Tyrosine-protein kinase Fes/Fps
protein autophosphorylation
Regulation of the human c-fes protein tyrosine kinase (p93c-fes) by its src homology 2 domain and major autophosphorylation site (Tyr-713).
The c-fes proto-oncogene product is expressed predominantly in hematopoietic cells of the myeloid lineage and has been implicated in the regulation of myeloid differentiation. The c-fes locus encodes a 93-kDa protein tyrosine kinase (p93c-fes) that possesses several structural features characteristic of the cytoplasmic class of protein tyrosine kinases, including a consensus sequence for autophosphorylation surrounding Tyr-713 and a src homology 2 (SH2) domain. To assess the effect of each of these potential regulatory sites on p93c-fes protein tyrosine kinase activity, we specifically deleted the c-fes SH2 domain using the polymerase chain reaction and replaced Tyr-713 with phenylalanine by oligonucleotide-directed mutagenesis (Y713F mutant). The resulting mutants were expressed in Escherichia coli and assayed for changes in protein tyrosine kinase activity using an immune complex kinase assay. Both mutations produced a marked decrease in the rate and extent of autophosphorylation and phosphorylation of the model substrate, enolase. To test whether the c-fes SH2 domain could interact with the autophosphorylated kinase domain, the SH2 domain was expressed as a fusion protein with glutathione S-transferase and immobilized on glutathione-agarose. The recombinant c-fes SH2 domain precipitated p93c-fes as readily as a monoclonal antibody. Binding of the SH2 domain to p93c-fes was completely dependent upon autophosphorylation, as a kinase-defective mutant of p93c-fes was not precipitated by the SH2 domain. High-affinity binding was also observed with recombinant SH2 domains from v-src and v-fps, raising the possibility of protein-protein interactions between various members of the cytoplasmic PTK family. These results indicate that the c-fes SH2 domain and consensus autophosphorylation site (Tyr-713) play major roles in the positive regulation of p93c-fes tyrosine kinase activity, possibly through intramolecular interaction.