The translocation of proteins across membranes is a fundamental cellular function. Bacteria have evolved a striking array of pathways for delivering proteins into or across cytoplasmic membranes and, when present, outer membranes. Translocated proteins can form part of the membrane landscape, reside in the periplasmic space situated between the inner and outer membranes of Gram-negative bacteria, deposit on the cell surface, or be released to the extracellular milieu or injected directly into target cells. One protein translocation system, the general secretory pathway, is conserved in all domains of life. A second, the twin-arginine translocation pathway, is also phylogenetically distributed among most bacteria and plant chloroplasts. While all cell types have evolved additional systems dedicated to the translocation of protein cargoes, the number of such systems in bacteria is now known to exceed nine. These dedicated protein translocation systems, which include the types 1 through 9 secretion systems (T1SSs–T9SSs), the chaperone–usher pathway, and type IV pilus system, are the subject of this review. Most of these systems were originally identified and have been extensively characterized in Gram-negative or diderm (two-membrane) species. It is now known that several of these systems also have been adapted to function in Gram-positive or monoderm (single-membrane) species, and at least one pathway is found only in monoderms. This review briefly summarizes the distinctive mechanistic and structural features of each dedicated pathway, as well as the shared properties, that together account for the broad biological diversity of protein translocation in bacteria.

中文翻译：

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细菌蛋白质易位系统的丰富挂毯。

蛋白质跨膜转运是一项基本的细胞功能。细菌已经进化出一系列惊人的途径，用于将蛋白质输送到或穿过细胞质膜，如果存在，还可以输送到外膜。易位蛋白可以形成膜景观的一部分，位于革兰氏阴性菌内外膜之间的周质空间中，沉积在细胞表面，或释放到细胞外环境或直接注射到靶细胞中。一种蛋白质易位系统，即一般分泌途径，在生命的所有领域都是保守的。第二种，双精氨酸易位途径，在系统发育上也分布在大多数细菌和植物叶绿体中。虽然所有细胞类型都进化出了专门用于蛋白质转运的额外系统，现在已知细菌中此类系统的数量超过九个。这些专用的蛋白质易位系统，包括 1 到 9 型分泌系统 (T1SSs–T9SSs)、伴侣-引导通路和 IV 型菌毛系统，是本综述的主题。这些系统中的大多数最初是在革兰氏阴性或双膜（双膜）物种中发现的，并已被广泛表征。现在已知，这些系统中的一些也已适应在革兰氏阳性或单皮动物（单膜）物种中发挥作用，并且至少有一种途径仅在单皮动物中发现。本综述简要总结了每个专用途径的独特机制和结构特征，以及共同的特性，它们共同解释了细菌中蛋白质易位的广泛生物多样性。