Determinants of the functional interaction between the soluble GM-CSF receptor and the GM-CSF receptor β-subunit

The GM-CSF receptor consists of a GM-CSF specific low affinity alpha-subunit (GMR alpha) and a beta-subunit (beta c) that associates with GMR alpha in the presence of GM-CSF to form a high-affinity complex. A splice variant soluble isoform of GMR alpha (sol alpha) consists of the extracellular domain of GMR alpha and a unique 16-amino acid C-terminal domain. Exogenously administered sol alpha is unable to associate with beta c on the cell surface either in the presence or absence of GM-CSF, However, paradoxically, co-expression of solo with beta c results in the ligand-independent association of solo with beta c on the cell surface via the G-terminal domain of sol alpha. To study the interaction and functional characteristics of the sol alpha-beta c complex we engineered a soluble beta c-subunit (ECD beta) and expressed it alone and with sol alpha. Co-expressed but not independent sources of solo and ECD beta could be co-precipitated in the absence of ligand demonstrating the extracellular domain of beta c was sufficient for association with solo upon co-expression. However, independent sources of sol alpha could associate with ECD beta in the presence of GM-CSF as could a C-terminal deficient sol alpha mutant (EGD alpha) and the addition of ECD beta to ECD alpha and GM-CSF was associated with a conversion from a low- to high-affinity ligand-receptor complex. (C) 2000 Academic Press.