Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA.
Department of Medicine, University of Washington, Seattle, WA, USA.
Nat Commun. 2022 Aug 16;13(1):4812. doi: 10.1038/s41467-022-32437-3.
Production of high density lipoprotein (HDL) requires ATP-binding cassette transporter A1 (ABCA1) to drive phospholipid (PL) from the plasma membrane into extracellular apolipoprotein A-I. Here, we use simulations to show that domains of ABCA1 within the plasma membrane remove PL from the membrane's outer leaflet. In our simulations, after the lipid diffuses into the interior of ABCA1's outward-open cavity, PL extracted by the gateway passes through a ring-shaped domain, the annulus orifice, which forms the base of an elongated hydrophobic tunnel in the transporter's extracellular domain. Engineered mutations in the gateway and annulus strongly inhibit lipid export by ABCA1 without affecting cell-surface expression levels. Our finding that ABCA1 extracts lipid from the outer face of the plasma membrane and forces it through its gateway and annulus into an elongated hydrophobic tunnel contrasts with the alternating access model, which proposes that ABCA1 flops PL substrate from the inner leaflet to the outer leaflet of the membrane. Consistent with our model, ABCA1 lacks the charged amino acid residues in the transmembrane domain found in the floppase members of the ABC transporter family.
高密度脂蛋白（HDL）的生成需要 ATP 结合盒转运蛋白 A1（ABCA1）将磷脂（PL）从质膜转运到细胞外载脂蛋白 A-I。在这里，我们使用模拟表明 ABCA1 质膜内的结构域可将 PL 从膜的外叶层中去除。在我们的模拟中，脂质扩散到 ABCA1 的外向开放腔内部后，由门户提取的 PL 通过一个环形结构域（环孔），该环孔形成了位于跨膜蛋白胞外结构域中的长疏水通道的底部。对门户和环孔进行工程改造的突变强烈抑制了 ABCA1 的脂质外排，而不影响细胞表面表达水平。我们的发现表明，ABCA1 从质膜的外叶层提取脂质，并将其通过门户和环孔强行推入长疏水通道，这与交替访问模型相反，该模型提出 ABCA1 将 PL 底物从质膜的内叶层翻转到外叶层。与我们的模型一致，ABCA1 缺乏 ABC 转运蛋白家族中的 floppase 成员的跨膜结构域中的带电荷氨基酸残基。
