In 1993, Huber and co-workers published the structure of an N-terminally truncated version of human annexin A1 lacking the first 32 amino acid residues (PDB code: 1AIN). In 2001, we reported the ...structure of full-length porcine annexin A1 including the N-terminal domain in the absence of calcium ions (PDB code: 1HM6). The latter structure did not reflect a typical annexin core fold, but rather a surprising interaction of the N-terminal domain and the core domain. Comparing these two structures revealed that in the full-length structure the first 12 residues of the N-terminal domain insert into the core of the protein, thereby replacing and unwinding one of the α-helices (helix D in repeat 3) that is involved in calcium binding. We hypothesized that this structure in the absence of calcium ions represents the inactive form of the protein. Furthermore, we proposed that upon calcium binding, the N-terminal domain would be expelled from the core domain and that the core D-helix would reform in the proper conformation for calcium coordination. Herein, we report the X-ray structure of full-length porcine annexin A1 in the presence of calcium. This new structure shows a typical annexin core structure as we hypothesized, with the D-helix back in place for calcium coordination while parts of the now exposed N-terminal domain are disordered. We could locate eight calcium ions in this structure, two of which are octa-coordinated and two of which were not observed in the structure of the N-terminally truncated annexin A1. Possible implications of this calcium-induced conformational switch for the membrane aggregation properties of annexin A1 will be discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Annexins comprise a multigene family of Ca2+ and phospholipid-binding proteins. They consist of a conserved C-terminal or core domain that confers Ca2+-dependent phospholipid binding and an ...N-terminal domain that is variable in sequence and length and responsible for the specific properties of each annexin. Crystal structures of various annexin core domains have revealed a high degree of similarity. From these and other studies it is evident that the core domain harbors the calcium-binding sites that interact with the phospholipid headgroups. However, no structure has been reported of an annexin with a complete N-terminal domain. We have now solved the crystal structure of such a full-length annexin, annexin 1. Annexin 1 is active in membrane aggregation and its refined 1.8 Å structure shows an α-helical N-terminal domain connected to the core domain by a flexible linker. It is surprising that the two α-helices present in the N-terminal domain of 41 residues interact intimately with the core domain, with the amphipathic helix 2–12 of the N-terminal domain replacing helix D of repeat III of the core. In turn, helix D is unwound into a flap now partially covering the N-terminal helix. Implications for membrane aggregation will be discussed and a model of aggregation based on the structure will be presented.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
3′-Uridylylation of RNA is emerging as a phylogenetically widespread phenomenon involved in processing events as diverse as uridine insertion/deletion RNA editing in mitochondria of trypanosomes and ...small nuclear RNA (snRNA) maturation in humans. This reaction is catalyzed by terminal uridylyltransferases (TUTases), which are template-independent RNA nucleotidyltransferases that specifically recognize UTP and belong to a large enzyme superfamily typified by DNA polymerase β. Multiple TUTases, recently identified in trypanosomes, as well as a U6 snRNA-specific TUTase enzyme in humans, are highly divergent at the protein sequence level. However, they all possess conserved catalytic and UTP recognition domains, often accompanied by various auxiliary modules present at the termini or between conserved domains. Here we report identification, structural and biochemical analyses of a novel trypanosomal TUTase, TbTUT4, which represents a minimal catalytically active RNA uridylyltransferase. The TbTUT4 consists of only two domains that define the catalytic center at the bottom of the nucleoside triphosphate and RNA substrate binding cleft. The 2.0 Å crystal structure reveals two significantly different conformations of this TUTase: one molecule is in a relatively open apo conformation, whereas the other displays a more compact TUTase–UTP complex. A single nucleoside triphosphate is bound in the active site by a complex network of interactions between amino acid residues, a magnesium ion and highly ordered water molecules with the UTP's base, ribose and phosphate moieties. The structure-guided mutagenesis and cross-linking studies define the amino acids essential for catalysis, uracil base recognition, ribose binding and phosphate coordination by uridylyltransferases. In addition, the cluster of positively charged residues involved in RNA binding is identified. We also report a 2.4 Å crystal structure of TbTUT4 with the bound 2′ deoxyribonucleoside, which provides the structural basis of the enzyme's preference toward ribonucleotides.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Cyclooxygenase-2 (COX-2) can oxygenate the endocannabinoids, arachidonyl ethanolamide (AEA) and 2-arachidonylglycerol (2-AG), to prostaglandin-H2-ethanolamide (PGH2-EA) and -glycerol ester (PGH2-G), ...respectively. Further metabolism of PGH2-EA and PGH2-G by prostaglandin synthases produces a variety of prostaglandin-EA's and prostaglandin-G's nearly as diverse as those derived from arachidonic acid. Thus, COX-2 may regulate endocannabinoid levels in neurons during retrograde signaling or produce novel endocannabinoid metabolites for receptor activation. Endocannabinoid-metabolizing enzymes are important regulators of their action, so we tested whether PG-G levels may be regulated by monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH). We found that PG-Gs are poor substrates for purified MGL and FAAH compared to 2-AG and/or AEA. Determination of substrate specificity demonstrates a 30−100- and 150−200-fold preference of MGL and FAAH for 2-AG over PG-Gs, respectively. The substrate specificity of AEA compared to those of PG-Gs was ∼200−300 fold higher for FAAH. Thus, PG-Gs are poor substrates for the major endocannabinoid-degrading enzymes, MGL and FAAH.
