Extracellular domain of the boss transmembrane ligand acts as an antagonist of the sev receptor

The fate of the R7 photoreceptor cell in the Drosophila compound eye is established by a specific inductive interaction between the R8 photoreceptor neuron and the R7 precursor cell¹. This induction is mediated by two cell-surface proteins: the ligand, bride of sevenless² (boss), and sevenless (sev), a tyrosine-kinase receptor^3-5. The structure of boss is unique for a ligand of a tyrosine-kinase receptor. It contains a large extracellular domain, seven transmembrane segments, and a carboxy-terminal cytoplasmic tail^6,7. Here we report that: (1) boss activates tyrosine phosphorylation of the sev receptor; (2) the seven transmembrane domain of boss is necessary for its function; and (3) a soluble form of boss acts as an antagonist of the sev receptor both in vivo and in vitro.