Proprotein convertase subtilisin/kexin type-9 (PCSK9) is a ligand of low-density lipoprotein (LDL) receptor (LDLR) that promotes LDLR degradation in late endosomes/lysosomes. In human plasma, 30-40% of PCSK9 is bound to LDL particles; however, the physiological significance of this interaction remains unknown. LDL binding in vitro requires a disordered N-terminal region in PCSK9's prodomain. Here, we report that peptides corresponding to a predicted amphipathic α-helix in the prodomain N terminus adopt helical structure in a membrane-mimetic environment. This effect was greatly enhanced by an R46L substitution representing an atheroprotective PCSK9 loss-of-function mutation. A helix-disrupting proline substitution within the putative α-helical motif in full-length PCSK9 lowered LDL binding affinity >5-fold. Modeling studies suggested that the transient α-helix aligns multiple polar residues to interact with positively charged residues in the C-terminal domain. Gain-of-function PCSK9 mutations associated with familial hypercholesterolemia (FH) and clustered at the predicted interdomain interface (R469W, R496W, and F515L) inhibited LDL binding, which was completely abolished in the case of the R496W variant. These findings shed light on allosteric conformational changes in PCSK9 required for high-affinity binding to LDL particles. Moreover, the initial identification of FH-associated mutations that diminish PCSK9's ability to bind LDL reported here supports the notion that PCSK9-LDL association in the circulation inhibits PCSK9 activity.

中文翻译：

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PCSK9前域中的瞬时两亲性螺旋促进了与低密度脂蛋白颗粒的结合。

前蛋白转化酶枯草杆菌蛋白酶/ 9型kexin（PCSK9）是低密度脂蛋白（LDL）受体（LDLR）的配体，可促进晚期内体/溶酶体中的LDLR降解。在人类血浆中，30％至40％的PCSK9与LDL颗粒结合；然而，这种相互作用的生理意义仍然未知。体外LDL结合需要PCSK9前域的N端无序区域。在这里，我们报告说，对应于前域N末端中预测的两亲性α-螺旋的肽在膜模拟环境中采用螺旋结构。代表动脉粥样硬化保护性PCSK9功能丧失突变的R46L取代大大增强了这种效果。全长PCSK9中推定的α-螺旋基序内破坏螺旋的脯氨酸取代将LDL结合亲和力降低了5倍以上。建模研究表明，瞬时α螺旋可使多个极性残基对齐，以与C末端结构域中带正电荷的残基相互作用。与家族性高胆固醇血症（FH）相关并聚集在预测的域间界面（R469W，R496W和F515L）上的功能性PCSK9突变抑制LDL结合，在R496W变体的情况下完全消除了LDL结合。这些发现揭示了高亲和力与LDL粒子结合所需的PCSK9的构构构象变化。此外，本文报道的减少PCSK9结合LDL能力的FH相关突变的初步鉴定支持循环中PCSK9-LDL结合抑制PCSK9活性的观点。