To survive in hostile environments, organisms activate stress-responsive transcriptional regulators that coordinately increase production of protective factors. Hypoxia changes cellular metabolism and thus activates redox-sensitive as well as oxygen-dependent signal transducers. We demonstrate that Sirtuin 1 (Sirt1), a redox-sensing deacetylase, selectively stimulates activity of the transcription factor hypoxia-inducible factor 2 alpha (HIF-2alpha) during hypoxia. The effect of Sirt1 on HIF-2alpha required direct interaction of the proteins and intact deacetylase activity of Sirt1. Select lysine residues in HIF-2alpha that are acetylated during hypoxia confer repression of Sirt1 augmentation by small-molecule inhibitors. In cultured cells and mice, decreasing or increasing Sirt1 activity or levels affected expression of the HIF-2alpha target gene erythropoietin accordingly. Thus, Sirt1 promotes HIF-2 signaling during hypoxia and likely other environmental stresses.
Neuronal PAS domain protein 2 (NPAS2) is a mammalian transcription factor that binds DNA as an obligate dimeric partner of BMAL1 and is implicated in the regulation of circadian rhythm. Here we show that both PAS domains of NPAS2 bind heme as a prosthetic group and that the heme status controls DNA binding in vitro. NPAS2-BMAL1 heterodimers, existing in either the apo (heme-free) or holo (heme-loaded) state, bound DNA avidly under favorably reducing ratios of the reduced and oxidized forms of nicotinamide adenine dinucleotide phosphate. Low micromolar concentrations of carbon monoxide inhibited the DNA binding activity of holo-NPAS2 but not that of apo-NPAS2. Upon exposure to carbon monoxide, inactive BMAL1 homodimers were formed at the expense of NPAS2-BMAL1 heterodimers. These results indicate that the heterodimerization of NPAS2, and presumably the expression of its target genes, are regulated by a gas through the heme-based sensor described here.
To evaluate the contributions of the Gβ-2 arginine to signal transduction in oxygen-sensing heme-PAS domains, we replaced this residue with alanine in Bradyrhizobium japonicum FixL and examined the results on heme-domain structure, ligand binding, and kinase regulation. In the isolated R220A BjFixL heme-PAS domain, the iron−histidine bond was increased in length by 0.31 Å, the heme flattened even without a ligand, and the interaction of a presumed regulatory loop (the FG loop) with the helix of heme attachment was weakened. Binding of carbon monoxide was similar for ferrous BjFixL and R220A BjFixL. In contrast, the level of binding of oxygen was dramatically lower (K d ∼ 1.5 mM) for R220A BjFixL, and this was manifested as 60- and 3-fold lower on- and off-rate constants, respectively. Binding of cyanide followed the same pattern as binding of oxygen. The catalytic activity was 3−4-fold higher in the “on-state” unliganded forms of R220A BjFixL than in the corresponding BjFixL species. Cyanide regulation of this activity was strongly impaired, but some inhibition was nevertheless preserved. Carbon monoxide and nitric oxide regulation, although weak in BjFixL, were abolished from R220A BjFixL. We conclude that the Gβ-2 arginine assists in the binding of oxygen to BjFixL but does not accomplish this by stabilizing the oxy form. This arginine is not absolutely required for regulation, although it is important for shifting a pre-existing kinase equilibrium toward the inactive state on binding of regulatory ligands. These findings support a regulatory model in which the heme-PAS domain operates as an ensemble that couples to the kinase rather than a mechanism driven by a single central switch.
Phosphorylation of the transcription factor RmFixJ is the key step in the hypoxic induction of Sinorhizobium meliloti nitrogen fixation genes. Oxygen regulates this process by binding reversibly to RmFixL, a heme protein kinase whose deoxy form catalyzes the phosphoryl transfer from ATP to RmFixJ. Here we present the first quantitative measure of the extent by which various heme ligands inhibit the turnover of RmFixJ to phospho-RmFixJ. We also quantitate the inhibition by ligands of the reaction of RmFixL with ATP, in the absence of RmFixJ, to form phospho-RmFixL, i.e., the "autophosphorylation". Phospho-RmFixL formed from autophosphorylation will transfer its phosphoryl group to RmFixJ in an oxygen-independent "phosphotransfer." Here we show that the mode of substrate presentation, i.e., simultaneous versus sequential, influences the regulation of phosphoryl transfer by heme status. Inhibition factors for O(2), CO, NO, CN(-), and imidazole in the presence of RmFixJ are drastically different from the inhibition of autophosphorylation by the same ligands. Oxidation of the heme iron in unliganded RmFixL is known to have no effect on either of the sequential reactions; yet oxidation causes a 100-fold decrease in RmFixJ turnover when ATP and RmFixJ are presented simultaneously. The profound difference between the regulation of isolated RmFixL versus the complex of RmFixL with RmFixJ shows that interaction of a response regulator with its histidine-kinase partner need not be limited to the enzymatic regions of the histidine kinase, but can extend also to its sensory domain.
