Molecular characterization of a granulocyte macrophage–colony-stimulating factor receptor α subunit-associated protein, GRAP

Abstract

The granulocyte macrophage–colony-stimulating factor receptor (GM-CSF-R) is a heterodimer composed of 2 subunits,  and β, and ligand binding to the high-affinity receptor leads to signalling for the multiple actions of GM-CSF on target cells. In order to explore the role of the  subunit in signalling, we used a yeast-2-hybrid system to identify proteins interacting with the intracellular domain of the GMR-. A cDNA encoding a predicted protein of 198 amino acids, designated GRAP (GM-CSFreceptor  subunit-associatedprotein), was isolated in experiments using the intracellular portion of GMR- as bait. The interaction between GRAP and GMR- was confirmed by coimmunoprecipitation in mammalian cells. GRAP mRNA is widely expressed in normal human and mouse tissues and in neoplastic human cell lines, but it is not restricted to cells or tissues that express GM-CSF receptors. Three discrete GRAP mRNA species were detected in human tissues and cells, with estimated sizes of 3.3, 3.1, and 1.3 kb. GRAP is highly conserved throughout evolution, and homologues are found in yeast. The GRAP locus in Saccharomyces cerevisiae was disrupted, and mutant yeast cells showed an inappropriate stress response under normal culture conditions, manifested by early accumulation of glycogen during the logarithmic growth phase. GRAP is, therefore, a highly conserved and widely expressed protein that binds to the intracellular domain of GMR-, and it appears to play an important role in cellular metabolism.