The anticoagulant human plasma serine protease, activated protein C (APC), inhibits blood coagulation by specific inactivation of the coagulation cofactors factor Va (FVa) and factor VIIIa. Site-directed mutagenesis of residues in three surface loops of a positive exosite located on APC was used to identify residues that play a significant role in binding to FVa. Eighteen different residues were mutated to alanine singly, in pairs, or in triple mutation combinations. Mutant APC proteins were purified and characterized for their inactivation of FVa. Three APC residues were identified that provide major contributions to
Two series of peptides that specifically bind to the extracellular domain of the ␣ chain of the human interleukin-5 receptor (IL-5R␣), but share no primary sequence homology to IL-5, were identified from libraries of random recombinant peptides. Affinity maturation procedures generated a 19-aa peptide that binds to the IL-5 receptor ␣͞ heterodimer complex with an affinity equal to that of IL-5 and is a potent and specific antagonist of IL-5 activity in a human eosinophil adhesion assay. The active form of the peptide is a disulfide-crosslinked dimer that forms spontaneously in solution. Gel filtration analysis, receptor-binding studies, and analytical ultracentrifugation reveal that the dimeric peptide binds simultaneously to two receptor ␣ chains in solution. Furthermore, the dimer peptide, but not IL-5, can activate a chimeric receptor consisting of the IL-5R␣ extracellular domain fused to the intracellular domain of the epidermal growth factor receptor, thus demonstrating that the peptide also promotes receptor dimerization in a cellular context. The functional antagonism produced by the bivalent interaction of the dimeric peptide with two IL-5R ␣ chains represents a distinctive mechanism for the antagonism of cytokines that use heteromeric receptors.I nterleukin-5 (IL-5) is a T cell-derived hematopoietic cytokine that acts exclusively on cells of the eosinophil and basophil lineage (1, 2). Although IL-5 can regulate many of the functions of mature, terminally differentiated eosinophils such as cell survival (3), adhesion (4), and activation (5), its primary function is to promote the differentiation and expansion of eosinophil precursors in the bone marrow (6).Transgenic mice that constitutively overexpress IL-5 in their lungs exhibit systemic and airway eosinophilia, bronchial hyperreactivity, and histopathological features characteristic of human asthma (7). Administration of recombinant IL-5 into the airways of either guinea pigs (8) or mildly asthmatic individuals (9) promotes a lung eosinophilia and bronchial hyperreactivity. Moreover, blocking the activity of IL-5 through administration of neutralizing anti-IL-5 antibodies (10-12), or through IL-5 gene deletion (13), prevents allergen-induced airway eosinophilia and bronchial hyperreactivity. Together, these data demonstrate that IL-5 plays a central role in the development of allergen-induced eosinophilia in animals and suggest that an anti-IL-5 therapeutic could provide an alternative approach to the treatment of human asthma perhaps more specific than current anti-inf lammatory therapies such as inhaled corticosteroids.The IL-5 receptor is a heterodimer consisting of an ␣ chain that specifically binds IL-5 and a signal-transducing  c chain shared with the receptors for two structurally related cytokines: granulocyte-macrophage colony-stimulating factor (GM-CSF) and . A number of protein-based IL-5 antagonists have been reported, including soluble IL-5 receptor ␣ chains (IL5R␣s) that sequester the ligand in solution (15), single point mutants of IL...
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