Identification of conserved amino acids in the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit critical for function. Evidence for formation of a heterodimeric receptor complex prior to ligand binding.

A superfamily of growth factor and cytokine receptors has recently been identified, which is characterized by four spatially conserved cysteine residues, a tryptophan-serine motif (WSXWS) in the extracellular domain, and a proline-rich cytoplasmic domain. The high affinity human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (hGM-CSFR) consists of two subunits, alpha (hGM-CSFR alpha) and beta (hGM-CSFR beta), both of which are members of the receptor superfamily. In this study, we prepared mutations in conserved amino acids of the receptor subunit necessary for GM-CSF binding (hGM-CSFR alpha) and analyzed mutant receptors for low affinity binding, internalization, and high affinity binding when complexed with the beta subunit. Mutations in the cytoplasmic domain did not affect GM-CSF binding or receptor internalization. Mutation of a single conserved serine residue within the WSXWS motif diminishes cell surface receptor expression but not ligand binding. Mutation of either the second or third conserved cysteine residue of hGM-CSFR alpha resulted in complete loss of low affinity binding; however, co-expression of the cysteine 2 mutant with hGM-CSFR beta yielded a high affinity receptor complex. Since neither the cysteine 2 mutant nor the beta subunit can bind ligand alone, this result suggests that hGM-CSFR alpha and hGM-CSFR beta exist in a preformed heterodimeric protein complex on the plasma membrane.