Mapping of Human Granulocyte-Macrophage-Colony-Stimulating-Factor Domains Interacting with the Human Granulocyte-Macrophage-Colony-Stimulating-Factor-Receptor alpha-Subunit

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Abstract
The high-affinity granulocyte-macrophage-colony-stimulating-factor (GM-CSF) receptor (R) is composed of at least two subunits, termed alpha and beta. The alpha subunit is crucial for initiating ligand/receptor interaction and for ligand specificity. The experiments reported in this study sought to identify the domains of human (h)GM-CSF which are responsible for interaction with hGM-CSFR alpha. Anti-(human-GM-CSF) mAb were used as competitors in a 125I-GM-CSF receptor-binding assay on cells expressing the recombinant hGM-CSFR alpha chain. Inhibition of 125I-GM-CSF binding to the GM-CSFR alpha chain was demonstrated by mAb which mapped to the middle third of the third alpha helix (amino acids 78-83), the distal two-thirds of the third alpha helix and the initial 7 residues of the loop between the third and fourth helix (amino acids 78-94), and the extreme carboxy terminus. No inhibition of binding occured with an antibody whose domain begins in the first beta-pleated sheet (amino acids, 39-43), continues through the second helix (amino acids 55-64) and into the proximal third of the third helix (to amino acid 77). mAb which mapped to the distal half of the fourth helix and the carboxy-terminal tail (amino acids 110-127) increased the binding of 125I-GM-CSF to GM-CSFR alpha. Due to the known discontinous epitopes the possibility that the distal portion of the fourth helix contributes to binding cannot be eliminated. However, the domains of hGM-CSF most clearly involved in binding to the hGM-CSFR alpha are the distal two-thirds of the third alpha helix, the immediate downstream residues and the extreme carboxy terminus of hGM-CSF.

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