Proteolysis Lab, Department of Structural Biology, Molecular Biology Institute of Barcelona, Consejo Superior de Investigaciones Científicas, Barcelona Science Park, Helix Building, E-08028 Barcelona, Spain.
Proc Natl Acad Sci U S A. 2011 Feb 1;108(5):1856-61. doi: 10.1073/pnas.1012173108. Epub 2011 Jan 13.
Enterotoxigenic Bacteroides fragilis is the most frequent disease-causing anaerobe in the intestinal tract of humans and livestock and its specific virulence factor is fragilysin, also known as B. fragilis toxin. This is a 21-kDa zinc-dependent metallopeptidase existing in three closely related isoforms that hydrolyze E-cadherin and contribute to secretory diarrhea, and possibly to inflammatory bowel disease and colorectal cancer. Here we studied the function and zymogenic structure of fragilysin-3 and found that its activity is repressed by a ∼170-residue prodomain, which is the largest hitherto structurally characterized for a metallopeptidase. This prodomain plays a role in both the latency and folding stability of the catalytic domain and it has no significant sequence similarity to any known protein. The prodomain adopts a novel fold and inhibits the protease domain via an aspartate-switch mechanism. The catalytic fragilysin-3 moiety is active against several protein substrates and its structure reveals a new family prototype within the metzincin clan of metallopeptidases. It shows high structural similarity despite negligible sequence identity to adamalysins/ADAMs, which have only been described in eukaryotes. Because no similar protein has been found outside enterotoxigenic B. fragilis, our findings support that fragilysins derived from a mammalian adamalysin/ADAM xenolog that was co-opted by B. fragilis through a rare case of horizontal gene transfer from a eukaryotic cell to a bacterial cell. Subsequently, this co-opted peptidase was provided with a unique chaperone and latency maintainer in the time course of evolution to render a robust and dedicated toxin to compromise the intestinal epithelium of mammalian hosts.
产肠毒素脆弱拟杆菌是人类和家畜肠道中最常见的致病厌氧菌，其特定的毒力因子是脆弱拟杆菌蛋白酶，也称为脆弱拟杆菌毒素。这是一种 21kDa 的锌依赖性金属蛋白酶，存在三种密切相关的同工型，可水解 E-钙粘蛋白，导致分泌性腹泻，并可能导致炎症性肠病和结直肠癌。在这里，我们研究了脆弱拟杆菌蛋白酶 3 的功能和酶原结构，发现其活性受一个约 170 个残基的前导肽抑制，这是迄今为止结构上表征的最大金属蛋白酶前导肽。该前导肽在催化结构域的潜伏和折叠稳定性中都发挥作用，并且与任何已知的蛋白质没有明显的序列相似性。该前导肽采用一种新的折叠方式，并通过天冬氨酸开关机制抑制蛋白酶结构域。催化的脆弱拟杆菌蛋白酶 3 部分对几种蛋白质底物具有活性，其结构揭示了金属蛋白酶中天冬氨酸蛋白酶家族原型中的一个新家族。尽管与仅在真核生物中描述的 adamalysins/ADAMs 相比，其序列同一性微不足道，但它表现出很高的结构相似性。由于在产肠毒素脆弱拟杆菌之外没有发现类似的蛋白质，我们的发现支持这样的观点，即脆弱拟杆菌蛋白酶来源于哺乳动物 adamalysin/ADAM 的一个同源物，通过罕见的从真核细胞到细菌细胞的水平基因转移被脆弱拟杆菌获得。随后，在进化过程中，这种被获得的肽酶被赋予了一个独特的伴侣和潜伏期维持者，以产生一种强大而专门的毒素，破坏哺乳动物宿主的肠道上皮细胞。
