Department of Pharmacology, Vanderbilt University, Nashville, TN, 37232, USA.
Cell Mol Life Sci. 2019 Nov;76(22):4413-4421. doi: 10.1007/s00018-019-03272-5. Epub 2019 Aug 17.
Mammalian arrestins are a family of four highly homologous relatively small ~ 45 kDa proteins with surprisingly diverse functions. The most striking feature is that each of the two non-visual subtypes can bind hundreds of diverse G protein-coupled receptors (GPCRs) and dozens of non-receptor partners. Through these interactions, arrestins regulate the G protein-dependent signaling by the desensitization mechanisms as well as control numerous signaling pathways in the G protein-dependent or independent manner via scaffolding. Some partners prefer receptor-bound arrestins, some bind better to the free arrestins in the cytoplasm, whereas several show no apparent preference for either conformation. Thus, arrestins are a perfect example of a multi-functional signaling regulator. The result of this multi-functionality is that reduction (by knockdown) or elimination (by knockout) of any of these two non-visual arrestins can affect so many pathways that the results are hard to interpret. The other difficulty is that the non-visual subtypes can in many cases compensate for each other, which explains relatively mild phenotypes of single knockouts, whereas double knockout is lethal in vivo, although cultured cells lacking both arrestins are viable. Thus, deciphering the role of arrestins in cell biology requires the identification of specific signaling function(s) of arrestins involved in a particular phenotype. This endeavor should be greatly assisted by identification of structural elements of the arrestin molecule critical for individual functions and by the creation of mutants where only one function is affected. Reintroduction of these biased mutants, or introduction of monofunctional stand-alone arrestin elements, which have been identified in some cases, into double arrestin-2/3 knockout cultured cells, is the most straightforward way to study arrestin functions. This is a laborious and technically challenging task, but the upside is that specific function of arrestins, their timing, subcellular specificity, and relations to one another could be investigated with precision.
哺乳动物 arrestins 是一个由四个高度同源的相对较小（~45 kDa）蛋白组成的家族，具有惊人多样的功能。最显著的特点是，两种非视觉亚型中的每一种都可以结合数百种不同的 G 蛋白偶联受体（GPCR）和数十种非受体伴侣。通过这些相互作用，arrestins 通过脱敏机制调节 G 蛋白依赖性信号转导，并通过支架以 G 蛋白依赖性或非依赖性方式控制许多信号通路。一些伴侣更喜欢与受体结合的 arrestins，一些与细胞质中游离的 arrestins 结合更好，而有一些则对任何一种构象都没有明显偏好。因此，arrestins 是一个多功能信号调节剂的完美范例。这种多功能性的结果是，任何两种非视觉 arrestins 的减少（通过敲低）或消除（通过敲除）都会影响如此多的途径，以至于结果难以解释。另一个困难是，非视觉亚型在许多情况下可以相互补偿，这解释了单敲除的相对温和的表型，而双敲除在体内是致命的，尽管缺乏两种 arrestins 的培养细胞是有活力的。因此，解析 arrestins 在细胞生物学中的作用需要确定参与特定表型的 arrestins 的特定信号转导功能。通过鉴定对单个功能至关重要的 arrestin 分子的结构元件，并创建只有一个功能受到影响的突变体，将极大地帮助这项工作。将这些偏向性突变体重新引入或引入已在某些情况下鉴定的单功能独立 arrestin 元件，是研究 arrestin 功能的最直接方法。这是一项费力且具有技术挑战性的任务，但好处是可以精确研究 arrestins 的特定功能、它们的时间、亚细胞特异性以及彼此之间的关系。
