LIGAND-INDEPENDENT CELL-SURFACE EXPRESSION OF THE HUMAN SOLUBLE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR-RECEPTOR ALPHA-SUBUNIT DEPENDS ON COEXPRESSION OF THE MEMBRANE-ASSOCIATED RECEPTOR-BETA SUBUNIT

The hematopoietic cytokine granulocyte-macrophage colony-stimulating f actor (GM-CSF) mediates its activity through binding to cell surface r eceptors, The receptor for GM-CSF belongs to a superfamily of cytokine receptors characterized by a conserved extracellular motif. The high affinity GM-CSF receptor (GMR) consists of two transmembrane anchored subunits; a ligand binding alpha subunit (transmembrane GMR alpha) and a signal transducing beta subunit (GMR beta), both of which belong to the cytokine receptor superfamily, The human GM-CSF receptor alpha su bunit also exists in a soluble form (sol-GMR alpha), which antagonizes GM-CSF activity in vitro, We directly tested the potential for solGMR alpha to interact with GMR beta in vitro, Our experiments demonstrate d that exogenous solGMR alpha, even in the presence of GM-CSF, does no t interact with GMR beta on the cell surface, However, when solGMR alp ha and GMR beta are co expressed in baby hamster kidney cells, solGMR alpha is retained on the cell surface and forms a functional intermedi ate affinity GM-CSF binding complex (K-d = 331 pM). In addition, the c ell surface expression of solGMR alpha is independent of the presence of GM-CSF as demonstrated using flow cytometry, Cells expressing only solGMR alpha do not show cell surface retention or form functional GM- CSF cell surface binding complexes. Sequencing of our GMR beta clone r evealed a nucleotide substitution (A --> C) resulting in the substitut ion of Ala for Glu at position 9 from the amino terminus of the mature GMR beta peptide. Because the GMR beta (A --> C) clone is capable of forming functional high affinity receptors with transmembrane GMR alph a (K-d = 64 pM), we feel that the cell surface retention of solGMR alp ha is independent of the GMR beta mutation, We suggest that the co-exp ression and interaction of solGMR alpha and GMR beta represents a prev iously unrecognized GM-CSF receptor complex and a novel, ligand-indepe ndent mechanism of cytokine receptor assembly.