Sharma Onkar, Cramer William A
Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA.
J Bacteriol. 2007 Jan;189(2):363-8. doi: 10.1128/JB.01344-06. Epub 2006 Nov 3.
The 315-residue N-terminal T domain of colicin E3 functions in translocation of the colicin across the outer membrane through its interaction with outer membrane proteins including the OmpF porin. The first 83 residues of the T domain are known from structure studies to be disordered. This flexible translocation subdomain contains the TolB box (residues 34 to 46) that must cross the outer membrane in an early translocation event, allowing the colicin to bind to the TolB protein in the periplasm. In the present study, it was found that cytotoxicity of the colicin requires a minimum length of 19 to 23 residues between the C terminus (residue 46) of the TolB box and the end of the flexible subdomain (residue 83). Colicin E3 molecules of sufficient length display normal binding to TolB and occlusion of OmpF channels in vitro. The length of the N-terminal subdomain is critical because it allows the TolB box to cross the outer membrane and interact with TolB. It is proposed that the length constraint is a consequence of ordered structure in the downstream segment of the T domain (residues 84 to 315) that prevents its insertion through the outer membrane via a translocation pore that includes OmpF.
大肠杆菌素E3的315个残基的N端T结构域通过与包括OmpF孔蛋白在内的外膜蛋白相互作用，在大肠杆菌素跨外膜转运中发挥作用。结构研究表明，T结构域的前83个残基是无序的。这个灵活的转运亚结构域包含TolB盒（第34至46位残基），在早期转运过程中它必须穿过外膜，使大肠杆菌素能够与周质中的TolB蛋白结合。在本研究中，发现大肠杆菌素的细胞毒性要求TolB盒的C端（第46位残基）与灵活亚结构域末端（第83位残基）之间的残基长度至少为19至23个。足够长度的大肠杆菌素E3分子在体外显示出与TolB的正常结合以及对OmpF通道的阻断。N端亚结构域的长度至关重要，因为它使TolB盒能够穿过外膜并与TolB相互作用。有人提出，长度限制是T结构域下游片段（第84至315位残基）中有序结构的结果，该结构阻止其通过包括OmpF的转运孔插入外膜。
