Laboratoire Pathogenèse des Bactéries Anaérobies, Institut Pasteur, Paris, France.
PLoS Pathog. 2012;8(6):e1002727. doi: 10.1371/journal.ppat.1002727. Epub 2012 Jun 7.
The pathogenesis of Clostridium difficile, the major cause of antibiotic-associated diarrhea, is mainly associated with the production and activities of two major toxins. In many bacteria, toxins are released into the extracellular environment via the general secretion pathways. C. difficile toxins A and B have no export signature and their secretion is not explainable by cell lysis, suggesting that they might be secreted by an unusual mechanism. The TcdE protein encoded within the C. difficile pathogenicity locus (PaLoc) has predicted structural features similar to those of bacteriophage holin proteins. During many types of phage infection, host lysis is driven by an endolysin that crosses the cytoplasmic membrane through a pore formed by holin oligomerization. We demonstrated that TcdE has a holin-like activity by functionally complementing a λ phage deprived of its holin. Similar to λ holin, TcdE expressed in Escherichia coli and C. difficile formed oligomers in the cytoplamic membrane. A C. difficile tcdE mutant strain grew at the same rate as the wild-type strain, but accumulated a dramatically reduced amount of toxin proteins in the medium. However, the complemented tcdE mutant released the toxins efficiently. There was no difference in the abundance of tcdA and tcdB transcripts or of several cytoplasmic proteins in the mutant and the wild-type strains. In addition, TcdE did not overtly affect membrane integrity of C. difficile in the presence of TcdA/TcdB. Thus, TcdE acts as a holin-like protein to facilitate the release of C. difficile toxins to the extracellular environment, but, unlike the phage holins, does not cause the non-specific release of cytosolic contents. TcdE appears to be the first example of a bacterial protein that releases toxins into the environment by a phage-like system.
艰难梭菌是抗生素相关性腹泻的主要病原体，其发病机制主要与两种主要毒素的产生和活性有关。在许多细菌中，毒素通过一般分泌途径释放到细胞外环境中。艰难梭菌毒素 A 和 B 没有出口特征，其分泌不能用细胞裂解来解释，这表明它们可能通过一种不寻常的机制进行分泌。编码在艰难梭菌致病基因座（PaLoc）内的 TcdE 蛋白具有与噬菌体溶素蛋白相似的预测结构特征。在许多类型的噬菌体感染中，宿主裂解是由一种内切酶驱动的，该内切酶通过溶素寡聚化形成的孔穿过细胞质膜。我们通过功能性互补缺失其溶素的 λ 噬菌体证明了 TcdE 具有溶素样活性。与 λ 溶素相似，在大肠杆菌和艰难梭菌中表达的 TcdE 在细胞质膜中形成寡聚体。与野生型菌株相比，艰难梭菌 tcdE 突变株的生长速度相同，但在培养基中积累的毒素蛋白量明显减少。然而，互补的 tcdE 突变株有效地释放了毒素。突变株和野生型菌株中 tcdA 和 tcdB 转录物的丰度或几种细胞质蛋白没有差异。此外，在存在 TcdA/TcdB 的情况下，TcdE 并没有明显影响艰难梭菌的膜完整性。因此，TcdE 作为一种溶素样蛋白，促进艰难梭菌毒素释放到细胞外环境，但与噬菌体溶素不同，它不会导致细胞质内容物的非特异性释放。TcdE 似乎是第一个通过类似噬菌体的系统将毒素释放到环境中的细菌蛋白。
