Finsen Laboratory, Copenhagen University Hospital-Rigshospitalet, DK-2200 Copenhagen N, Denmark.
Finsen Laboratory, Biotech Research and Innovation Centre, University of Copenhagen, DK-2200 Copenhagen N, Denmark.
Proc Natl Acad Sci U S A. 2023 May 2;120(18):e2221888120. doi: 10.1073/pnas.2221888120. Epub 2023 Apr 24.
The lipolytic processing of triglyceride-rich lipoproteins (TRLs) by lipoprotein lipase (LPL) is crucial for the delivery of dietary lipids to the heart, skeletal muscle, and adipose tissue. The processing of TRLs by LPL is regulated in a tissue-specific manner by a complex interplay between activators and inhibitors. Angiopoietin-like protein 4 (ANGPTL4) inhibits LPL by reducing its thermal stability and catalyzing the irreversible unfolding of LPL's α/β-hydrolase domain. We previously mapped the ANGPTL4 binding site on LPL and defined the downstream unfolding events resulting in LPL inactivation. The binding of LPL to glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 protects against LPL unfolding. The binding site on LPL for an activating cofactor, apolipoprotein C2 (APOC2), and the mechanisms by which APOC2 activates LPL have been unclear and controversial. Using hydrogen-deuterium exchange/mass spectrometry, we now show that APOC2's C-terminal α-helix binds to regions of LPL surrounding the catalytic pocket. Remarkably, APOC2's binding site on LPL overlaps with that for ANGPTL4, but their effects on LPL conformation are distinct. In contrast to ANGPTL4, APOC2 increases the thermal stability of LPL and protects it from unfolding. Also, the regions of LPL that anchor the lid are stabilized by APOC2 but destabilized by ANGPTL4, providing a plausible explanation for why APOC2 is an activator of LPL, while ANGPTL4 is an inhibitor. Our studies provide fresh insights into the molecular mechanisms by which APOC2 binds and stabilizes LPL-and properties that we suspect are relevant to the conformational gating of LPL's active site.
富含甘油三酯的脂蛋白 (TRLs) 的脂肪酶 (LPL) 脂解加工对于膳食脂质向心脏、骨骼肌和脂肪组织的输送至关重要。LPL 对 TRL 的加工受激活剂和抑制剂之间复杂相互作用的组织特异性调节。血管生成素样蛋白 4 (ANGPTL4) 通过降低其热稳定性并催化 LPL 的α/β-水解酶结构域的不可逆展开来抑制 LPL。我们之前在 LPL 上绘制了 ANGPTL4 结合位点，并定义了导致 LPL 失活的下游展开事件。LPL 与糖基磷脂酰肌醇锚定高密度脂蛋白结合蛋白 1 的结合可防止 LPL 展开。激活辅因子载脂蛋白 C2 (APOC2) 在 LPL 上的结合位点以及 APOC2 激活 LPL 的机制一直不清楚且存在争议。使用氘代/质谱法，我们现在表明 APOC2 的 C 末端α-螺旋与 LPL 周围的催化口袋区域结合。值得注意的是，APOC2 在 LPL 上的结合位点与 ANGPTL4 重叠，但它们对 LPL 构象的影响是不同的。与 ANGPTL4 相反，APOC2 增加了 LPL 的热稳定性并保护其免于展开。此外，APOC2 稳定了锚定盖的 LPL 区域，但 ANGPTL4 使其不稳定，这为为什么 APOC2 是 LPL 的激活剂，而 ANGPTL4 是抑制剂提供了合理的解释。我们的研究提供了关于 APOC2 结合和稳定 LPL 的分子机制的新见解-我们怀疑这些特性与 LPL 的活性位点构象门控相关。
