Understanding What Small Heat Shock Proteins Do for Bacterial Cells

Abstract

Being initially discovered due to the increased transcription of their encoding genes under heat shock conditions, heat shock proteins (HSPs, interchangeably named as 'stress proteins' or 'molecular chaperones') are known for their dramatic increase in amount when an organism is exposed to a variety of stress conditions. The major known activity for HSPs is to prevent the aggregation of nascent polypeptide chains that are yet-folded or mature proteins that are denatured under stress conditions. By transiently binding to and subsequently releasing the client proteins, HSPs function to promote their folding and/or assembly in living organisms. Small heat shock proteins (sHSPs), being relatively small among the HSPs in terms of the molecular weight of a single polypeptide chain, was found to present in bacteria only years after they were fi rst identifi ed in animals and plants. In this chapter, I will provide a historical perspective on what we have learned about the structure, function and regulation of sHSPs in bacteria. Main aspects covered in this chapter include the following. sHSPs exist as large dynamic homo-oligomers and regulate their activities by modulating their oligomeric status in a stress-responsive manner; sHSPs exhibit effective chaperone-like activities under in vitro and in vivo conditions; The monomeric small heat shock proteins possess an immunoglobulinlike folding pattern; sHSPs associate with and affect the physical state of cellular membranes; sHSPs play a potential role for bacterial cells to enter the non-growing dormant state; The biological functions of sHSPs are explored via gene knockout studies. In the end, I will also briefl y discuss some of the unresolved issues regarding the structure, function and regulation of sHSPs.

Dodecameric structure of a small heat shock protein from Mycobacterium marinum M

“…Apart from their molecular chaperoning abilities, sHSPs have various other roles. In bacteria, protozoans, and fungi, sHSPs are usually less in number and are essential for cell fitness and survival during harsh conditions, while in plants, a large number of sHSPs targeted to different organelles are known to exist which help them cope with conditions like drought, salinity, floods etc . In case of humans, the scenario is more complicated, since different sHSPs are present in varying concentrations in different tissues and are expressed in different developmental stages.…”

Exploring proteins in living cells

“…基于我们对 DegP 蛋白在极端酸性条件下被抗酸分子伴侣蛋白 HdeA 保护这一 活细胞非天然氨基酸光交联研究结果 [46,47] ，我们进一步研究了 DegP 蛋白在细菌 抗酸过程中究竟发挥何种生物学功能。结果表明，DegP 蛋白的确可以帮助经历 了酸性条件的细菌细胞在恢复到中性条件时对膜间质中一些底物蛋白质予以降 解清除 [52] 。这似乎通过一种独特机制进行：这样的底物蛋白质在酸性条件下即与 同样处于变性状态的 DegP 蛋白进行预结合，当细胞恢复到中性条件时，DegP 蛋 白恢复其结构并降解所结合的底物蛋白，从而实现其抵抗酸性胁迫的功能。 9. 活细胞中小分子热休克蛋白的结构和活性响应温度的变化而变化 小分子热休克蛋白是一类存在于所有生物种类中的分子伴侣蛋白 [53] 。我们对 该家族蛋白质的多个成员(如结核杆菌中的 Hsp16.3、大肠杆菌中的 IbpB 等)进 行的体外研究表明， 这类蛋白质的三级和四级结构对包括热休克在内的胁迫条件 具有高度的敏感性和响应能力， 这些细微结构变化导致其分子伴侣活性明显改变 [54][55][56][57][58][59][60] 。我们一直好奇的是，这样的体外胁迫条件诱导发生的结构和活性改变是否 反映活细胞中的真实情况？ 通过非天然氨基酸介导的蛋白质光交联活细胞分析，我们观察到，随着温度 的逐步升高(20 o C-50 o C) ，细菌中的 IbpB 小分子热休克蛋白参与和底物蛋白质结 合的残基数目也逐渐增多，因此，分子伴侣活性也逐渐提高(图 10A) [61] 。显然， 这种在活细胞中体现出来的小分子热休克蛋白的层级式激活机制与我们在体外 观察到的结果高度吻合。同时，我们也对细菌小分子热休克蛋白在活细胞中所结 合的底物蛋白质的种类进行了质谱分析，结果表明，在 IbpB 所保护的底物蛋白 质中，与蛋白质翻译过程相关的蛋白质因子相对比较富集 [62] ，这一现象背后的生 物学意义有待深入认识。 同样重要的是，我们在体外实验中观察到小分子热休克蛋白形成寡聚体的亚 基之间相互作用的动态性 [55][56][57][58][59] ， 也通过非天然氨基酸蛋白质交联分析在活细胞内 得到了验证 [22,63] 。这样的分析结果还表明，这种分子伴侣蛋白质在活细胞中构象 的动态性似乎远超出我们在体外所观察到的程度(图 10B) [22,63] 。对活细胞的研 究，大大加深了我们对蛋白质发挥功能机制的认识。 图 10. 小分子热休克蛋白在活细胞中响应温度变化以层级方式被激活 [61] 并 具高度的结构动态性 [63] 。 Figure 10.…”

Small Heat Shock Protein16.3 of Mycobacterium tuberculosis: After Two Decades of Functional Characterization

“…In addition, sHSP plays a vital role in growth, survival and virulence of bacteria [93,94]. As the fact that a sHSP interacts with a number of proteins (of diverse origin and nature) / regulatory partners (co-chaperones) during performing its action as a molecular chaperone; so, targeting a particular sHSP with potential inhibitor may lead to impair the function(s) of proteins supposed to be in its network.…”