Department of Pathology, University of Cambridge, Cambridge CB2 1QP, United Kingdom.
Proc Natl Acad Sci U S A. 2022 Sep 6;119(36):e2208662119. doi: 10.1073/pnas.2208662119. Epub 2022 Aug 29.
In gram-negative bacteria, lipoproteins are vital structural components of the outer membrane (OM) and crucial elements of machineries central to the physiology of the cell envelope. A dedicated apparatus, the Lol system, is required for the correct localization of OM lipoproteins and is essential for viability. The periplasmic chaperone LolA is central to this trafficking pathway, accepting triacylated lipoproteins from the inner membrane transporter LolCDE, before carrying them across the periplasm to the OM receptor LolB. Here, we report a crystal structure of liganded LolA, generated in vivo, revealing the molecular details of lipoprotein association. The structure highlights how LolA, initially primed to receive lipoprotein by interaction with LolC, further opens to accommodate the three ligand acyl chains in a precise conformation within its cavity. LolA forms extensive interactions with the acyl chains but not with any residue of the cargo, explaining the chaperone's ability to transport structurally diverse lipoproteins. Structural characterization of a ligandedLolA variant incapable of lipoprotein release reveals aberrant association, demonstrating the importance of the LolCDE-coordinated, sequential opening of LolA for inserting lipoprotein in a manner productive for subsequent trafficking. Comparison with existing structures of LolA in complex with LolC or LolCDE reveals substantial overlap of the lipoprotein and LolC binding sites within the LolA cavity, demonstrating that insertion of lipoprotein acyl chains physically disengages the chaperone protein from the transporter by perturbing interaction with LolC. Taken together, our data provide a key step toward a complete understanding of a fundamentally important trafficking pathway.
在革兰氏阴性菌中，脂蛋白是外膜（OM）的重要结构组成部分，也是细胞包膜生理学中关键机制的重要元素。一个专门的装置，Lol 系统，是正确定位 OM 脂蛋白所必需的，也是生存所必需的。周质伴侣蛋白 LolA 是这个运输途径的核心，它从内膜转运蛋白 LolCDE 接收三酰化脂蛋白，然后将其穿过周质转运到 OM 受体 LolB。在这里，我们报告了一个结合配体的 LolA 的晶体结构，该结构是在体内产生的，揭示了脂蛋白结合的分子细节。该结构突出了 LolA 如何通过与 LolC 的相互作用最初被激活以接收脂蛋白，然后进一步打开以在其腔中精确构象容纳三个配体酰基链。LolA 与酰基链形成广泛的相互作用，但与货物的任何残基都没有相互作用，这解释了伴侣蛋白能够转运结构多样的脂蛋白的能力。不能释放脂蛋白的结合配体 LolA 变体的结构特征揭示了异常的关联，证明了 LolCDE 协调的、顺序打开 LolA 对于以有利于随后运输的方式插入脂蛋白的重要性。与 LolA 与 LolC 或 LolCDE 复合物的现有结构进行比较，揭示了脂蛋白和 LolC 结合位点在 LolA 腔中的大量重叠，证明插入脂蛋白的酰基链通过扰乱与 LolC 的相互作用，使伴侣蛋白与转运体物理分离。总之，我们的数据为全面理解一个基本重要的运输途径提供了关键的一步。