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IJS, KILJ, NUK, PNG, UL, UM
The tumor suppressor protein p53 plays a key role in cell‐cycle regulation by triggering DNA repair, cell‐cycle arrest and apoptosis when the appropriate signal is received. p53 has the classic ...architecture of a transcription factor, with an amino‐terminal transactivation domain, a core DNA‐binding domain and carboxy‐terminal tetramerization and regulatory domains. The crystal structure of the p53 core domain, which includes the amino acids from residue 96 to residue 289, has been determined in the absence of DNA to a resolution of 2.05 Å. Crystals grew in a new monoclinic space group (P21), with unit‐cell parameters a = 68.91, b = 69.36, c = 84.18 Å, β = 90.11°. The structure was solved by molecular replacement and has been refined to a final R factor of 20.9% (Rfree = 24.6%). The final model contains four molecules in the asymmetric unit with four zinc ions and 389 water molecules. The non‐crystallographic tetramers display different protein contacts from those in other p53 crystals, giving rise to the question of how p53 arranges as a tetramer when it binds its target DNA.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Oleylethanolamide (OEA) is a naturally occurring lipid that regulates satiety and body weight. Although structurally related to the endogenous cannabinoid anandamide, OEA does not bind to cannabinoid ...receptors and its molecular targets have not been defined. Here we show that OEA binds with high affinity to the peroxisome-proliferator-activated receptor-α (PPAR-α), a nuclear receptor that regulates several aspects of lipid metabolism. Administration of OEA produces satiety and reduces body weight gain in wild-type mice, but not in mice deficient in PPAR-α. Two distinct PPAR-α agonists have similar effects that are also contingent on PPAR-α expression, whereas potent and selective agonists for PPAR-γ and PPAR-β/δ are ineffective. In the small intestine of wild-type but not PPAR-α-null mice, OEA regulates the expression of several PPAR-α target genes: it initiates the transcription of proteins involved in lipid metabolism and represses inducible nitric oxide synthase, an enzyme that may contribute to feeding stimulation. Our results, which show that OEA induces satiety by activating PPAR-α, identify an unexpected role for this nuclear receptor in regulating behaviour, and raise possibilities for the treatment of eating disorders.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Annexin I, a member of the annexin family of Ca2+‐ and phospholipid‐binding proteins, has been crystallized with the complete N‐terminus. Annexins are structurally divided into a conserved protein ...core and an N‐terminal domain that is variable in sequence and length. Three‐dimensional structures of annexins comprising the protein core and a short N‐terminal domain (annexins III, IV, V, VI, XII) or a truncated form almost completely lacking the N‐terminal domain (annexins I and II) have been published so far. Here, the crystallization of annexin I comprising not only the core but also the complete N‐terminal domain is reported. The crystals belong to the space group P212121, with unit‐cell parameters a = 63.6, b = 96.3, c = 127.4 Å, and diffract to better than 2 Å. Assuming a molecular weight of 38.7 kDa for annexin I and an average value of 2.5 Å3 Da−1 for VM, two molecules per asymmetric unit are present.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In order to understand how isomerization of the retinal drives unidirectional transmembrane ion transport in bacteriorhodopsin, we determined the atomic structures of the BR state and M ...photointermediate of the E204Q mutant, to 1.7 and 1.8 Å resolution, respectively. Comparison of this M, in which proton release to the extracellular surface is blocked, with the previously determined M in the D96N mutant indicates that the changes in the extracellular region are initiated by changes in the electrostatic interactions of the retinal Schiff base with Asp85 and Asp212, but those on the cytoplasmic side originate from steric conflict of the 13-methyl retinal group with Trp182 and distortion of the π-bulge of helix G. The structural changes suggest that protonation of Asp85 initiates a cascade of atomic displacements in the extracellular region that cause release of a proton to the surface. The progressive relaxation of the strained 13-cis retinal chain with deprotonated Schiff base, in turn, initiates atomic displacements in the cytoplasmic region that cause the intercalation of a hydrogen-bonded water molecule between Thr46 and Asp96. This accounts for the lowering of the pKa of Asp96, which then reprotonates the Schiff base via a newly formed chain of water molecules that is extending toward the Schiff base.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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