Vetter I R, Parker M W, Tucker A D, Lakey J H, Pattus F, Tsernoglou D
European Molecular Biology Laboratory, Heidelberg, Germany.
Structure. 1998 Jul 15;6(7):863-74. doi: 10.1016/s0969-2126(98)00088-4.
Pore-forming colicins are water-soluble bacteriocins capable of binding to and translocating through the Escherichia coli cell envelope. They then undergo a transition to a transmembrane ion channel in the cytoplasmic membrane leading to bacterial death. Colicin N is the smallest pore-forming colicin known to date (40 kDa instead of the more usual 60 kDa) and the crystal structure of its membrane receptor, the porin OmpF, is already known. Structural knowledge of colicin N is therefore important for a molecular understanding of colicin toxicity and is relevant to toxic mechanisms in general.
The crystal structure of colicin N reveals a novel receptor-binding domain containing a six-stranded antiparallel beta sheet wrapped around the 63 A long N-terminal alpha helix of the pore-forming domain. The pore-forming domain adopts a ten alpha-helix bundle that has been observed previously in the pore-forming domains of colicin A, la and E1. The translocation domain, however, does not appear to adopt any regular structure. Models for receptor binding and translocation through the outer membrane are proposed based on the structure and biochemical data.
The colicin N-ompF system is now the structurally best-defined translocation pathway. Knowledge of the colicin N structure, coupled with the structure of its receptor, OmpF, and previously published biochemical data, limits the numerous possibilities of translocation and leads to a model in which the translocation domain inserts itself through the porin pore, the receptor-binding domain stays outside and the pore-forming domain translocates along the outer wall of the trimeric porin channel.
成孔大肠杆菌素是一种水溶性细菌素，能够结合并穿过大肠杆菌细胞膜。然后，它们会转变为细胞质膜中的跨膜离子通道，导致细菌死亡。大肠杆菌素N是迄今为止已知的最小的成孔大肠杆菌素（40 kDa，而非通常的60 kDa），其膜受体孔蛋白OmpF的晶体结构已经为人所知。因此，大肠杆菌素N的结构知识对于从分子层面理解大肠杆菌素毒性至关重要，并且总体上与毒性机制相关。
大肠杆菌素N的晶体结构揭示了一个新的受体结合结构域，该结构域包含一个六链反平行β折叠，环绕在成孔结构域63 Å长的N端α螺旋周围。成孔结构域采用了一个十螺旋束，这在大肠杆菌素A、Ia和E1的成孔结构域中也曾被观察到。然而，转运结构域似乎并未呈现出任何规则结构。基于该结构和生化数据，提出了受体结合和穿过外膜的转运模型。
大肠杆菌素N-OmpF系统现在是结构上定义最明确的转运途径。大肠杆菌素N的结构知识，连同其受体OmpF的结构以及先前发表的生化数据，限制了众多可能的转运方式，并得出一个模型，即转运结构域通过孔蛋白孔插入自身，受体结合结构域留在外部，而成孔结构域沿着三聚体孔蛋白通道的外壁进行转运。
