Miller C E, Majewski J, Watkins E B, Weygand M, Kuhl T L
Manuel Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
Biophys J. 2008 Jul;95(2):641-7. doi: 10.1529/biophysj.107.120808. Epub 2008 Mar 21.
The structure of cholera toxin (CTAB(5)) bound to its putative ganglioside receptor, galactosyl-N-acetylgalactosaminyl (N-acetyl-neuraminyl) galactosylglucosylceramide (GM(1)), in a lipid monolayer at the air-water interface has been studied utilizing grazing incidence x-ray diffraction. Cholera toxin is one of very few proteins to be crystallized in two dimensions and characterized in a fully hydrated state. The observed grazing incidence x-ray diffraction Bragg peaks indicated cholera toxin was ordered in a hexagonal lattice and the order extended 600-800 A. The pentameric binding portion of cholera toxin (CTB(5)) improved in-plane ordering over the full toxin (CTAB(5)) especially at low pH. Disulfide bond reduction (activation of the full toxin) also increased the protein layer ordering. These findings are consistent with A-subunit flexibility and motion, which cause packing inefficiencies and greater disorder of the protein layer. Corroborative out-of-plane diffraction (Bragg rod) analysis indicated that the scattering units in the cholera layer with CTAB(5) shortened after disulfide bond reduction of the A subunit. These studies, together with Part I results, revealed key changes in the structure of the cholera toxin-lipid system under different pH conditions.
利用掠入射X射线衍射研究了霍乱毒素（CTAB(5)）与其假定的神经节苷脂受体半乳糖基-N-乙酰半乳糖胺基（N-乙酰神经氨酰基）半乳糖基葡萄糖基神经酰胺（GM(1)）在气-水界面脂质单层中的结构。霍乱毒素是极少数能在二维结晶并在完全水合状态下进行表征的蛋白质之一。观察到的掠入射X射线衍射布拉格峰表明霍乱毒素以六方晶格排列，且排列延伸600 - 800 Å。霍乱毒素的五聚体结合部分（CTB(5)）相较于完整毒素（CTAB(5)），尤其是在低pH值下，改善了面内排列。二硫键还原（完整毒素的激活）也增加了蛋白质层的排列。这些发现与A亚基的灵活性和运动性一致，这会导致堆积效率低下以及蛋白质层更大的无序性。辅助的面外衍射（布拉格棒）分析表明，在A亚基二硫键还原后，含有CTAB(5)的霍乱层中的散射单元缩短。这些研究与第一部分的结果一起，揭示了在不同pH条件下霍乱毒素-脂质系统结构的关键变化。