The EcDos protein belongs to a group of heme-based sensors that detect their ligands with a heme-binding PAS domain. Among these various heme-PAS proteins, EcDos is unique in having its heme iron coordinated at both axial positions to residues of the protein. To achieve its high affinities for ligands, one of the axial heme-iron residues in EcDos must be readily displaceable. Here we present evidence from mutagenesis, ligand-binding measurements, and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance spectroscopies about the nature of the displaceable residue in the heme-PAS domain of EcDos, i.e., EcDosH. The magnetic circular dichroism spectra in the near-infrared region establish histidine-methionine coordination in met-EcDos. To determine whether in deoxy-EcDos coordination of the sixth axial position is also to methionine, methionine 95 was substituted with isoleucine. This substitution caused the ferrous heme iron to change from an exclusively hexacoordinate low-spin form (EcDosH) to an exclusively pentacoordinate high-spin form (M95I EcDosH). This was accompanied by a modest acceleration of the dissociation rates of ligands but a dramatic increase (60-1300-fold) in the association rate constants for binding of O(2), CO, and NO. As a result, the affinity for O(2) was enhanced 10-fold in M95I EcDosH, but the partition constant M = [K(d)(O(2))/K(d)(CO)] between CO and O(2) was raised to about 30 from the extraordinarily low EcDosH value of 1. Thus a major consequence of the increased O(2) affinity of this sensor was the loss of its unusually strong ligand discrimination.
Hypoxia-inducible factors (HIFs) are stress-responsive transcriptional regulators of cellular and physiological processes involved in oxygen metabolism. Although much is understood about the molecular machinery that confers HIF responsiveness to oxygen, far less is known about HIF isoform-specific mechanisms of regulation, despite the fact that HIF-1 and HIF-2 exhibit distinct biological roles. We recently determined that the stress-responsive genetic regulator sirtuin 1 (Sirt1) selectively augments HIF-2 signaling during hypoxia. However, the mechanism by which Sirt1 maintains activity during hypoxia is unknown. In this report, we demonstrate that Sirt1 gene expression increases in a HIF-dependent manner during hypoxia in Hep3B and in HT1080 cells. Impairment of HIF signaling affects Sirt1 deacetylase activity as decreased HIF-1 signaling results in the appearance of acetylated HIF-2␣, which is detected without pharmacological inhibition of Sirt1. We also find that Sirt1 augments HIF-2 mediated, but not HIF-1 mediated, transcriptional activation of the isolated Sirt1 promoter. These data in summary reveal a bidirectional link of HIF and Sirt1 signaling during hypoxia.The ability to sense and respond to changes in oxygen content, conserved in almost all eukaryotic organisms, is conferred at the cellular level and is dictated by changes in gene expression, including by de novo transcriptional events (1). Members of the hypoxia-inducible factor (HIF) 2 family of transcription factors are key regulators of genes whose expression is altered during hypoxia. HIFs, obligate heterodimeric protein complexes, are composed of an oxygen-labile ␣-subunit and a shared, oxygen-stable -subunit also referred to as ARNT (2). Whereas invertebrates contain a single HIF-␣ member, mammals contain three HIF-␣ genes: HIF-1␣, HIF-2␣ (also called endothelial PAS domain protein 1 (EPAS1)) (3-5), and HIF-3␣. HIF-␣ proteins have similar domain structures with conserved sequence identity in some regions, particularly for HIF-1␣ and HIF-2␣. The N termini of HIF-␣ and HIF- proteins contain the highly conserved basic helix-loop-helix and Per/ARNT/Sim (PAS) domains involved in DNA binding and protein-protein interactions, respectively. The PAS domain may also contribute to HIF target gene specificity and may serve as a target for small molecules that disrupt specific HIF complexes (6, 7).The levels of HIF-␣ subunits increase during hypoxia due to impaired modifications of two proline residues (8, 9) situated within the oxygen-dependent degradation domain (10), part of a larger domain known as the N-terminal activation domain (NTAD) located in the midportion of HIF-␣ proteins. These two proline residues are otherwise selectively hydroxylated under normoxic conditions by oxygen-dependent prolyl hydroxylases (8, 9, 11) and subsequently target the HIF-␣ proteins for proteasomal degradation by the von Hippel-Lindau (VHL) ubiquitin-protein ligase complex (12-16).A second oxygen-dependent hydroxylation by asparaginyl hydroxylases (17, 18) targets an ...
Background: Nutrients stimulate calcium dependent activation of energy metabolism, in pancreatic beta cells.Results: Glucose-induced ATP synthase-dependent respiration is strictly calcium-dependent, with little or no effect of calcium on the NAD(P)H response.Conclusion: Calcium coordinates oxidative metabolism and respiration in pancreatic beta cells.Significance: Calcium has novel mitochondrial targets downstream of mitochondrial dehydrogenases.
Hemoglobins are ubiquitous in Eukarya and Bacteria but, until now, have not been found in Archaea. A phylogenetic analysis of the recently revealed microbial family of globin-coupled hemebased sensors suggests that these sensors descended from an ancient globin-only progenitor, or a protoglobin (Pgb). Here, we report the discovery and characterization of two Pgbs from the Archaea: ApPgb from the obligately aerobic hyperthermophile Aeropyrum pernix, and MaPgb from the strictly anaerobic methanogen Methanosarcina acetivorans. Both ApPgb and MaPgb bind molecular oxygen, nitric oxide, and carbon monoxide by means of a heme moiety that is coordinated to the protein through the F8 histidine (histidine 120). We postulate that these archaeal globins are the ancestors of contemporary hemoglobins.globin-coupled sensor ͉ oxygen sensor ͉ NO sensor ͉ myoglobin
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