School of Life Sciences, Gwangju Institute of Science and Technology, Gwangju 61005, Korea.
Center for Cell Mechanobiology, Gwangju Institute of Science and Technology, Gwangju 61005, Korea.
Cells. 2020 Jul 6;9(7):1625. doi: 10.3390/cells9071625.
Apoptotic cells expressing phosphatidylserine (PS) on their cell surface are directly or indirectly recognized by phagocytes through PS-binding proteins. The PS-binding protein Tim-4 secures apoptotic cells to phagocytes to facilitate the engulfment of apoptotic cells. However, the molecular mechanism by which Tim-4 transduces signals to phagocytes during Tim-4-mediated efferocytosis is incompletely understood. Here, we report that Tim-4 collaborates with Mertk during efferocytosis through a biochemical interaction with Mertk. Proximal localization between the two proteins in phagocytes was observed by immunofluorescence and proximal ligation assays. Physical association between Tim-4 and Mertk, which was mediated by an interaction between the IgV domain of Tim-4 and the fibronectin type-III domain of Mertk, was also detected with immunoprecipitation. Furthermore, the effect of Mertk on Tim-4-mediated efferocytosis was abolished by GST-Mertk, a soluble form of the fibronectin type-III domain of Mertk that disrupts the interaction between Tim-4 and Mertk. Taken together, the results from our study suggest that a physical interaction between Tim-4 and Mertk is necessary for Mertk to enhance efferocytosis mediated by Tim-4.
凋亡细胞表面表达的磷脂酰丝氨酸（PS）可被吞噬细胞直接或间接识别，通过 PS 结合蛋白。PS 结合蛋白 Tim-4 将凋亡细胞固定在吞噬细胞上，以促进凋亡细胞的吞噬作用。然而，Tim-4 在 Tim-4 介导的胞吐作用过程中向吞噬细胞传递信号的分子机制尚不完全清楚。在这里，我们报道 Tim-4 通过与 Mertk 的生化相互作用，在胞吐作用过程中与 Mertk 协作。通过免疫荧光和邻近连接测定观察到在吞噬细胞中两种蛋白之间的近端定位。还通过免疫沉淀检测到 Tim-4 和 Mertk 之间的物理关联，该关联由 Tim-4 的 IgV 结构域与 Mertk 的纤连蛋白 III 结构域之间的相互作用介导。此外，GST-Mertk（Mertk 的纤连蛋白 III 结构域的可溶性形式）可破坏 Tim-4 和 Mertk 之间的相互作用，从而消除了 Mertk 对 Tim-4 介导的胞吐作用的影响。总之，我们的研究结果表明，Tim-4 和 Mertk 之间的物理相互作用对于 Mertk 增强 Tim-4 介导的胞吐作用是必需的。
