A way to prevent deadly interaction Many metazoan proteins form oligomers, which is often mediated by modular domains such as BTB domains. Mena et al. now describe a quality control pathway they term dimerization quality control (DQC) (see the Perspective by Herhaus and Dikic). DQC monitors and prevents aberrant dimerization of BTB domain–containing proteins. The system relies on FBXL17, an adaptor protein that recruits an E3 ligase that specifically ubiquitylates nonfunctional BTB heterodimers, triggering their degradation. FBXL17 accesses a degradation signal at the BTB dimer interface in nonphysiological, nonfunctional complexes. The loss of DQC from Xenopus laevis embryos leads to lethal neurodevelopmental defects. Science, this issue p. eaap8236; see also p. 151 A quality control system ensures functional dimerization of a widespread protein interaction module by eliminating nonfunctional assemblies. INTRODUCTION Protein complex formation is at the heart of all metazoan signal transduction networks. Facilitating cellular information flow, modular BTB domains, leucine zippers, or coiled coils have been reused in many proteins, where they often mediate crucial homodimerization events. While mutation of a single allele encoding homodimeric proteins might poison signaling complexes, aberrant heterodimerization between related modules can also inhibit or alter the output of signal transduction cascades. Whether cells detect and eliminate protein complexes of aberrant composition has remained unknown. The globular BTB domain is found in ~220 human proteins that function as substrate adapters of CUL3 E3 ligases, transcription factors, or membrane channels. Proteins containing homodimeric BTB domains, such as KEAP1, KLHL3, KBTBD8, or BCL6, are essential for metazoan development, and their mutation or aberrant expression causes hypertension, cancer, or neurodegeneration. As it is not understood how organisms control the expression or activity of homodimeric BTB proteins, the BTB domain provides a physiologically important model to dissect the regulation of recurrent interaction modules. RATIONALE To identify regulatory mechanisms that impinge on modular interaction domains, we searched for shared binding partners of BTB proteins. Having found an E3 ligase that targets multiple BTB proteins for proteasomal degradation, we used biochemical reconstitution and protein complex engineering to dissect the underlying molecular control mechanism. Finally, we relied on Xenopus laevis embryos to study the organismal consequences of aberrant regulation of recurrent protein interaction modules. RESULTS Affinity purification and mass spectrometry experiments revealed that many BTB proteins heterodimerize, but also interacted with FBXL17, the substrate adapter of the SCFFBXL17 E3 ligase. SCFFBXL17 catalyzed the polyubiquitylation of BTB proteins to trigger their proteasomal degradation. SCFFBXL17 is therefore a rare example of an E3 ligase that targets a domain shared by many proteins, rather than a specific substrate. As shown by biochemical reconstitution and affinity purification from cells and animals, SCFFBXL17 is a quality control enzyme that detects and ubiquitylates inactive BTB heterodimers, yet ignores active homodimers of the same domains. Accordingly, the loss of FBXL17 increased heterodimerization of BTB proteins, yet at the same time reduced the ability of BTB proteins to engage their downstream targets. SCFFBXL17 therefore ensures that only functional BTB dimers are present in cells, an activity that we refer to as dimerization quality control (DQC). Depletion of FBXL17 in differentiating human embryonic stem cells showed that DQC prevented heterodimerization of KBTBD8, a BTB protein that is an essential regulator of neural crest specification. In line with this observation, the loss of DQC from Xenopus laevis embryos interfered with the differentiation, function, and survival of cells of the central and peripheral nervous system, including the neural crest. By contrast, somitogenesis or general body plan formation were initially unaffected. Similar to other quality control networks, the loss of DQC thus caused specific neuronal phenotypes. However, in addition to the known consequence of muted quality control, i.e. premature neuronal death, the effects of aberrant DQC were already observed early during differentiation. CONCLUSION We discovered DQC as a surveillance pathway that detects protein complexes of aberrant composition, rather than protein misfolding. We speculate that other recurrent interaction modules, such as leucine zippers or coiled coils, are monitored by similar DQC networks that rely on distinct E3 ligases. The neuronal phenotypes caused by DQC inactivation point to an active role of quality control in fate decisions in the nervous system. During evolution, DQC appeared at the same time as BTB domains multiplied in the vertebrate genome, suggesting that the ability to eliminate inactive heterodimers formed by related BTB domains contributed to the widespread use of this domain as a dimerization module. Dimerization quality control eliminates inactive heterodimers of a recurrent interaction module but leaves functional homodimers intact. SCFFBXL17 selectively ubiquitylates inactive BTB dimers, such as BTB heterodimers or dimers containing mutant BTB domains, which triggers their proteasomal degradation. Functional BTB homodimers escape detection by SCFFBXL17. Aberrant complex formation by recurrent interaction modules, such as BTB domains, leucine zippers, or coiled coils, can disrupt signal transduction, yet whether cells detect and eliminate complexes of irregular composition is unknown. By searching for regulators of the BTB family, we discovered a quality control pathway that ensures functional dimerization [dimerization quality control (DQC)]. Key to this network is the E3 ligase SCFFBXL17, which selectively binds and ubiquitylates BTB dimers of aberrant composition to trigger their clearance by proteasomal degradation. Underscoring the physiological importance of DQC, SCFFBXL17 is required for the differentiation, function, and survival of neural crest and neuronal cells. We conclude that metazoan organisms actively monitor BTB dimerization, and we predict that distinct E3 ligases similarly control complex formation by other recurrent domains.

