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

Abstract

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 cell. This induction is mediated by two cell-surface proteins: the ligand, bride of sevenless (boss), and sevenless (sev), a tyrosine-kinase receptor. 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. 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.