DIMERIZATION OF GRANULOCYTE-COLONY-STIMULATING FACTOR-RECEPTOR - THE IG PLUS CRH CONSTRUCT OF GRANULOCYTE-COLONY-STIMULATING FACTOR-RECEPTOR FORMS A 2 2 COMPLEX WITH A LIGAND/

We have previously shown that the extracellular domain of granulocyte- colony stimulating factor receptor (soluble G-CSFR), prepared from CHO cell conditioned media, dimerizes upon binding its ligand, G-CSF. The most stable ligand-receptor complex occurs at a 2:2 stoichiometry, un like the growth hormone and erythropoietin systems. In the latter case s, each ligand uses two binding sites to bring two receptors together. In this study, ire have generated a truncated human G-CSF receptor, k nown to be sufficient for high affinity ligand binding, which consists of an Ig-like domain and a cytokine receptor homology module. With an affinity purified receptor, sedimentation equilibrium experiments cle arly demonstrated that this truncated form of the receptor behaves ver y similarly to the entire extracellular domain. The sedimentation equi librium data are consistent with the model that the truneated receptor has a weak tendency to self-associate into a dimer in the absence of a ligand, this receptor-receptor interaction is enhanced by ligand bin ding, and the most stable complex occurs at a 2:2 stoichiometry. These results ape very different from those described by others for various murine G-CSF receptor constructs from either Escherichia coli or inse ct expression systems.