中文翻译：

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二聚化质量控制确保神经元发育和存活

一种防止致命相互作用的方法 许多后生动物蛋白形成寡聚体，这通常由模块结构域（例如 BTB 结构域）介导。梅纳等人。现在描述他们称为二聚化质量控制 (DQC) 的质量控制途径（参见 Herhaus 和 Dikic 的观点）。DQC 监测并防止含有 BTB 结构域的蛋白质的异常二聚化。该系统依赖于 FBXL17，这是一种衔接蛋白，可以招募一种 E3 连接酶，该酶特异性泛素化无功能的 BTB 异二聚体，从而引发它们的降解。FBXL17 在非生理、无功能复合物中的 BTB 二聚体界面处获取降解信号。非洲爪蟾胚胎 DQC 的丧失会导致致命的神经发育缺陷。科学，这个问题 p。eaap8236; 另见第。151 质量控制系统通过消除非功能性组装来确保广泛的蛋白质相互作用模块的功能性二聚化。介绍 蛋白质复合物的形成是所有后生动物信号转导网络的核心。促进细胞信息流、模块化 BTB 结构域、亮氨酸拉链或卷曲线圈已在许多蛋白质中重复使用，它们通常介导关键的同源二聚化事件。虽然编码同源二聚体蛋白的单个等位基因的突变可能会毒化信号复合物，但相关模块之间的异常异源二聚化也可以抑制或改变信号转导级联的输出。细胞是否检测和消除异常组成的蛋白质复合物仍然未知。球状 BTB 结构域存在于约 220 种人类蛋白质中，这些蛋白质充当 CUL3 E3 连接酶、转录因子或膜通道的底物接头。含有同源二聚体 BTB 结构域的蛋白质，如 KEAP1、KLHL3、KBTBD8 或 BCL6，对于后生动物的发育至关重要，它们的突变或异常表达会导致高血压、癌症或神经变性。由于不了解生物体如何控制同源二聚体 BTB 蛋白的表达或活性，BTB 结构域提供了一个生理学上重要的模型来剖析循环相互作用模块的调节。基本原理为了确定影响模块化相互作用域的调节机制，我们搜索了 BTB 蛋白的共享结合伙伴。发现了一种针对多种 BTB 蛋白进行蛋白酶体降解的 E3 连接酶，我们使用生化重组和蛋白质复合工程来剖析潜在的分子控制机制。最后，我们依靠非洲爪蟾胚胎来研究周期性蛋白质相互作用模块异常调节的有机体后果。结果 亲和纯化和质谱实验表明，许多 BTB 蛋白异二聚化，但也与 FBXL17（SCFFBXL17 E3 连接酶的底物接头）相互作用。SCFFBXL17 催化 BTB 蛋白的多泛素化以触发它们的蛋白酶体降解。因此，SCFFBXL17 是 E3 连接酶的一个罕见例子，它靶向许多蛋白质共享的域，而不是特定的底物。如细胞和动物的生化重组和亲和纯化所示，SCFFBXL17 是一种质量控制酶，可检测和泛素化无活性的 BTB 异源二聚体，但忽略相同域的活性同源二聚体。因此，FBXL17 的缺失增加了 BTB 蛋白的异二聚化，但同时降低了 BTB 蛋白与其下游目标结合的能力。因此 SCFFBXL17 确保细胞中只存在功能性 BTB 二聚体，我们将这种活动称为二聚化质量控制 (DQC)。在分化人类胚胎干细胞中消耗 FBXL17 表明 DQC 阻止了 KBTBD8 的异二聚化，KBTBD8 是一种 BTB 蛋白，是神经嵴特化的重要调节因子。与这一观察结果一致，非洲爪蟾胚胎 DQC 的丧失干扰了中枢和外周神经系统细胞的分化、功能和存活，包括神经嵴。相比之下，体节发生或一般身体计划的形成最初不受影响。