Pihl Rasmus, Jensen Rasmus K, Poulsen Emil C, Jensen Lisbeth, Hansen Annette G, Thøgersen Ida B, Dobó József, Gál Péter, Andersen Gregers R, Enghild Jan J, Thiel Steffen
Department of Biomedicine, Aarhus University, Aarhus, Denmark.
Department of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark.
Sci Adv. 2021 Jan 8;7(2). doi: 10.1126/sciadv.aba7381. Print 2021 Jan.
Inter-α-inhibitor heavy chain 4 (ITIH4) is a poorly characterized plasma protein that is proteolytically processed in multiple pathological conditions. However, no biological function of ITIH4 has been identified. Here, we show that ITIH4 is cleaved by several human proteases within a protease-susceptible region, enabling ITIH4 to function as a protease inhibitor. This is exemplified by its inhibition of mannan-binding lectin-associated serine protease-1 (MASP-1), MASP-2, and plasma kallikrein, which are key proteases for intravascular host defense. Mechanistically, ITIH4 acts as bait that, upon cleavage, forms a noncovalent, inhibitory complex with the executing protease that depends on the ITIH4 von Willebrand factor A domain. ITIH4 inhibits the MASPs by sterically preventing larger protein substrates from accessing their active sites, which remain accessible and fully functional toward small substrates. Thus, we demonstrate that ITIH4 functions as a protease inhibitor by a previously undescribed inhibitory mechanism.
α-抑制因子重链4（ITIH4）是一种特征不明的血浆蛋白，在多种病理条件下会发生蛋白水解加工。然而，ITIH4的生物学功能尚未明确。在此，我们表明ITIH4在一个蛋白酶敏感区域内被几种人类蛋白酶切割，从而使ITIH4能够作为一种蛋白酶抑制剂发挥作用。这以其对甘露聚糖结合凝集素相关丝氨酸蛋白酶-1（MASP-1）、MASP-2和血浆激肽释放酶的抑制作用为例，这些是血管内宿主防御的关键蛋白酶。从机制上讲，ITIH4作为诱饵，在被切割后与执行蛋白酶形成一种非共价抑制复合物，该复合物依赖于ITIH4的血管性血友病因子A结构域。ITIH4通过空间位阻阻止较大的蛋白质底物接近其活性位点来抑制MASP，而这些活性位点对小底物仍然可及且功能完全正常。因此，我们证明ITIH4通过一种先前未描述的抑制机制发挥蛋白酶抑制剂的作用。
