Human erythropoietin is a haematopoietic cytokine required for the differentiation and proliferation of precursor cells into red blood cells. It activates cells by binding and orientating two cell-surface erythropoietin receptors (EPORs) which trigger an intracellular phosphorylation cascade. The half-maximal response in a cellular proliferation assay is evoked at an erythropoietin concentration of 10 pM, 10(-2) of its Kd value for erythropoietin-EPOR binding site 1 (Kd approximately equal to nM), and 10(-5) of the Kd for erythropoietin-EPOR binding site 2 (Kd approximately equal to 1 microM). Overall half-maximal binding (IC50) of cell-surface receptors is produced with approximately 0.18 nM erythropoietin, indicating that only approximately 6% of the receptors would be bound in the presence of 10 pM erythropoietin. Other effective erythropoietin-mimetic ligands that dimerize receptors can evoke the same cellular responses but much less efficiently, requiring concentrations close to their Kd values (approximately 0.1 microM). The crystal structure of erythropoietin complexed to the extracellular ligand-binding domains of the erythropoietin receptor, determined at 1.9 A from two crystal forms, shows that erythropoietin imposes a unique 120 degrees angular relationship and orientation that is responsible for optimal signalling through intracellular kinase pathways.

Cellular and physiological responses to changes in dioxygen levels in metazoans are mediated via the posttranslational oxidation of hypoxia-inducible transcription factor (HIF). Hydroxylation of conserved prolyl residues in the HIF-␣ subunit, catalyzed by HIF prolyl-hydroxylases (PHDs), signals for its proteasomal degradation. The requirement of the PHDs for dioxygen links changes in dioxygen levels with the transcriptional regulation of the gene array that enables the cellular response to chronic hypoxia; the PHDs thus act as an oxygen-sensing component of the HIF system, and their inhibition mimics the hypoxic response. We describe crystal structures of the catalytic domain of human PHD2, an important prolyl-4-hydroxylase in the human hypoxic response in normal cells, in complex with Fe(II) and an inhibitor to 1.7 Å resolution. PHD2 crystallizes as a homotrimer and contains a double-stranded ␤-helix core fold common to the Fe(II) and 2-oxoglutarate-dependant dioxygenase family, the residues of which are well conserved in the three human PHD enzymes (PHD 1-3). The structure provides insights into the hypoxic response, helps to rationalize a clinically observed mutation leading to familial erythrocytosis, and will aid in the design of PHD selective inhibitors for the treatment of anemia and ischemic disease.erythropoietin ͉ dioxygenase ͉ hypoxic response ͉ 2-oxoglutarate I n metazoans the ␣͞␤ heterodimeric hypoxia-inducible transcription factor (HIF) (1) regulates the transcription of an array of genes including those coding for glycolytic enzymes, erythropoietin, and VEGF. The levels and transcriptional activity of the HIF-␣, but not the HIF-␤, subunit are regulated by oxygen. Hydroxylation of either Pro-402 or Pro-564 in human HIF-1␣ (2, 3) within the C-terminal oxygen-dependent degradation domain (CODDD) enables its binding to the von Hippel-Lindau protein (pVHL), a targeting element of the E3-ubiquitin ligase; subsequent ubiquitylation leads to proteasomal degradation of HIF-␣ (for reviews, see refs. 4 -7). In humans, this mechanism is augmented by hydroxylation of an asparagine residue in the C-terminal transcriptional activation domain (8); this modification blocks interaction of HIF-1␣ with the CBP͞p300 coactivator, thereby disabling HIFmediated transcription.Hydroxylation of HIF-1␣ is catalyzed by four 2-oxoglutarate (2OG) dioxygenases: three prolyl hydroxlyases (PHD 1, 2, and 3) (also known as HPH 3, 2, and 1 and EGLN 2, 1, and 3; refs. 9-11) and an asparaginyl hydroxylase [factor inhibiting HIF (FIH); refs. 12 and 13]. The available evidence implicates PHD2 as the most important HIF hydroxylase in down-regulating the hypoxic response during normoxia (5,7,14,15).The HIF hydroxylases are Fe(II) and 2OG-dependent dioxygenases (16, 17); their requirement for dioxygen has led to their characterization as cellular oxygen sensors (refs. 9 -11, 18, and 19; Fig. 1a). The first 2OG dioxygenase to be identified was procollagen prolyl-hydroxylase, which like the PHDs catalyzes trans-4-hydroxylation reactions. Pro...

The three‐dimensional structure of the Sabin strain of type 3 poliovirus has been determined at 2.4 A resolution. Significant structural differences with the Mahoney strain of type 1 poliovirus are confined to loops and terminal extensions of the capsid proteins, occur in all of the major antigenic sites of the virion and typically involve insertions, deletions or the replacement of prolines. Several newly identified components of the structure participate in assembly‐dependent interactions which are relevant to the biologically important processes of viral assembly and uncoating. These include two sites of lipid substitution, two putative nucleotides and a beta sheet formed by the N‐termini of capsid proteins VP4 and VP1. The structure provides an explanation for the temperature sensitive phenotype of the P3/Sabin strain. Amino acids that regulate temperature sensitivity in type 3 poliovirus are located in the interfaces between promoters, in the binding site for a lipid substituent and in an assembly‐dependent extended beta sheet that stabilizes the association of pentamers. Several lines of evidence indicate that these structural components also control conformational transitions at various stages of the viral life cycle.

The crystal structure of the Pl/Mahoney poliovirus empty capsid has been determined at 2.9 A resolution. The empty capsids differ from mature virions in that they lack the viral RNA and have yet to undergo a stabilizing maturation cleavage of VPO to yield the mature capsid proteins VP4 and VP2. The outer surface and the bulk of the protein shell are very similar to those of the mature virion. The major differences between the 2 structures are focused in a network formed by the N-terminal extensions of the capsid proteins on the inner surface of the shell. In the empty capsids, the entire N-terminal extension of V P l , as well as portions corresponding to VP4 and the N-terminal extension of VP2, are disordered, and many stabilizing interactions that are present in the mature virion are missing. In the empty capsid, the VPO scissile bond is located some 20 A away from the positions in the mature virion of the termini generated by VPO cleavage. The scissile bond is located on the rim of a trefoilshaped depression in the inner surface of the shell that is highly reminiscent of an RNA binding site in bean pod mottle virus. The structure suggests plausible (and ultimately testable) models for the initiation of encapsidation, for the RNA-dependent autocatalytic cleavage of VPO, and for the role of the cleavage in establishing the ordered N-terminal network and in generating stable virions.Keywords: capsid proteins; empty capsid; encapsidation initiation; maturation cleavage; poliovirus; viral RNA; virion stability Poliovirus is a member of the picornavirus family. This family includes a large number of related small spherical viruses consisting entirely of a protein shell (approximately 300 A in diameter) and a single-stranded RNA genome (-7,500 nucleotides) of positive polarity. The family is subdivided into several genera, including the enteroviruses (polioviruses, coxsackieviruses,

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