Horstman Amanda L, Bauman Susanne J, Kuehn Meta J
Duke University Medical Center, Department of Biochemistry, Durham, North Carolina 27710, USA.
J Biol Chem. 2004 Feb 27;279(9):8070-5. doi: 10.1074/jbc.M308633200. Epub 2003 Dec 4.
In contrast to cholera toxin (CT), which is secreted solubly by Vibrio cholerae across the outer membrane, heat-labile enterotoxin (LT) is retained on the surface of enterotoxigenic Escherichia coli (ETEC) via an interaction with lipopolysaccharide (LPS). We examined the nature of the association between LT and LPS. Soluble LT binds to the surface of LPS deep-rough biosynthesis mutants but not to lipid A, indicating that only the Kdo (3-deoxy-d-manno-octulosonic acid) core is required for binding. Although capable of binding truncated LPS and Kdo, LT has a higher affinity for longer, more complete LPS species. A putative LPS binding pocket is proposed based on the crystal structure of the toxin. The ability to bind LPS and remain associated with the bacterial surface is not unique to LT, as CT also binds to E. coli LPS. However, neither LT nor CT is capable of binding to the surface of Vibrio. The core structures of Vibrio and E. coli LPS differ in that Vibrio contains a phosphorylated single Kdo-lipid A, and E. coli LPS contains unphosphorylated Kdo2-lipid A. We determined that the phosphate group on the Kdo core of Vibrio LPS prevents CT from binding, resulting in the secretion of soluble toxin. Because LT binds E. coli LPS, it remains associated with the extracellular bacterial surface and is released in association with outer membrane vesicles. We propose that difference in the extracellular fates of LT and CT contribute to the differences in disease caused by ETEC and Vibrio cholerae.
与霍乱弧菌通过外膜可溶性分泌的霍乱毒素（CT）不同，热不稳定肠毒素（LT）通过与脂多糖（LPS）相互作用而保留在产肠毒素大肠杆菌（ETEC）表面。我们研究了LT与LPS之间结合的性质。可溶性LT可与LPS深粗糙生物合成突变体表面结合，但不与脂质A结合，这表明只有Kdo（3-脱氧-D-甘露糖辛酸）核心是结合所必需的。尽管LT能够结合截短的LPS和Kdo，但它对更长、更完整的LPS种类具有更高的亲和力。基于毒素的晶体结构提出了一个假定的LPS结合口袋。结合LPS并与细菌表面保持结合的能力并非LT所独有，因为CT也能结合大肠杆菌LPS。然而，LT和CT都不能结合霍乱弧菌表面。霍乱弧菌和大肠杆菌LPS的核心结构不同，在于霍乱弧菌含有磷酸化的单个Kdo-脂质A，而大肠杆菌LPS含有未磷酸化的Kdo2-脂质A。我们确定霍乱弧菌LPS的Kdo核心上的磷酸基团会阻止CT结合，从而导致可溶性毒素的分泌。由于LT结合大肠杆菌LPS，它会与细胞外细菌表面保持结合，并与外膜囊泡一起释放。我们认为LT和CT在细胞外命运的差异导致了ETEC和霍乱弧菌所引起疾病的差异。
