Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
Proc Natl Acad Sci U S A. 2010 Jun 15;107(24):10990-5. doi: 10.1073/pnas.1004153107. Epub 2010 Jun 1.
Lipopolysaccharide (LPS) of Gram-negative bacteria is a common pathogen-associated molecular pattern (PAMP) that induces potent innate immune responses. The host immune response against LPS is triggered by myeloid differentiation factor 2 (MD-2) in association with Toll-like receptor 4 (TLR4) on the cell surface. The MD-2/TLR4-mediated LPS response is regulated by the evolutionarily related complex of MD-1 and Toll-like receptor homolog RP105. Here, we report crystallographic and biophysical data that demonstrate a previously unidentified direct interaction of MD-1 with LPS. The crystal structure of chicken MD-1 (cMD-1) at 2.0 A resolution exhibits a beta-cup-like fold, similar to MD-2, that encloses a hydrophobic cavity between the two beta-sheets. A lipid-like moiety was observed inside the cavity, suggesting the possibility of a direct MD-1/LPS interaction. LPS was subsequently identified as an MD-1 ligand by native gel electrophoresis and gel filtration analyses. The crystal structure of cMD-1 with lipid IVa, an LPS precursor, at 2.4 A resolution revealed that the lipid inserts into the deep hydrophobic cavity of the beta-cup-like structure, but with some important differences compared with MD-2. These findings suggest that soluble MD-1 alone, in addition to its complex with RP105, can regulate host LPS sensitivity.
革兰氏阴性菌的脂多糖（LPS）是一种常见的病原体相关分子模式（PAMP），能诱导强烈的先天免疫反应。宿主对 LPS 的免疫反应是由细胞表面的髓样分化因子 2（MD-2）与 Toll 样受体 4（TLR4）共同触发的。MD-2/TLR4 介导的 LPS 反应受 MD-1 和 Toll 样受体同源物 RP105 的进化相关复合物调节。在这里，我们报告了晶体学和生物物理数据，这些数据证明了 MD-1 与 LPS 之间存在以前未被识别的直接相互作用。2.0Å分辨率的鸡 MD-1（cMD-1）晶体结构呈现出 β 杯状折叠，类似于 MD-2，在两个β 片层之间包围着一个疏水性腔。腔内观察到一个类脂样部分，提示可能存在 MD-1/LPS 直接相互作用。随后通过天然凝胶电泳和凝胶过滤分析鉴定 LPS 是 MD-1 的配体。2.4Å分辨率的 cMD-1 与 LPS 前体脂质 IVa 的晶体结构显示，脂质插入到 β 杯状结构的深疏水性腔中，但与 MD-2 相比存在一些重要差异。这些发现表明，可溶性 MD-1 本身，除了与 RP105 的复合物外，还可以调节宿主对 LPS 的敏感性。
