The high affinity receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF) is composed of at least two subunits, an 85-kDa low affinity GM-CSF-binding protein (alpha-GMR) and a 120-kDa beta-subunit (beta-GMR) necessary for high affinity binding and signal transduction. Previous studies have shown that deletion of the intracellular domain of alpha-GMR inactivates the receptor's ability to support proliferation, but has no effect on GM-CSF binding. Using anti-alpha-GMR- and anti-beta-GMR-specific antibodies, we show that alpha-GMR and beta-GMR coprecipitate only after GM-CSF binding, suggesting that binding of GM-CSF induces stabilization or assembly of an activated receptor complex involving recruitment of beta-GMR chains. To understand the contribution of each subunit of this receptor to the generation of an activated receptor complex, we attempted to construct minimal receptors with some or all of the functions of the wild-type heterodimer. We found that a hybrid human alpha/beta-GMR molecule in which the extracellular and transmembrane segments are composed of alpha-GMR sequences and the intracellular segment is composed of beta-GMR bound GM-CSF with low affinity, but activated tyrosine kinase activity, induced receptor internalization, and supported short- and long-term proliferation of transfected Ba/F3 cells. At least 1 ng/ml human GM-CSF was required for growth stimulation, and maximal proliferation occurred at a concentration of 10 ng/ml. This was 10-100-fold more than needed to stimulate growth of Ba/F3 cells expressing both full-length human alpha-GMR and beta-GMR and 1000-fold less than needed to stimulate growth of Ba/F3 cells expressing only human alpha-GMR. These results indicate that the cytoplasmic domain of alpha-GMR is not required to initiate a unique signaling event for proliferation in Ba/F3 cells, but can be functionally replaced by the cytoplasmic domain of beta-GMR.