Structure and activity of the sevenless protein: a protein tyrosine kinase receptor required for photoreceptor development in Drosophila

The sevenless gene encodes a putative protein tyrosine kinase receptor that is required for the proper differentiation of the R7 photoreceptor cells of the Drosophila eye. We have expressed the sevenless protein in Drosophila tissue culture cells and studied its synthesis, processing, and activity. Our results show that the sevenless protein possesses protein tyrosine kinase activity. The protein is first synthesized as a 280-kDa glycoprotein precursor that is subsequently cleaved into 220-kDa amino-terminal and 58-kDa carboxyl-terminal subunits that remain associated by noncovalent interactions. The 220-kDa subunit is glycosylated and contains most of the extracellular portion of the protein, and the 58-kDa subunit is composed of a small portion of the extracellular sequences and the intracellular protein tyrosine kinase domain. This complex is subsequently cleaved into either 49- or 48-kDa carboxyl-terminal fragments with concomitant degradation of the rest of the protein.