与其他质量控制网络类似，DQC 的丢失因此导致特定的神经元表型。然而，除了质量控制减弱的已知后果，即神经元过早死亡之外，在分化早期已经观察到异常 DQC 的影响。结论 我们发现 DQC 作为一种监测途径，可以检测组成异常的蛋白质复合物，而不是蛋白质错误折叠。我们推测其他循环交互模块，例如亮氨酸拉链或卷曲线圈，由依赖于不同 E3 连接酶的类似 DQC 网络监控。DQC 失活引起的神经元表型表明质量控制在神经系统的命运决定中发挥着积极作用。在进化过程中，DQC 与 BTB 域在脊椎动物基因组中成倍增加的同时出现，这表明消除相关 BTB 域形成的无活性异二聚体的能力有助于该域作为二聚化模块的广泛使用。二聚化质量控制消除了循环相互作用模块的非活性异二聚体，但保持功能性同源二聚体完好无损。SCFFBXL17 选择性地泛素化无活性的 BTB 二聚体，例如 BTB 异源二聚体或含有突变 BTB 结构域的二聚体，从而触发它们的蛋白酶体降解。功能性 BTB 同源二聚体逃脱了 SCFFBXL17 的检测。由重复性相互作用模块（例如 BTB 域、亮氨酸拉链或卷曲线圈）形成的异常复合物会破坏信号转导，然而，细胞是否能检测并消除不规则组成的复合物尚不清楚。通过寻找 BTB 家族的调节剂，我们发现了一种确保功能性二聚化 [二聚化质量控制 (DQC)] 的质量控制途径。该网络的关键是 E3 连接酶 SCFFBXL17，它选择性地结合并泛素化组成异常的 BTB 二聚体，以通过蛋白酶体降解触发它们的清除。强调 DQC 的生理重要性，SCFFBXL17 是神经嵴和神经元细胞的分化、功能和存活所必需的。我们得出结论，后生动物主动监测 BTB 二聚化，并且我们预测不同的 E3 连接酶类似地控制其他循环域的复合物形成。我们发现了一种确保功能性二聚化 [二聚化质量控制 (DQC)] 的质量控制途径。该网络的关键是 E3 连接酶 SCFFBXL17，它选择性地结合并泛素化组成异常的 BTB 二聚体，以通过蛋白酶体降解触发它们的清除。强调 DQC 的生理重要性，SCFFBXL17 是神经嵴和神经元细胞的分化、功能和存活所必需的。我们得出结论，后生动物主动监测 BTB 二聚化，并且我们预测不同的 E3 连接酶类似地控制其他循环结构域的复合物形成。我们发现了一种确保功能性二聚化 [二聚化质量控制 (DQC)] 的质量控制途径。该网络的关键是 E3 连接酶 SCFFBXL17，它选择性地结合并泛素化组成异常的 BTB 二聚体，以通过蛋白酶体降解触发它们的清除。强调 DQC 的生理重要性，SCFFBXL17 是神经嵴和神经元细胞的分化、功能和存活所必需的。我们得出结论，后生动物主动监测 BTB 二聚化，并且我们预测不同的 E3 连接酶类似地控制其他循环结构域的复合物形成。它选择性地结合并泛素化组成异常的 BTB 二聚体，以通过蛋白酶体降解触发它们的清除。强调 DQC 的生理重要性，SCFFBXL17 是神经嵴和神经元细胞的分化、功能和存活所必需的。我们得出结论，后生动物主动监测 BTB 二聚化，并且我们预测不同的 E3 连接酶类似地控制其他循环结构域的复合物形成。它选择性地结合并泛素化组成异常的 BTB 二聚体，以通过蛋白酶体降解触发它们的清除。强调 DQC 的生理重要性，SCFFBXL17 是神经嵴和神经元细胞的分化、功能和存活所必需的。我们得出结论，后生动物主动监测 BTB 二聚化，并且我们预测不同的 E3 连接酶类似地控制其他循环结构域的复合物形成。