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