The ß subunit of human granulocyte-macrophage colony-stimulating factor receptor forms a homodimer and is activated via association with the α subunit

Abstract

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) receptor (hGMR) consists of α and β subunits, and the precise stoichiometry of these subunits has remained to be determined. In this work, oligomerization of the β subunit was studied using a chemical crosslinker. In Ba/F3, a mouse interleukin-3-dependent cell line expressing both subunits of hGMR (Ba/F3-α,ß), a protein with a molecular mass corresponding to that of a homodimer of the β subunit (β homodimer) was detected only when cells were treated with the cross-linker. Dimerization of the β subunit was confirmed by coimmunoprecipitation of a tagged β subunit with the wild type β subunit in COS7 cells. The β homodimer had already formed in the absence of hGM-CSF, whereas stimulation with the ligand brought both α and βsubunits into a complex, the result being tyrosine phosphorylation of the β homodimer. Tyrosine phosphorylation of the α subunit was impaired by deletion of the cytoplasmic domain of the α subunit without interfering with the association of both subunits. These results indicate that the β homodimer, which alone is insufficient for signaling, forms the functional hGMR with the a subunit in response to hGM-CSF.