The extracellular region of granulocyte colony-stimulating factor receptor in solution has multiple oligomerization states without ligand

Expression and purification of the extracellular portion of granulocyte colony-stimulating factor (G-CSF) receptor, which contains an immunoglobulin-like (Ig) domain and the cytokine receptor homologous (CRH) region, using a baculovirus secretion system have shown that a tetrameric Ig-CRH protein (about 200 kDa) existed in addition to the dimer (85 kDa) [7]. Scatchard analysis revealed that the tetramer had ligand binding affinity, with a dissociation constant of about 2.5 nM. The tetramer dissociated into monomers at pH 2 and was re-formed at pH 7, in contrast, the dimer was re-dimerized with the same treatment. These observations led us to hypothesize the existence of conformational heterogeneity, which leads to tetramer as well as dimer formation, in the soluble state of the Ig-CRH protein.