Department of Pharmacology & Toxicology, University of Alabama at Birmingham, Birmingham, AL 35205, USA.
Department of Microbiology, Immunology, and Molecular Genetics, University of Kentucky, Lexington, KY 40536, USA.
Int J Mol Sci. 2023 Aug 25;24(17):13221. doi: 10.3390/ijms241713221.
The Toxin Complex (Tc) superfamily consists of toxin translocases that contribute to the targeting, delivery, and cytotoxicity of certain pathogenic Gram-negative bacteria. Membrane receptor targeting is driven by the A-subunit (TcA), which comprises IgG-like receptor binding domains (RBDs) at the surface. To better understand XptA2, an insect specific TcA secreted by the symbiont from the intestine of entomopathogenic nematodes, we determined structures by X-ray crystallography and cryo-EM. Contrary to a previous report, XptA2 is pentameric. RBD-B exhibits an indentation from crystal packing that indicates loose association with the shell and a hotspot for possible receptor binding or a trigger for conformational dynamics. A two-fragment XptA2 lacking an intact linker achieved the folded pre-pore state like wild type (wt), revealing no requirement of the linker for protein folding. The linker is disordered in all structures, and we propose it plays a role in dynamics downstream of the initial pre-pore state.
毒素复合物 (Tc) 超家族由毒素转运蛋白组成，这些蛋白有助于某些致病性革兰氏阴性菌的靶向、输送和细胞毒性。膜受体靶向由 A 亚基 (TcA) 驱动，A 亚基在表面包含 IgG 样受体结合结构域 (RBD)。为了更好地了解由共生菌从昆虫病原线虫肠道中分泌的昆虫特异性 TcA XptA2，我们通过 X 射线晶体学和 cryo-EM 确定了结构。与之前的报告相反，XptA2 是五聚体。RBD-B 表现出晶体包装的凹陷，表明与外壳的松散结合以及可能的受体结合热点或构象动力学的触发点。缺乏完整连接子的两段式 XptA2 像野生型 (wt) 一样实现了折叠前孔状态，表明连接子对于蛋白折叠不是必需的。连接子在所有结构中都是无序的，我们提出它在初始前孔状态之后的动力学中发挥作用。
