Biochemistry. 2014 Jun 24;53(24):4025-33. doi: 10.1021/bi500340z.
The human apolipoprotein (apo) E4 isoform, which differs from wild-type apoE3 by the single amino acid substitution C112R, is associated with elevated risk of cardiovascular and Alzheimer’s diseases, but the molecular basis for this variation between isoforms is not understood. Human apoE is a two-domain protein comprising an N-terminal helix bundle and a separately folded C-terminal region. Here, we examine the concept that the ability of the protein to bind to lipid surfaces is influenced by the stability (or readiness to unfold) of these domains. The lipid-free structures and abilities to bind to lipid and lipoprotein particles of a series of human and mouse apoE variants with varying domain stabilities and domain–domain interactions are compared. As assessed by urea denaturation, the two domains are more unstable in apoE4 than in apoE3. To distinguish the contributions of the destabilization of each domain to the greater lipid-binding ability of apoE4, the properties of the apoE4 R61T and E255A variants, which have the same helix bundle stabilities but altered C-terminal domain stabilities, are compared. In these cases, the effects on lipid-binding properties are relatively minor, indicating that the destabilization of the helix bundle domain is primarily responsible for the enhanced lipid-binding ability of apoE4. Unlike human apoE, mouse apoE behaves essentially as a single domain, and its lipid-binding characteristics are more similar to those of apoE4. Together, the results show that the overall stability of the entire apoE molecule exerts a major influence on its lipid- and lipoprotein-binding properties.
人类载脂蛋白（apo）E4 异构体与野生型 apoE3 不同，仅在单个氨基酸取代 C112R 处有所不同，与心血管疾病和阿尔茨海默病的风险增加有关，但这种异构体之间的差异的分子基础尚不清楚。人类 apoE 是一种由两个结构域组成的蛋白质，包括 N 端螺旋束和单独折叠的 C 端区域。在这里，我们研究了这样一种概念，即蛋白质与脂质表面结合的能力受这些结构域的稳定性（或展开准备状态）的影响。比较了一系列具有不同结构域稳定性和结构域间相互作用的人源和鼠源 apoE 变体的无脂结构和与脂质及脂蛋白颗粒结合的能力。如尿素变性评估所示，apoE4 中的两个结构域比 apoE3 中的更不稳定。为了区分每个结构域的去稳定化对 apoE4 更大的脂质结合能力的贡献，比较了 apoE4 R61T 和 E255A 变体的特性，这些变体具有相同的螺旋束稳定性但改变了 C 端结构域稳定性。在这些情况下，对脂质结合特性的影响相对较小，表明螺旋束结构域的去稳定化主要负责 apoE4 增强的脂质结合能力。与人类 apoE 不同，鼠源 apoE 基本上表现为单个结构域，其脂质结合特性与 apoE4 更为相似。总之，这些结果表明整个 apoE 分子的整体稳定性对其脂质和脂蛋白结合特性有很大影响。
