Liu Min, Yuan Cai, Jensen Jan K, Zhao Baoyu, Jiang Yunbin, Jiang Longguang, Huang Mingdong
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China; University of Chinese Academy of Sciences, Beijing 100049, China.
College of Bioscience and Biotechnology, Fuzhou University, Fuzhou, Fujian, 350108, China.
J Biol Chem. 2017 May 19;292(20):8412-8423. doi: 10.1074/jbc.M117.779256. Epub 2017 Mar 27.
Hepatocyte growth factor activator inhibitor 1 (HAI-1) is a membrane-bound multidomain protein essential to the integrity of the basement membrane during placental development and is also important in maintaining postnatal homeostasis in many tissues. HAI-1 is a Kunitz-type serine protease inhibitor, and soluble fragments of HAI-1 with variable lengths have been identified The full-length extracellular portion of HAI-1 (sHAI-1) shows weaker inhibitory activity toward target proteases than the smaller fragments, suggesting auto-inhibition of HAI-1. However, this possible regulatory mechanism has not yet been evaluated. Here, we solved the crystal structure of sHAI-1 and determined the solution structure by small-angle X-ray scattering. These structural analyses revealed that, despite the presence of long linkers, sHAI-1 exists in a compact conformation in which sHAI-1 active sites in Kunitz domain 1 are sterically blocked by neighboring structural elements. We also found that in the presence of target proteases, sHAI-1 adopts an extended conformation that disables the auto-inhibition effect. Our results also reveal the roles of non-inhibitory domains of this multidomain protein and explain the low activity of the full-length protein. The structural insights gained here improve our understanding of the regulation of HAI-1 inhibitory activities and point to new approaches for better controlling these activities.
肝细胞生长因子激活剂抑制剂1（HAI-1）是一种膜结合的多结构域蛋白，对胎盘发育过程中基底膜的完整性至关重要，在维持许多组织的产后内环境稳定方面也很重要。HAI-1是一种Kunitz型丝氨酸蛋白酶抑制剂，已鉴定出不同长度的HAI-1可溶性片段。HAI-1的全长细胞外部分（sHAI-1）对靶蛋白酶的抑制活性比小片段弱，提示HAI-1存在自我抑制作用。然而，这种可能的调节机制尚未得到评估。在此，我们解析了sHAI-1的晶体结构，并通过小角X射线散射确定了其溶液结构。这些结构分析表明，尽管存在长连接子，sHAI-1仍以紧凑构象存在，其中Kunitz结构域1中的sHAI-1活性位点被相邻结构元件在空间上阻断。我们还发现，在存在靶蛋白酶的情况下，sHAI-1会采取一种延伸构象，从而消除自我抑制作用。我们的结果还揭示了这种多结构域蛋白非抑制性结构域的作用，并解释了全长蛋白活性较低的原因。此处获得的结构见解增进了我们对HAI-1抑制活性调节的理解，并指出了更好控制这些活性的新方法。
