic dose responses of glycine betaine, glycerol, proline, and trehalose revealed a regulatory effect on the folding activities of individual and combinations of chaperones GroEL, DnaK, and ClpB. With the exception of trehalose, low physiological concentrations of proline, glycerol, and especially glycine betaine activated the molecular chaperones, likely by assisting local folding in chaperone-bound polypeptides and stabilizing the native end product of the reaction. High osmolyte concentrations, especially trehalose, strongly inhibited DnaK-dependent chaperone networks, such as DnaK+GroEL and DnaK+ClpB, likely because high viscosity affects dynamic interactions between chaperones and folding substrates and stabilizes protein aggregates. Thus, during combined salt and heat stresses, cells can specifically control protein stability and chaperone-mediated disaggregation and refolding by modulating the intracellular levels of different osmolytes.
盐胁迫和热胁迫在自然界中常常同时出现，它们会引发互补的防御机制。生物体通过积累称为渗透保护剂的小分子有机化合物来适应高外部盐度，这些化合物可平衡细胞渗透压。渗透保护剂还可以通过提高天然蛋白质的稳定性和协助未折叠多肽的重新折叠来充当“化学伴侣”。对热胁迫的适应取决于热休克蛋白的表达，其中许多是分子伴侣，它们可防止蛋白质聚集、拆解蛋白质聚集体并协助蛋白质重新折叠。我们在此表明，预先适应高盐度的大肠杆菌细胞中甘氨酸甜菜碱水平升高，可在热胁迫下防止蛋白质聚集。热休克后，未受保护而聚集的蛋白质在适应盐环境的细胞中与在低盐环境中一样有效地解聚。在此，我们研究了四种常见渗透保护剂对体外伴侣活性的影响。甘氨酸甜菜碱、甘油、脯氨酸和海藻糖的系统剂量反应揭示了对伴侣蛋白GroEL、DnaK和ClpB单独及组合的折叠活性具有调节作用。除海藻糖外，低生理浓度的脯氨酸、甘油，尤其是甘氨酸甜菜碱激活了分子伴侣，可能是通过协助伴侣结合多肽的局部折叠并稳定反应的天然终产物来实现的。高浓度的渗透保护剂，尤其是海藻糖，强烈抑制了依赖DnaK的伴侣网络，如DnaK+GroEL和DnaK+ClpB，这可能是因为高粘度影响了伴侣蛋白与折叠底物之间的动态相互作用并稳定了蛋白质聚集体。因此，在盐胁迫和热胁迫共同作用期间，细胞可以通过调节不同渗透保护剂的细胞内水平来特异性地控制蛋白质稳定性以及伴侣介导的解聚和重新折叠。
