Role of Ifitm/Fragilis proteins as intracellular shuttles during cell activation

描述（由申请人提供）：本项目的中心重点是测试Ifitm蛋白下调细胞激活并通过将修饰酶穿梭到细胞表面以降解激活复合物来预防病毒感染的假设。细胞免疫应答的激活往往需要通过细胞表面受体发出信号。这一事件将激活蛋白（激酶等）和底物（受体和细胞内信号伙伴）带入激活域，如脂筏。在某些细胞（吞噬细胞，如巨噬细胞和B细胞）中，许多这样的复合物被内化和降解。然而，这些细胞和其他细胞上的其他活化复合物仍留在细胞表面，需要修饰以阻止进一步的细胞活化。Ifitm蛋白的功能是帮助下调活化细胞，抑制粘附和增殖。此外，它们最近被认为是I型和II型干扰素激活后哺乳动物细胞抗病毒状态的主要决定因素。然而，Ifitm蛋白实现这些作用的机制尚未被描述。我们发现Ifitm蛋白可以与多种蛋白质结合，这些蛋白质的功能与通过添加底物（如泛素或SUMO）或酶切来修饰蛋白质有关。我们认为Ifitm蛋白被保存在细胞内，并在激活后释放到膜内。当Ifitm蛋白进行这种迁移时，它们携带这些修饰蛋白作为与膜内蛋白质结合的货物。这些货物蛋白进入活化复合物后，其作用是降解和/或修饰表面蛋白，从而抑制细胞活化。同样的途径也被认为具有控制病毒感染的功能。我们建议针对一种特定的货物蛋白，Bat5，它已被证明与人和小鼠的Ifitm蛋白结合。我们进一步提出，该通路的缺失将允许不受抑制的细胞激活，可能导致细胞增殖增加和缺乏免疫控制，从而导致自身免疫和组织损伤。

DESCRIPTION (provided by applicant): The central focus of this project is to test the hypothesis that the Ifitm proteins down-modulate cell activation and prevent viral infections by shuttling modifying enzymes to the cell surface to degrade activation complexes. The activation of cells of the immune response often requires signaling through cell surface receptors. This event brings into activation domains, such as lipid rafts, both the activating proteins (kinases, etc) and the substrates (receptors and intracellular signaling partners). In some cells (phagocytic cells such as macrophages and B cells) many such complexes are internalized and degraded. Other activation complexes on these and other cells, however, remain on the cell surface and require modification to block further cellular activation. The Ifitm proteins have been characterized as functioning to help down-regulate activated cells, depressing adhesion and proliferation. In addition they were recently characterized as the major determinants of the anti-viral state in mammalian cells after activation with type I and type II interferons. However, the mechanism by which the Ifitm proteins accomplish these effects has not been described. We have found that the Ifitm proteins can bind to a variety of proteins whose functions are associated with the modification of proteins by the addition of substrates (such as ubiquitin or SUMO) or enzymatic cleavage. We propose that the Ifitm proteins are held within the cell and are released following activation to intercalate within the membrane. As the Ifitm proteins make this migration, they carry these modifying proteins as cargo for association with proteins within the membrane. The delivery of these cargo proteins into activation complexes is followed by their action to degrade and/or modify surface proteins thus depressing cell activation. This same pathway is also proposed to function to control viral infection. We propose to target one specific cargo protein in particular, Bat5, which has been shown to bind to the Ifitm proteins of man and mouse. We further propose that the absence of this pathway will allow for uninhibited cellular activation potentially leading to increased cell proliferation and lack of immune control that could lead to autoimmunity and tissue damage. PUBLIC HEALTH RELEVANCE: The control of cell activation requires pathways that down regulate activating complexes and components. The Ifitm proteins have been implicated as helping depress cell activation and enhance innate immune defenses to certain viral infections but no mechanisms as to how they accomplish this have been demonstrated. We propose to test the hypothesis that the Ifitm proteins shuttle cargo enzymes into membrane activation complexes. These enzymes then target constituents of such sites for modification and degradation thus depressing activation and inhibiting infection.