Many bacteria are protected by different types of polysaccharide capsules, structures formed of long repetitive glycan chains that are sometimes free and sometimes anchored to the outer membrane via lipid tails. One type, called group 4 capsule, results from the expression of the gfcABCDE-etp-etk operon in Escherichia coli. Of the proteins encoded in this operon, GfcE is thought to provide the export pore for free polysaccharide chains, but none of the proteins has been implicated in the export of chains carrying a lipid anchor. For this function, GfcD has been a focus of attention as the only outer membrane β-barrel encoded in the operon. AlphaFold predicts two β-barrel domains in GfcD, a canonical N-terminal one of 12 strands and an unusual C-terminal one of 13 strands, which features a large lateral aperture between strands β1 and β13. This immediately suggests a lateral exit gate for hydrophobic molecules into the membrane, analogous to the one proposed for the lipopolysaccharide export pore LptD. Here, we report an unsteered molecular dynamics study of GfcD embedded in the bacterial outer membrane, with the common polysaccharide anchor, lipid A, inserted in the pore of the C-terminal barrel. Our results show that the lateral aperture does not collapse during simulations and membrane lipids nevertheless do not penetrate the barrel but the lipid chains of the lipid A molecule readily exit into the membrane.

中文翻译：

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外膜蛋白 GfcD 脂质锚定多糖输出的计算模型


许多细菌受到不同类型的多糖胶囊的保护，这些多糖胶囊的结构由长重复的聚糖链形成，这些聚糖链有时是游离的，有时通过脂质尾部锚定在外膜上。一种类型称为第 4 组胶囊，由大肠杆菌中 gfcABCDE-etp-etk 操纵子的表达产生。在该操纵子编码的蛋白质中，GfcE 被认为为游离多糖链提供输出孔，但没有一种蛋白质与携带脂质锚的链的输出有关。对于此功能，GfcD 作为操纵子中编码的唯一外膜β桶一直是关注的焦点。AlphaFold 预测 GfcD 中的两个 β 桶结构域，一个 12 条链的规范 N 端结构域和一个不寻常的 13 条链的 C 端结构域，其特点是在 β1 和 β13 链之间有一个大的横向孔径。这立即表明疏水分子进入膜的横向出口门，类似于为脂多糖输出孔 LptD 提出的出口。在这里，我们报告了嵌入细菌外膜中的 GfcD 的无控制分子动力学研究，其中常见的多糖锚点脂质 A 插入 C 端桶的孔中。我们的结果表明，在模拟过程中，侧孔不会塌陷，膜脂质仍然不会穿透桶，但脂质 A 分子的脂质链很容易进入膜。