Molecular mechanisms of low-density lipoprotein receptor regulation by the protease BMP1

哺乳动物体内胆固醇的稳态受其生物合成、饮食摄入和细胞从血液中摄取的严格控制。低密度脂蛋白受体（LDLR）负责细胞以低密度脂蛋白胆固醇（LDL-C）的形式摄取胆固醇。在肝细胞和其他细胞的表面，LDLR结合LDL-C，然后复合物通过受体介导的内吞作用内化。LDLR的活性通过其生物合成、表面定位、内化、再循环和降解受到严格调节，以维持细胞胆固醇稳态。最近，我们发现LDLR在其配体结合域内的生理相关位点被锌金属蛋白酶骨形态发生蛋白1（BMP 1）蛋白水解裂解（Banerjee et al. 2019）。BMP 1对LDLR的切割降低了LDL-C的结合和内吞作用。因此，BMP 1对LDLR的蛋白水解裂解是调节细胞LDL-C摄取的新机制。我们假设BMP 1、LDLR和其他细胞组分之间的分子相互作用调节BMP 1对LDLR的蛋白水解裂解，并影响LDL-C代谢。为了解决这一假设，本提案的目的是：1）确定BMP 1上的非催化蛋白质-蛋白质相互作用结构域在LDLR识别和裂解中的作用; 2）鉴定结合BMP 1或LDLR以调节BMP 1对LDLR的蛋白水解裂解并因此调节LDL-C内吞作用的蛋白质。这是一个好奇心驱动的生物科学项目，将提供对控制细胞摄取胆固醇的生物机制的新理解，并可能导致新的分子靶点来调节低密度脂蛋白受体（LDLR）作用和管理胆固醇稳态。因此，该项目在“推进生物科学发现前沿”的主题范围内处理生物科学与资源中心的优先领域“了解生命规则”。

Cholesterol homeostasis in mammals is tightly controlled by its biosynthesis, dietary intake and cellular uptake from the blood. The low-density lipoprotein receptor (LDLR) is responsible for the cellular uptake of cholesterol in the form of low-density lipoprotein cholesterol (LDL-C). At the surface of hepatocytes and other cells, LDLR binds LDL-C and the complex is then internalized by receptor-mediated endocytosis. The activity of LDLR is tightly regulated through its biosynthesis, surface localisation, internalisation, recycling and degradation, in order to maintain cellular cholesterol homeostasis. Recently we have identified that LDLR is proteolytically cleaved at a physiologically-relevant site within its ligand-binding domain by the zinc metalloprotease bone morphogenetic protein 1 (BMP1) (Banerjee et al. 2019). The cleavage of LDLR by BMP1 reduces the binding and endocytosis of LDL-C. Thus proteolytic cleavage of LDLR by BMP1 is a new mechanism to regulate cellular LDL-C uptake. We hypothesise that molecular interactions between BMP1, LDLR and other cellular components regulate the proteolytic cleavage of LDLR by BMP1 and impact on LDL-C metabolism. To address this hypothesis, the objectives of this proposal are to: 1) determine the role of the non-catalytic protein-protein interaction domains on BMP1 in the recognition and cleavage of LDLR; and 2) identify the proteins that bind BMP1 or LDLR to regulate the proteolytic cleavage of LDLR by BMP1 and hence LDL-C endocytosis. This is a curiosity-driven bioscience project that will provide new understanding of the biological mechanisms controlling the cellular uptake of cholesterol and could lead to new molecular targets to modulate low-density lipoprotein receptor (LDLR) action and manage cholesterol homeostasis. Thus, the project addresses the BBSRC priority areas of "Understanding the rules of life" within the "Advancing the frontiers of bioscience discovery" theme.