The nuclear receptors REV-ERBα (encoded by NR1D1) and REV-ERBβ (NR1D2) have remained orphans owing to the lack of identified physiological ligands. Here we show that heme is a physiological ligand of both receptors. Heme associates with the ligand-binding domains of the REV-ERB receptors with a 1:1 stoichiometry and enhances the thermal stability of the proteins. Results from experiments of heme depletion in mammalian cells indicate that heme binding to REV-ERB causes the recruitment of the co-repressor NCoR, leading to repression of target genes including BMAL1 (official symbol ARNTL), an essential component of the circadian oscillator. Heme extends the known types of ligands used by the human nuclear receptor family beyond the endocrine hormones and dietary lipids described so far. Our results further indicate that heme regulation of REV-ERBs may link the control of metabolism and the mammalian clock.REV-ERBα was originally identified as an orphan member of the nuclear hormone receptor (NHR) family on the basis of its canonical domain structure and sequence conservation 1,2 . REV-ERBβ was subsequently identified by its homology to other NHRs and its pattern of expression, which overlaps greatly with that of REV-ERBα. Both receptors have particularly high expression in the liver, adipose tissue, skeletal muscle and brain 3-8 , where they are transcribed in a circadian manner 9-11 . The REV-ERBs are unique in the NHR superfamily in that they lack the carboxy-terminal tail (helix 12) of the ligand-binding domain (LBD), which is required for coactivator recognition 12

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Human Protein C (HPC), an antithrombotic factor with potential clinical utility, is a vitamin K-dependent protein that has several complex post-translational modifications. In an effort to define the functional roles of these modifications, recombinant HPC (rHPC) was expressed in and characterized from 3 adenovirus-transformed cell lines. The rHPC in crude culture medium from the 3 cell lines displayed anticoagulant activities that were either higher, slightly lower or much lower than that of plasma HPC. The rHPC from each cell line was purified and characterized using a novel, but simple chromatographic method, termed "pseudo-affinity", capable of resolving molecules differing by only very slight modifications. We demonstrate the critical dependence of full gamma-carboxylation on the function of this protein. In addition, our data indicate that both the gamma-carboxyglutamate and glycosyl contents affect the functional activities of rHPC.

Phospholipases A2 (PLA2s) may be grouped into distinct families of proteins that catalyse the hydrolysis of the 2-acyl bond of phospholipids and perform a variety of biological functions. The best characterized are the small (relative molecular mass approximately 14,000) calcium-dependent, secretory enzymes of diverse origin, such as pancreatic and venom PLA2s. The structures and functions of several PLA2s are known. Recently, high-resolution crystal structures of complexes of secretory PLA2s with phosphonate phospholipid analogues have provided information about the detailed stereochemistry of transition-state binding, confirming the proposed catalytic mechanism of esterolysis. By contrast, studies on mammalian nonpancreatic secretory PLA2s (s-PLA2s) have only recently begun; s-PLA2s are scarce in normal cells and tissues but large amounts are found in association with local and systemic inflammatory processes and tissue injury in animals and man. Such s-PLAs have been purified from rabbit and rat inflammatory exudate, from synovial fluid from patients with rheumatoid arthritis and from human platelets. Cloning and sequencing shows that the primary structure of the human s-PLA2 has about 37% homology with that of bovine pancreatic PLA2 and 44% homology with that of Crotalus atrox PLA2. The human s-PLA2 is an unusually basic protein, yet contains most of the highly conserved amino-acid residues and sequences characteristic of the PLA2s sequenced so far. Here we report the refined, three-dimensional crystal structure at 2.2 A resolution of recombinant human rheumatoid arthritic synovial fluid PLA2. This may aid the development of potent and specific inhibitors of this enzyme using structure-based design.

AMP-activated protein kinase (AMPK) serves as an energy-sensing protein kinase that is activated by a variety of metabolic stresses that lower cellular energy levels. When activated, AMPK modulates a network of metabolic pathways that result in net increased substrate oxidation, generation of reduced nucleotide cofactors, and production of ATP. AMPK is activated by a high AMP:ATP ratio and phosphorylation on threonine 172 by an upstream kinase. Recent studies suggest that mechanisms that do not involve changes in adenine nucleotide levels can activate AMPK. Another sensor of the metabolic state of the cell is the NAD/NADH redox potential. To test whether the redox state might have an effect on AMPK activity, we examined the effect of ␤-NAD and NADH on this enzyme. The recombinant T172D-AMPK, which was mutated to mimic the phosphorylated state, was activated by ␤-NAD in a dose-dependent manner, whereas NADH inhibited its activity. We explored the effect of NADH on AMPK by systematically varying the concentrations of ATP, NADH, peptide substrate, and AMP. Based on our findings and established activation of AMPK by AMP, we proposed a model for the regulation by NADH. Key features of this model are as follows. (a) NADH has an apparent competitive behavior with respect to ATP and uncompetitive behavior with respect to AMP resulting in improved binding constant in the presence of AMP, and (b) the binding of the peptide is not significantly altered by NADH. In the absence of AMP, the binding constant of NADH becomes higher than physiologically relevant. We conclude that AMPK senses both components of cellular energy status, redox potential, and phosphorylation potential.The mammalian 5Ј-AMP-activated protein kinase (AMPK) 1 serves as an energy-sensing protein kinase. It is activated by a variety of metabolic stresses that lower cellular energy levels. These include exercise, nutrient starvation, ischemia/hypoxia, heat shock, and metabolic poisoning (1-4). AMPK is also activated by a number of pharmacological interventions. AICAR (5-aminoimidazole-4-carboxamide 1-b-D-ribonucleoside), which is converted to an AMP analog upon entering the cells, is a commonly used tool for activating AMPK (5). Leptin (6), adiponectin (7), metformin (8, 9), and rosiglitazone (8) activate AMPK through poorly characterized mechanisms that may not involve energy depletion. The activation of AMPK stimulates fatty acid oxidation, hexokinase activity, and uptake of glucose into skeletal muscle, which generates reduced nucleoside cofactors and ATP, and represses hepatic gluconeogenic and fatty acid synthesis enzymes, which consume them (10).AMPK is a heterotrimeric complex consisting of a catalytic (␣) subunit and two regulatory subunits (␤ and ␥) (11). The isoforms of all three subunits have been identified including two isoforms of the catalytic subunit, ␣ 1 and ␣ 2 (9), two of the regulatory subunits, ␤ 1 and ␤ 2 , and three of the regulatory subunits, ␥ 1 , ␥ 2 , and ␥ 3 (12). The formation of the trimeric complex is necessary for optimal kin...