Oligomerization of the soluble granulocyte-macrophage colony-stimulating factor receptor: identification of the functional ligand-binding species.

The ligand-specific alpha subunit of the dimeric human GM-CSF receptor exists in both transmembrane anchored (tmGM-CSFR alpha) and soluble (sGM-CSFR alpha) isoforms. sGM-CSFR alpha binds to GM-CSF in solution and antagonizes GM-CSF biological activity in vitro. In an effort to better understand the biological properties of sGM-CSFR alpha the authors have attempted to define the exact stoichiometry of the interaction between GM-CSF and sGM-CSFR alpha. Size separation of sGM-CSFR alpha by polyacrylamide gel electrophoresis (PAGE) under non-reducing conditions demonstrated that sGM-CSFR alpha can exist in solution not only in a monomeric state but also in higher order oligomers. FPLC analysis of ligand/sGM-CSFR alpha complexes suggested that only one of these sGM-CSFR alpha species could functionally bind GM-CSF. PAGE analysis of FPLC fractions demonstrated that the peak of GM-CSF binding activity corresponded to the presence of a monomeric form of sGM-CSFR alpha. The experiments demonstrate that while sGM-CSFR alpha can adopt oligomeric forms in solution, the binding of GM-CSF to sGM-CSFR alpha most likely occurs in a (GM-CSF)1 (sGM-CSFR alpha)1 configuration.