Lipopolysaccharide (LPS) is an essential component of the outer membrane of most Gram-negative bacteria that provides resistance to various toxic compounds and antibiotics. Newly synthesized LPS is extracted from the inner membrane by the ATP-binding cassette (ABC) transporter LptB₂FGC, which places the glycolipid onto a periplasmic protein bridge that connects to the outer membrane. This ABC transporter is structurally unusual in that it associates with an additional protein, LptC. The periplasmic domain of LptC is part of the transporter's bridge while its transmembrane α-helix intercalates into the LPS-binding cavity of the core LptB₂FG transporter. LptC’s transmembrane helix affects the in vitro ATPase activity of LptB₂FG, but its role in LPS transport in cells remains undefined. Here, we describe two roles of LptC’s transmembrane helix in Escherichia coli. We demonstrate that it is required to maintain proper levels of LptC and participates in coupling the activity of the ATPase LptB to that of its transmembrane partners LptF/LptG prior to loading LPS onto the periplasmic bridge. Our data support a model in which the association of LptC’s transmembrane helix with LptFG creates a nonessential step that slows down the LPS transporter.

中文翻译：

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LptC 的跨膜α-螺旋参与 LptB2FGC 转运蛋白的 LPS 提取

脂多糖 (LPS) 是大多数革兰氏阴性细菌外膜的重要组成部分，可提供对各种有毒化合物和抗生素的抗性。_{新合成的 LPS 由 ATP 结合盒 (ABC) 转运蛋白 LptB 2} FGC从内膜提取，该转运蛋白将糖脂置于连接外膜的周质蛋白桥上。这种 ABC 转运蛋白在结构上是不寻常的，因为它与一种额外的蛋白质 LptC 相关联。LptC 的周质结构域是转运蛋白桥的一部分，而其跨膜α-螺旋插入核心 LptB ₂ FG 转运蛋白的 LPS 结合腔中。_{LptC 的跨膜螺旋影响 LptB 2}的体外 ATPase 活性FG，但它在细胞内 LPS 转运中的作用仍未确定。在这里，我们描述了 LptC 的跨膜螺旋在大肠杆菌中的两个作用。我们证明，在将 LPS 加载到周质桥上之前，它需要保持适当的 LptC 水平并参与将 ATPase LptB 的活性与其跨膜伙伴 LptF/LptG 的活性偶联。我们的数据支持一个模型，其中 LptC 的跨膜螺旋与 LptFG 的关联产生了一个非必要的步骤，减缓了 LPS 转运蛋白。