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

The GM-CSF receptor consists of a GM-CSF specific low affinity alpha-subuni t (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 s oluble isoform of GMR alpha (sol alpha) consists of the extracellular domai n of GMR alpha and a unique 16-amino acid C-terminal domain. Exogenously ad ministered 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-ex pression 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 al pha. To study the interaction and functional characteristics of the sol alp ha-beta c complex we engineered a soluble beta c-subunit (ECD beta) and exp ressed it alone and with sol alpha. Co-expressed but not independent source s of solo and ECD beta could be co-precipitated in the absence of ligand de monstrating the extracellular domain of beta c was sufficient for associati on 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-termin al deficient sol alpha mutant (EGD alpha) and the addition of ECD beta to E CD alpha and GM-CSF was associated with a conversion from a low- to high-af finity ligand-receptor complex. (C) 2000 Academic Press.