Division of Cell and Molecular Biology, Centre for Molecular Microbiology and Infection, Imperial College London London, UK.
Front Microbiol. 2011 Jul 18;2:155. doi: 10.3389/fmicb.2011.00155. eCollection 2011.
Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment. They are used to release enzymes that hydrolyze complex carbon sources into usable compounds, or to release proteins that capture essential ions such as iron. They are also used to colonize and survive within eukaryotic hosts, causing acute or chronic infections, subverting the host cell response and escaping the immune system. In this article, the opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals. This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The striking similarities between secretion systems and type IV pili, flagella, bacteriophage tail, or efflux pumps is a nice illustration of this evolution. Differences are also needed since various secretion configurations call for diversity. For example, some proteins are released in the extracellular medium while others are directly injected into the cytosol of eukaryotic cells. Some proteins are folded before being released and transit into the periplasm. Other proteins cross the whole cell envelope at once in an unfolded state. However, the secretion system requires conserved basic elements or features. For example, there is a need for an energy source or for an outer membrane channel. The structure of this review is thus quite unconventional. Instead of listing secretion types one after each other, it presents a melting pot of concepts indicating that secretion types are in constant evolution and use basic principles. In other words, emergence of new secretion systems could be predicted the way Mendeleïev had anticipated characteristics of yet unknown elements.
蛋白质分泌系统是革兰氏阴性菌在其环境中生存所使用的分子纳米机器。它们被用来释放水解复杂碳源的酶，将其转化为可利用的化合物，或者释放捕获铁等必需离子的蛋白质。它们还被用来在真核宿主中定殖和存活，引起急性或慢性感染，颠覆宿主细胞反应并逃避免疫系统。在本文中，以机会性病原体铜绿假单胞菌作为模型，综述了细菌为实现这些目标而进化出的各种分泌系统的多样性。这种多样性可能是由于细胞包膜复合物的逐渐转化，这些复合物最初可能不是专门用于分泌的。分泌系统与 IV 型菌毛、鞭毛、噬菌体尾部或外排泵之间惊人的相似之处很好地说明了这种进化。差异也是必要的，因为各种分泌构型需要多样性。例如，一些蛋白质被释放到细胞外培养基中，而另一些则直接注射到真核细胞的细胞质中。一些蛋白质在被释放之前折叠并穿过周质。其他蛋白质则以未折叠状态一次穿过整个细胞包膜。然而，分泌系统需要保守的基本元件或特征。例如，需要能量源或外膜通道。因此，这篇综述的结构非常非常规。它没有按照分泌类型逐一列出，而是呈现了一个概念的大熔炉，表明分泌类型在不断进化，并使用基本原理。换句话说，可以根据门捷列夫预测尚未发现元素的特征的方式，预测新分泌系统的出现。
