The Heme-Regulated Inhibitor Kinase Requires Dimerization for Heme- Sensing Activity

Cryo-EM Structure of HRSL Domain Reveals Activating Crossed Helices at the Core of GCN2

GCN2 is a conserved receptor kinase activating the Integrated Stress Response (ISR) in eukaryotic cells. The ISR kinases detect accumulation of stress molecules and reprogram translation from basal tasks to preferred production of cytoprotective proteins. GCN2 stands out evolutionarily among all protein kinases due to the presence of a h istidyl t R NA s ynthetase-like (HRSL) domain, which arises only in GCN2 and is located next to the kinase domain. How HRSL contributes to GCN2 signaling remains unknown. Here we report a 3.2 Å cryo-EM structure of HRSL from thermotolerant yeast Kluyveromyces marxianus . This structure shows a constitutive symmetrical homodimer featuring a compact helical-bundle structure at the junction between HRSL and kinase domains, in the core of the receptor. Mutagenesis demonstrates that this junction structure activates GCN2 and indicates that our cryo-EM structure captures the active signaling state of HRSL. Based on these results, we put forward a GCN2 regulation mechanism, where HRSL drives the formation of activated kinase dimers. Remaining domains of GCN2 have the opposite role and in the absence of stress they help keep GCN2 basally inactive. This autoinhibitory activity is relieved upon stress ligand binding. We propose that the opposing action of HRSL and additional GCN2 domains thus yields a regulated ISR receptor.

Significance statement

Regulation of protein synthesis (translation) is a central mechanism by which eukaryotic cells adapt to stressful conditions. In starving cells, this translational adaptation is achieved via the receptor kinase GCN2, which stays inactive under normal conditions, but is switched on under stress. The molecular mechanism of GCN2 switching is not well understood due to the presence of a structurally and biochemically uncharacterized h istidyl t R NA s ynthetase-like domain (HRSL) at the core of GCN2. Here we use single-particle cryo-EM and biochemistry to elucidate the structure and function of HRSL. We identify a structure at the kinase/HRSL interface, which forms crossed helices and helps position GCN2 kinase domains for activation. These data clarify the molecular mechanism of GCN2 regulation.

Signaling plasticity in the integrated stress response

The Integrated Stress Response (ISR) is an essential homeostatic signaling network that controls the cell's biosynthetic capacity. Four ISR sensor kinases detect multiple stressors and relay this information to downstream effectors by phosphorylating a common node: the alpha subunit of the eukaryotic initiation factor eIF2. As a result, general protein synthesis is repressed while select transcripts are preferentially translated, thus remodeling the proteome and transcriptome. Mounting evidence supports a view of the ISR as a dynamic signaling network with multiple modulators and feedback regulatory features that vary across cell and tissue types. Here, we discuss updated views on ISR sensor kinase mechanisms, how the subcellular localization of ISR components impacts signaling, and highlight ISR signaling differences across cells and tissues. Finally, we consider crosstalk between the ISR and other signaling pathways as a determinant of cell health.

Cellular Stress: Modulator of Regulated Cell Death

Cellular stress response activates a complex program of an adaptive response called integrated stress response (ISR) that can allow a cell to survive in the presence of stressors. ISR reprograms gene expression to increase the transcription and translation of stress response genes while repressing the translation of most proteins to reduce the metabolic burden. In some cases, ISR activation can lead to the assembly of a cytoplasmic membraneless compartment called stress granules (SGs). ISR and SGs can inhibit apoptosis, pyroptosis, and necroptosis, suggesting that they guard against uncontrolled regulated cell death (RCD) to promote organismal homeostasis. However, ISR and SGs also allow cancer cells to survive in stressful environments, including hypoxia and during chemotherapy. Therefore, there is a great need to understand the molecular mechanism of the crosstalk between ISR and RCD. This is an active area of research and is expected to be relevant to a range of human diseases. In this review, we provided an overview of the interplay between different cellular stress responses and RCD pathways and their modulation in health and disease.

The integrated stress response in cancer progression: a force for plasticity and resistance

During their quest for growth, adaptation, and survival, cancer cells create a favorable environment through the manipulation of normal cellular mechanisms. They increase anabolic processes, including protein synthesis, to facilitate uncontrolled proliferation and deplete the tumor microenvironment of resources. As a dynamic adaptation to the self-imposed oncogenic stress, cancer cells promptly hijack translational control to alter gene expression. Rewiring the cellular proteome shifts the phenotypic balance between growth and adaptation to promote therapeutic resistance and cancer cell survival. The integrated stress response (ISR) is a key translational program activated by oncogenic stress that is utilized to fine-tune protein synthesis and adjust to environmental barriers. Here, we focus on the role of ISR signaling for driving cancer progression. We highlight mechanisms of regulation for distinct mRNA translation downstream of the ISR, expand on oncogenic signaling utilizing the ISR in response to environmental stresses, and pinpoint the impact this has for cancer cell plasticity during resistance to therapy. There is an ongoing need for innovative drug targets in cancer treatment, and modulating ISR activity may provide a unique avenue for clinical benefit.

Is ATP the Only Nucleoside Triphosphate among ATP, CTP, GTP, and UTP to Have a Role in Kinase Catalysis of Heme-Regulated Inhibitor toward eIF2α during Lung Cancer Development?

The heme-regulated eukaryotic initiation factor 2α (eIF2α) kinase, also known as heme-regulated inhibitor (HRI), detects misfolded proteins and induces cytoprotective response to stress, mainly caused by heme-shortage. The nucleoside triphosphate ATP serves as the main donor of phosphate for the phosphorylation of eIF2α by HRI in human cells. However, the other main nucleoside triphosphates (CTP, GTP, UTP) are also present at relatively high concentrations, especially in human tumor cells. Therefore, in this short communication we evaluate the role of four substrates (namely ATP, CTP, GTP, and UTP) on human HRI kinase activity. Additionally, for the first time, we perform a detailed kinetics study of the HRI G202S mutant, whose presence in the human lung is associated with cancer development. Here, the role of all four tested nucleoside triphosphates during cancer development is discussed from the point of view of the HRI activity. The results showed that the kcat value of GTP was lower than that of ATP but was significantly higher than those of CTP and UTP. Additionally, the kcat value of GTP for G202S was approximately 20% higher than that for wild-type, while the kcat values of ATP, CTP, and UTP for G202S were lower than those for wild-type.