INVOLVEMENT OF THE 4TH ALPHA-HELIX OF MOUSE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR IN BINDING TO THE ALPHA-SUBUNIT OF THE RECEPTOR COMPLEX

Mouse granulocyte-macrophage colony-stimulating factor (mGM-CSF) prote ins with sub stitutions for residues located within alpha-helix D were examined for biological activity and receptor binding properties. Ala nine substitutions of the surface exposed positions indicated that sev eral residues contribute to the ligand-receptor interface. Position K- 108 and particularly D-102 appeared to dominate the binding epitope re cognized by mGM-R alpha. Several amino acid substitutions were made fo r K which reduced binding with concommitant losses in bioactivity. Sub stitutions for D-102 resulted in binding affinities less than 0.1% tha t of the wild-type mGM-CSF and bioactivity decreased to 1.0%. Comparat ive analysis using high and low affinity binding conditions indicated that mGM-R beta binding was unaffected by these mutations.