Krebs Institute, University of Sheffield, Sheffield, United Kingdom.
Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield, United Kingdom.
mSphere. 2020 Jul 1;5(4):e00424-20. doi: 10.1128/mSphere.00424-20.
Spores, the infectious agents of many , are remarkably resilient cell forms. Even distant relatives can have similar spore architectures although some display unique features; they all incorporate protective proteinaceous envelopes. We previously found that spores can achieve these protective properties through extensive disulfide cross-linking of self-assembled arrays of cysteine-rich proteins. We predicted that this could be a mechanism employed by spore formers in general, even those from other genera. Here, we tested this by revealing in nanometer detail how the outer envelope (exosporium) in (surrogate for group I), and in other clostridial relatives, forms a hexagonally symmetric semipermeable array. A cysteine-rich protein, CsxA, when expressed in , self-assembles into a highly thermally stable structure identical to that of the native exosporium. Like the exosporium, CsxA arrays require harsh "reducing" conditions for disassembly. We conclude that , CsxA self-organizes into a highly resilient, disulfide cross-linked array decorated with additional protein appendages enveloping the forespore. This pattern is remarkably similar to that in spores, despite a lack of protein homology. In both cases, intracellular disulfide formation is favored by the high lattice symmetry. We have identified cysteine-rich proteins in many distantly related spore formers and propose that they may adopt a similar strategy for intracellular assembly of robust protective structures. Bacteria such as those causing botulism and anthrax survive harsh conditions and spread disease as spores. Distantly related species have similar spore architectures with protective proteinaceous layers aiding adhesion and targeting. The structures that confer these common properties are largely unstudied, and the proteins involved can be very dissimilar in sequence. We identify CsxA as a cysteine-rich protein that self-assembles in a two-dimensional lattice enveloping the spores of several species. We show that apparently unrelated cysteine-rich proteins from very different species can self-assemble to form remarkably similar and robust structures. We propose that diverse cysteine-rich proteins identified in the genomes of a broad range of spore formers may adopt a similar strategy for assembly.
孢子是许多微生物的传染性因子，是一种具有惊人弹性的细胞形式。即使是远缘的微生物，它们的孢子结构也具有相似性，尽管有些微生物的孢子具有独特的特征，但它们都包含保护性的蛋白质外壳。我们之前发现，孢子可以通过富含半胱氨酸的蛋白质自组装形成的复杂结构的广泛二硫键交联来获得这些保护特性。我们预测，这可能是孢子形成者普遍采用的一种机制，甚至是来自其他属的孢子形成者也会采用这种机制。在这里，我们通过揭示纳米级细节来测试这一点，这些细节包括（替代 组 I）的外孢囊（外层）以及其他梭状芽孢杆菌亲缘微生物如何形成六方对称的半渗透排列。一种富含半胱氨酸的蛋白质 CsxA，在 中表达时，会自组装成一种与天然外孢囊高度热稳定的结构。与外孢囊一样，CsxA 结构需要苛刻的“还原”条件才能解组装。我们得出结论，CsxA 自我组织成一个高度有弹性的、二硫键交联的阵列，这些阵列上还装饰有额外的蛋白质附属物，包裹着前孢子。尽管缺乏蛋白质同源性，但这种模式与 孢子非常相似。在这两种情况下，高晶格对称性有利于细胞内二硫键的形成。我们在许多亲缘关系较远的孢子形成者中发现了富含半胱氨酸的蛋白质，并提出它们可能采用类似的策略，用于细胞内组装坚固的保护性结构。像导致肉毒中毒和炭疽病的细菌一样，它们以孢子的形式在恶劣条件下生存并传播疾病。亲缘关系较远的物种具有相似的孢子结构，带有保护性的蛋白质层，有助于附着和靶向。赋予这些共同特性的结构在很大程度上尚未得到研究，并且涉及的蛋白质在序列上可能非常不同。我们鉴定出 CsxA 是一种富含半胱氨酸的蛋白质，它在二维晶格中自组装，包裹着几种 物种的孢子。我们表明，来自非常不同物种的明显不相关的富含半胱氨酸的蛋白质可以自我组装，形成非常相似和坚固的结构。我们提出，在广泛的孢子形成者的基因组中鉴定出的多种多样的富含半胱氨酸的蛋白质可能会采用类似的组装策略。
