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 receptor3-5. 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 tail6,7. 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.
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