Department of Biology, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany.
Biochem J. 2011 May 1;435(3):733-42. doi: 10.1042/BJ20102131.
Cyanobacteria require efficient protein-quality-control mechanisms to survive under dynamic, often stressful, environmental conditions. It was reported that three serine proteases, HtrA (high temperature requirement A), HhoA (HtrA homologue A) and HhoB (HtrA homologue B), are important for survival of Synechocystis sp. PCC 6803 under high light and temperature stresses and might have redundant physiological functions. In the present paper, we show that all three proteases can degrade unfolded model substrates, but differ with respect to cleavage sites, temperature and pH optima. For recombinant HhoA, and to a lesser extent for HtrA, we observed an interesting shift in the pH optimum from slightly acidic to alkaline in the presence of Mg2+ and Ca2+ ions. All three proteases formed different homo-oligomeric complexes with and without substrate, implying mechanistic differences in comparison with each other and with the well-studied Escherichia coli orthologues DegP (degradation of periplasmic proteins P) and DegS. Deletion of the PDZ domain decreased, but did not abolish, the proteolytic activity of all three proteases, and prevented substrate-induced formation of complexes higher than trimers by HtrA and HhoA. In summary, biochemical characterization of HtrA, HhoA and HhoB lays the foundation for a better understanding of their overlapping, but not completely redundant, stress-resistance functions in Synechocystis sp. PCC 6803.
蓝藻需要有效的蛋白质质量控制机制来在动态的、通常是有压力的环境条件下生存。据报道，三种丝氨酸蛋白酶，HtrA（高温需求 A）、HhoA（HtrA 同源物 A）和 HhoB（HtrA 同源物 B），对于集胞藻 PCC 6803 在高光和高温胁迫下的生存是重要的，并且可能具有冗余的生理功能。在本文中，我们表明这三种蛋白酶都可以降解未折叠的模型底物，但在切割位点、最适温度和 pH 值方面存在差异。对于重组 HhoA，以及在较小程度上对于 HtrA，我们观察到在存在 Mg2+和 Ca2+离子的情况下，pH 最适值从略酸性向碱性转移。所有三种蛋白酶都与底物形成不同的同型寡聚复合物，这表明与彼此以及与研究充分的大肠杆菌同源物 DegP（周质蛋白 P 的降解）和 DegS 相比，它们在机制上存在差异。PDZ 结构域的缺失降低了，但没有完全消除，三种蛋白酶的蛋白水解活性，并且防止了 HtrA 和 HhoA 诱导的复合物形成高于三聚体的底物。总之，HtrA、HhoA 和 HhoB 的生化特性为更好地理解它们在集胞藻 PCC 6803 中的重叠但不完全冗余的抗应激功能奠定了基础。
