Tata Institute of Fundamental Research, Center for Interdisciplinary Sciences, Hyderabad 500107, India.
J Phys Chem B. 2021 May 27;125(20):5285-5295. doi: 10.1021/acs.jpcb.1c03011. Epub 2021 May 12.
Apolipoprotein E (apoE), a major determinant protein for lipid metabolism, actively participates in lipid transport in the central nervous system via high-affinity interaction with the low-density lipoprotein receptor (LDLR). Prior evidences indicate that the phospholipids first need to assemble around apoE before the protein can recognize its receptor. However, despite multiple attempts via spectroscopic and biochemical investigations, it is unclear what are the impacts of lipid assembly on the globular structure of apoE. Here, using a combination of all-atom and coarse-grained molecular dynamics simulations, we demonstrate that an otherwise compact tertiary fold of monomeric apoE3 spontaneously unwraps in an aqueous phospholipid solution in two distinct stages. Interestingly, these structural reorganizations are triggered by an initial localized binding of lipid molecules to the C-terminal domain of the protein, which induce a rapid separation of the C-terminal domain of apoE3 from the rest of its tertiary fold. This is followed by a slow lipid-induced interhelix separation event within the N-terminal domain of the protein, as seen in an extensively long coarse-grained simulation. Remarkably, the resultant complex takes the shape of an "open conformation" of the lipid-stabilized unwrapped protein, which intriguingly coincides with an earlier proposal by a small-angle X-ray scattering (SAXS) experiment. The lipid-binding activity and the lipid-induced protein conformation are found to be robust across a monomeric mutant and wild-type sequence of apoE3. The "open" complex derived in coarse-grained simulation retains its structural morphology after reverse-mapping to the all-atom representation. Collectively, the investigation puts forward a plausible structure of currently elusive conformationally activated state of apoE3, which is primed for recognition by the lipoprotein receptor and can be exploited for eventual lipid transport.
载脂蛋白 E（apoE）是脂质代谢的主要决定蛋白，通过与低密度脂蛋白受体（LDLR）的高亲和力相互作用，积极参与中枢神经系统中的脂质转运。先前的证据表明，磷脂首先需要在蛋白质识别其受体之前围绕 apoE 组装。然而，尽管通过光谱和生化研究进行了多次尝试，但仍不清楚脂质组装对 apoE 球蛋白结构的影响。在这里，我们使用全原子和粗粒度分子动力学模拟的组合，证明了单体 apoE3 的原本紧凑的三级结构在水性磷脂溶液中以两个不同的阶段自发展开。有趣的是，这些结构重排是由最初脂质分子与蛋白质 C 末端结构域的局部结合引发的，这导致 apoE3 的 C 末端结构域与其三级结构的其余部分快速分离。接下来，在蛋白质的 N 末端结构域内发生缓慢的脂质诱导的螺旋间分离事件，如在广泛的粗粒度模拟中所见。值得注意的是，所得复合物呈现出脂质稳定的展开蛋白的“开放构象”，这与小角度 X 射线散射（SAXS）实验的早期提议相吻合。脂质结合活性和脂质诱导的蛋白质构象在 apoE3 的单体突变体和野生型序列中均表现出稳健性。在粗粒度模拟中得出的“开放”复合物在反向映射到全原子表示后保留其结构形态。总的来说，该研究提出了 apoE3 目前难以捉摸的构象激活状态的合理结构，为脂蛋白受体的识别做好准备，并可用于最终的脂质转运。
