p-S349 SQSTM1 oligomer binds KEAP1:CUL3:RBX1

SQSTM1 binds to the KEAP1 subunit of the CRL3 ubiquitin ligase complex through the KEAP1 interacting region (KIR) located at residues 339-358 (Jain et al, 2010). Like NFE2L2, SQSTM1 has a KELCH1-domain binding STGE motif that is thought to interact with the KEAP1 complex in the 'open' conformation. Binding is enhanced by phosphorylation of the SQSTM1 STGE motif (Ichimura et al, 2013; reviewed in Baird and Yamamoto, 2020).<br>SQSTM1 is a transcriptional target of NFE2L2 and is upregulated in the presence of cellular stressors like reactive oxygen species (ROS) and electrophiles (reviewed in Baird and Yamamoto, 2020). Upregulation of SQSTM1 under stress conditions leads to competition between NFE2L2 and SQSTM1 for KEAP1 binding and results in stabilization of NFE2L2 protein levels. This establishes a positive feedback loop as NFE2L2 is able to translocate into the nucleus to promote expression of its target genes (Jain et al, 2010).<br>SQSTM1 has also been shown to be a substrate for KEAP1:CUL3:RBX1-mediated ubiquitination. Ubiquitination at lysine 420 increases the ability of SQSTM1 to sequester and degrade target ubiquitinated proteins through selective autophagy (Lee et al, 2017). Some evidence shows that in addition to competing with NFE2L2 for KEAP1-binding, SQSTM1 also targets KEAP1 for sequestration and degradation through the autophagy pathway (Copple et al, 2010; reviewed in Jiang et al, 2015). Sestrin proteins may also play a role in targeting KEAP1 for SQSTM1-dependent autophagy (Bae et al, 2013; Ro et al, 2014).

SQSTM1通过与位于残基339-358处的KEAP1相互作用区域（KIR）结合至CRL3泛素连接酶复合物的KEAP1亚基（Jain等，2010年）。与NFE2L2类似，SQSTM1具有KELCH1结构域结合的STGE基序，该基序被认为在与KEAP1复合物的'开放'构象中相互作用。SQSTM1 STGE基序的磷酸化增强了其结合能力（Ichimura等，2013年；参见Baird和Yamamoto，2020年的综述）。SQSTM1是NFE2L2的转录靶点，在存在细胞应激原如活性氧（ROS）和亲电体的情况下被上调（参见Baird和Yamamoto，2020年的综述）。在应激条件下，SQSTM1的上调导致NFE2L2与SQSTM1对KEAP1结合的竞争，从而稳定NFE2L2蛋白水平。这形成了一个正反馈环路，因为NFE2L2能够转移至细胞核以促进其靶基因的表达（Jain等，2010年）。此外，SQSTM1已被证明是KEAP1:CUL3:RBX1介导的泛素化的底物。在赖氨酸420位点的泛素化增加了SQSTM1通过选择性自噬降解靶泛素化蛋白的能力（Lee等，2017年）。一些证据表明，除了与NFE2L2竞争KEAP1结合外，SQSTM1还通过自噬途径靶向KEAP1以进行隔离和降解（Copple等，2010年；参见Jiang等，2015年的综述）。Sestrin蛋白可能在靶向KEAP1进行SQSTM1依赖性自噬中也发挥作用（Bae等，2013年；Ro等，2014年）。