We have shown that cyanobacterial chaperonins have pH-dependent anti-aggregation activity. The pH in cyanobacterial cytosol increases by one pH unit following a shift from darkness to light. In this study, we examined whether other major chaperones such as Hsp90 (HtpG) and Hsp70 (DnaK2) from the cyanobacterium Synechococcus elongatus PCC 7942 also display pH-dependent activity. Suppressing aggregation of various heat-denatured proteins, especially lactate dehydrogenase, at an equimolar ratio of cyanobacterial Hsp90 to protein substrate was found to be pH-dependent. Hsp90 showed the highest activity at pH 8.5 over the examined pH range of 7.0 to 8.5. pH affected the anti-aggregation activity of DnaK2 in a similar manner to that of Hsp90 in the presence of half equimolar DnaK2 to the protein substrate. The ATPase activity of cyanobacterial Hsp90 was pH-dependent, with a fourfold increase in activity when the pH was raised from 7.0 to 8.5. The ATPase activity of DnaK2 was also regulated by pH in a similar manner. Finally, an increase in pH from 7.0 to 8.5 enhanced activities of both Hsp90 and Hsp70 in protein-folding assistance by two- to threefold. These results suggest that changes in pH may regulate chaperone function during a light–dark cycle in cyanobacterial cells.

中文翻译：

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蓝藻 Hsp90 和 Hsp70 的 pH 调节伴侣功能：对光/暗调节的影响

我们已经证明蓝藻伴侣蛋白具有 pH 依赖性抗聚集活性。随着从黑暗到光明的转变，蓝藻胞质溶胶中的 pH 值增加一个 pH 单位。在这项研究中，我们检查了来自蓝藻细长聚球藻 PCC 7942 的其他主要伴侣蛋白，如 Hsp90 (HtpG) 和 Hsp70 (DnaK2) 是否也显示出 pH 依赖性活性。发现以蓝藻 Hsp90 与蛋白质底物的等摩尔比抑制各种热变性蛋白质，尤其是乳酸脱氢酶的聚集是 pH 依赖性的。Hsp90 在 7.0 至 8.5 的检测 pH 范围内在 pH 8.5 显示最高活性。在与蛋白质底物存在半等摩尔 DnaK2 的情况下，pH 以与 Hsp90 相似的方式影响 DnaK2 的抗聚集活性。蓝藻 Hsp90 的 ATP 酶活性是 pH 依赖性的，当 pH 从 7.0 升高到 8.5 时，活性增加了四倍。DnaK2 的 ATP 酶活性也以类似的方式受 pH 调节。最后，pH 从 7.0 增加到 8.5 将 Hsp90 和 Hsp70 在蛋白质折叠辅助中的活性提高了两到三倍。这些结果表明，pH 值的变化可能在蓝藻细胞的明暗循环期间调节伴侣功能。