Agonist-induced endocytosis is a key regulatory mechanism for controlling the responsiveness of the cell by changing the density of cell surface receptors. In addition to the role of endocytosis in signal termination, endocytosed G protein–coupled receptors (GPCRs) have been found to signal from intracellular compartments of the cell. Arrestins are generally believed to be the master regulators of GPCR endocytosis by binding to both phosphorylated receptors and adaptor protein 2 (AP-2) or clathrin, thus recruiting receptors to clathrin-coated pits to facilitate the internalization process. However, many other functions have been described for arrestins that do not relate to their role in terminating signaling. Additionally, there are now more than 30 examples of GPCRs that internalize independently of arrestins. Here we review the methods, pharmacological tools, and cellular backgrounds used to determine the role of arrestins in receptor internalization, highlighting their advantages and caveats. We also summarize key examples of arrestin-independent GPCR endocytosis in the literature and their suggested alternative endocytosis pathway (e.g., the caveolae-dependent and fast endophilin-mediated endocytosis pathways). Finally, we consider the possible function of arrestins recruited to GPCRs that are endocytosed independently of arrestins, including the catalytic arrestin activation paradigm. Technological improvements in recent years have advanced the field further, and, combined with the important implications of endocytosis on drug responses, this makes endocytosis an obvious parameter to include in molecular pharmacological characterization of ligand-GPCR interactions.

中文翻译：

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G蛋白偶联受体的arrestin依赖和独立的内在化：方法，机制和对细胞信号传导的启示

激动剂诱导的内吞作用是通过改变细胞表面受体的密度来控制细胞反应性的关键调节机制。除了内吞作用在信号终止中的作用外，还发现内吞的G蛋白偶联受体（GPCR）从细胞的细胞内区室发出信号。通常认为，抑制素通过与磷酸化受体和衔接蛋白2（AP-2）或网格蛋白结合，从而成为GPCR内吞作用的主要调节剂，从而将受体募集到网格蛋白包被的凹坑中以促进内化过程。但是，已经描述了抑制蛋白的许多其他功能，与它们在终止信号传导中的作用无关。另外，现在有30多个独立于抑制蛋白而内在化的GPCR实例。在这里，我们回顾一下这些方法，用来确定抑制蛋白在受体内在化中的作用的药理学工具和细胞背景，突出了它们的优势和警告。我们还总结了文献中与抑制蛋白无关的GPCR内吞作用的关键实例及其建议的替代内吞途径（例如，小窝依赖和快速的内啡肽介导的内吞途径）。最后，我们考虑了募集到GPCR的抑制蛋白的可能功能，这些蛋白独立于抑制蛋白而被内吞，包括催化抑制蛋白激活范例。近年来的技术进步进一步推动了该领域的发展，并结合内吞作用对药物反应的重要影响，这使内吞作用成为包括在配体-GPCR相互作用的分子药理学表征中的明显参数。