Core Viewpoint - The article discusses the molecular mechanisms of the SIFI protein in the integrated stress response (ISR), highlighting its role in managing cellular stress and preventing neurodegenerative diseases [4][5][6]. Group 1: Stress Response Mechanism - Chronic stress activation can damage cell survival and lead to severe degenerative diseases, prompting organisms to deploy factors like E3 ubiquitin ligase SIFI to terminate stress signaling and maintain cellular homeostasis [2][3]. - When cells encounter stress, such as mitochondrial damage or protein misfolding, they activate ISR to pause non-essential activities and focus resources on repair. If the stress response is not timely deactivated, it can lead to cell starvation and diseases like cerebellar ataxia and early-onset dementia [5][6]. Group 2: Role of SIFI - SIFI is an E3 ubiquitin ligase complex responsible for marking HRI and damaged proteins for degradation after stress is alleviated, thus restarting normal cellular functions [7][8]. - The research team utilized cryo-electron microscopy to capture the high-resolution structure of SIFI, revealing a giant scaffold structure composed of UBR4, KCMF1, and calmodulin, which is comparable in size to ribosomes (1.3 MDa) [9]. Group 3: SIFI's Mechanism of Action - SIFI operates through a multi-step process: 1. It performs a broad-spectrum quality check by capturing various stress-related proteins [12]. 2. KCMF1 initiates the tagging of substrates with the first ubiquitin label [13]. 3. UBR4 facilitates a chain reaction to form a degradation signal chain, essential for controlling stress signaling [14]. Group 4: Implications for Disease and Therapy - Mutations in UBR4 found in patients disrupt SIFI's function, leading to neurodegenerative conditions, but restoring SIFI function or inhibiting HRI can reverse pathological phenotypes in mouse models [15]. - The broad substrate binding capability of SIFI provides a template for designing new PROTAC molecules, potentially overcoming challenges in targeting "undruggable" proteins in cancer therapy [16].