Leka Oneda, Vallese Francesca, Pirazzini Marco, Berto Paola, Montecucco Cesare, Zanotti Giuseppe
Department of Biomedical Sciences, University of Padua, Italy.
FEBS J. 2014 May;281(9):2115-22. doi: 10.1111/febs.12783. Epub 2014 Apr 1.
Diphtheria toxin (DT), the etiological agent of the homonymous disease, like other bacterial toxins, has to undergo a dramatic structural change in order to be internalized into the cytosol, where it finally performs its function. The molecular mechanism of toxin transit across the membrane is not well known, but the available experimental evidence indicates that one of the three domains of the toxin, called the central α-helical domain, inserts into the lipid bilayer, so favoring the translocation of the catalytic domain. This process is driven by the acidic pH of the endosomal lumen. Here, we describe the crystal structure of DT grown at acidic pH in the presence of bicelles. We were unable to freeze the moment of DT insertion into the lipid bilayer, but our crystal structure indicates that the low pH causes the unfolding of the TH2, TH3 and TH4 α-helices. This event gives rise to the exposure of a hydrophobic surface that includes the TH5 and TH8 α-helices, and the loop region connecting the TH8 and TH9 α-helices. Their exposure is probably favored by the presence of lipid bilayers in the crystallization solution, and they appear to be ready to insert into the membrane.
Coordinates and structure factors have been deposited in the Protein Data Bank under accession number 4OW6.
白喉毒素（DT）是同名疾病的病原体，与其他细菌毒素一样，必须经历剧烈的结构变化才能内化到胞质溶胶中，最终在那里发挥其功能。毒素跨膜转运的分子机制尚不清楚，但现有的实验证据表明，毒素的三个结构域之一，称为中央α-螺旋结构域，插入脂质双层，从而促进催化结构域的转运。这个过程由内体腔的酸性pH驱动。在这里，我们描述了在存在双分子层的情况下在酸性pH下生长的DT的晶体结构。我们无法冻结DT插入脂质双层的瞬间，但我们的晶体结构表明低pH导致TH2、TH3和TH4α-螺旋展开。这一事件导致包括TH5和TH8α-螺旋以及连接TH8和TH9α-螺旋的环区域的疏水表面暴露。它们的暴露可能受到结晶溶液中脂质双层的存在的促进，并且它们似乎准备好插入膜中。
坐标和结构因子已存入蛋白质数据库，登录号为4OW6。
