IN-VIVO ANALYSIS OF ARGOS STRUCTURE-FUNCTION - SEQUENCE REQUIREMENTS FOR INHIBITION OF THE DROSOPHILA EPIDERMAL GROWTH-FACTOR RECEPTOR

The Drosophila Argos protein is the only known extracellular inhibitor of the epidermal growth factor receptor (EGFR), It is structurally re lated to the activating ligands, in that it is a secreted protein with a single epidermal growth factor (EGF) domain, To understand the mech anism of Argos inhibition, we have investigated which regions of the p rotein are essential, A series of deletions were made and tested in vi vo; furthermore, by analyzing chimeric proteins between Argos and the activating ligand, Spitz (a transforming growth factor-alpha-like fact or), we have examined what makes one inhibitory and the other activati ng, Our results reveal that Argos has structural requirements that dif fer from all known EGFR activating ligands; domains flanking the EGF d omain are essential for its function, We have also defined the importa nt regions of the atypical Argos EGF domain, The extended B-loop is ne cessary, whereas the C-loop can be replaced with the equivalent Spitz region without substantially affecting Argos function, Comparison of t he argos genes from Drosophila melanogaster and the housefly, Musca do mestica, supports our structure-function analysis, These studies are a prerequisite for understanding how Argos inhibits the Drosophila EGFR and provide a basis for designing mammalian EGFR inhibitors.