Recruitment of the protein tyrosine phosphatase CSW by DOS is an essential step during signaling by the sevenless receptor tyrosine kinase.

The pleckstrin homology (PH) domain-containing protein Daughter of Sevenless (DOS) is an essential component of the Sevenless receptor tyrosine kinase (SEV) signaling cascade, which specifies R7 photoreceptor development in the Drosophila eye. Previous results have suggested that DOS becomes tyrosine phosphorylated during SEV signaling and collaborates with the protein tyrosine phosphatase CSW. We have investigated this possibility by identifying tyrosine residues 801 and 854 of DOS as the phosphorylated binding sites for the CSW SH2 domains. We show that these sites become phosphorylated in response to SEV activation and that phosphorylation of both sites is required to allow CSW to bind DOS. Mutant DOS proteins in which either Y801 or Y854 of DOS has been changed to phenylalanine are unable to function during signaling by SEV and other receptor tyrosine kinases. In contrast, we find that a mutant DOS protein in which all tyrosine phosphorylation sites except Y801 and Y854 have been removed is able effectively to provide DOS function during SEV signaling and to rescue the lethality associated with dos loss-of-function mutations. These results indicate that a primary role for DOS during signaling by SEV and other receptor tyrosine kinases is to become phosphorylated at Y801 and Y854 and then recruit CSW.