Evidence for the ligand-induced conversion from a dimer to a tetramer of the granulocyte colony-stimulating factor receptor

An extracellular portion of granulocyte colony-stimulating factor (G-CSF) receptor, which contains an immunoglobulin-like (Ig) domain and cytokine receptor homologous (CRH) region, was secreted into the medium using Trichoplusia ni-Autographa californica nuclear polyhedrosis virus system. The gene product was purified to homogeneity mainly as a dimer (85 kDa) using G-CSF affinity column chromatography and gel filtration HPLC, although the product existed as a monomer (45 kDa) in the medium. Scatchard analyses suggested that only the dimer had high affinity ligand binding ( K d = about 100 pM), which is comparable with the K d value of the cell surface receptor. The binding of G-CSF to Ig-CRH induced its tetramerization (200–250 kDa). The molecular composition of the tetrameric complex showed a stoichiometry of four ligands bound to four Ig-CRH. These results suggested that the oligomeric mechanism of the G-CSF receptor differs from that reported for growth hormone (GH) receptor, although CD spectrum spectroscopy suggested that the Ig-CRH has a GH receptor-like structure.