HERC2 Targets the Iron Regulator FBXL5 for Degradation and Modulates Iron Metabolism
HERC2 Targets the Iron Regulator FBXL5 for Degradation and Modulates Iron Metabolism
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DOI：
10.1074/jbc.m113.541490
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发表时间：
2014-06-06
影响因子：
4.8
通讯作者：
Nakayama, Keiichi I.
中科院分区：
文献类型：
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作者：
Moroishi, Toshiro;Yamauchi, Takayoshi;Nakayama, Keiichi I.
FBXL5（F-box 和富含亮氨酸的重复蛋白 5）是 SCFFBXL5 泛素连接酶复合物的 F-box 蛋白亚基，因此负责 SCFFBXL5 泛素连接酶复合物的底物识别，该复合物靶向铁调节蛋白 2 (IRP2) 进行蛋白酶体降解。 IRP2 通过转录后调节有助于控制细胞内铁浓度的蛋白质，在维持哺乳动物细胞铁稳态中发挥着核心作用。 FBXL5-IRP2 轴对于体内铁代谢的控制是不可或缺的，因为缺乏 FBXL5 的小鼠在早期胚胎发生过程中会因 IRP2 活性不受限制和铁积累过多导致的氧化应激而死亡。然而，尽管 FBXL5 在控制铁稳态中发挥着关键作用，但人们对 FBXL5 活性的上游调节知之甚少。我们现在证明 FBXL5 在稳定状态下会经历组成型泛素依赖性降解。通过使用蛋白质组学方法发现调节 FBXL5 稳定性的蛋白质，我们将大 HECT 型泛素连接酶 HERC2（包含 E3 泛素蛋白连接酶 2 的 HECT 和 RLD 结构域）鉴定为 FBXL5 相关蛋白。通过 RNA 干扰抑制 HERC2-FBXL5 相互作用或消耗内源性 HERC2 导致 FBXL5 稳定并随之增加其丰度。 FBXL5 的这种积累反过来导致细胞内二价铁含量的减少。因此，我们的结果表明 HERC2 调节 FBXL5 的基础周转，并且这种泛素依赖性降解途径有助于控制哺乳动物铁代谢。
FBXL5 (F-box and leucine-rich repeat protein 5) is the F-box protein subunit of, and therefore responsible for substrate recognition by, the SCFFBXL5 ubiquitin-ligase complex, which targets iron regulatory protein 2 (IRP2) for proteasomal degradation. IRP2 plays a central role in the maintenance of cellular iron homeostasis in mammals through posttranscriptional regulation of proteins that contribute to control of the intracellular iron concentration. The FBXL5-IRP2 axis is integral to control of iron metabolism in vivo, given that mice lacking FBXL5 die during early embryogenesis as a result of unrestrained IRP2 activity and oxidative stress attributable to excessive iron accumulation. Despite its pivotal role in the control of iron homeostasis, however, little is known of the upstream regulation of FBXL5 activity. We now show that FBXL5 undergoes constitutive ubiquitin-dependent degradation at the steady state. With the use of a proteomics approach to the discovery of proteins that regulate the stability of FBXL5, we identified the large HECT-type ubiquitin ligase HERC2 (HECT and RLD domain containing E3 ubiquitin protein ligase 2) as an FBXL5-associated protein. Inhibition of the HERC2-FBXL5 interaction or depletion of endogenous HERC2 by RNA interference resulted in the stabilization of FBXL5 and a consequent increase in its abundance. Such accumulation of FBXL5 in turn led to a decrease in the intracellular content of ferrous iron. Our results thus suggest that HERC2 regulates the basal turnover of FBXL5, and that this ubiquitin-dependent degradation pathway contributes to the control of mammalian iron metabolism.