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

哺乳动物体内的胆固醇稳态受到其生物合成、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表面，LDLR与LDL-C结合，然后通过受体介导的内吞作用将复合物内化。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被锌金属蛋白酶骨形态发生蛋白1 （BMP1）在其配体结合域中的生理相关位点进行蛋白水解裂解（Banerjee etal . 2019）。BMP1对LDLR的裂解降低了LDL-C的结合和内吞作用。因此，BMP1蛋白裂解ldl - lr是调节细胞LDL-C摄取的一种新机制。我们假设BMP1、LDLR和其他细胞成分之间的分子相互作用调节了BMP1对LDLR的蛋白水解裂解，并影响了LDL-C代谢。为了解决这一假设，本提案的目标是：1)确定BMP1上的非催化蛋白-蛋白相互作用结构域在LDLR识别和切割中的作用；2)鉴定结合BMP1或LDLR的蛋白，以调节BMP1对LDLR的蛋白水解裂解，从而调节LDL-C内吞作用。这是一个好奇心驱动的生物科学项目，将为控制细胞摄取胆固醇的生物学机制提供新的理解，并可能导致调节低密度脂蛋白受体（LDLR）作用和管理胆固醇稳态的新分子靶点。因此，该项目解决了BBSRC在“推进生物科学发现前沿”主题下的“理解生命规则”优先领域。

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.