A pathway of multi-chaperone interactions common to diverse regulatory proteins: estrogen receptor, Fes tyrosine kinase, heat shock transcription factor Hsf1, and the aryl hydrocarbon receptor.

A variety of regulatory proteins, including different classes of transcription factors and protein kinases, have been identified in complexes with Hsp90. On careful examination of unactivated progesterone receptor complexes, eight different protein participants have been identified, and each can be considered a component of the cytoplasmic molecular chaperone machinery. These proteins are Hsp90, Hsp70, Hip, p60, p23, FKBP51, FKBP52 and Cyp40. Studies in a cell-free assembly system have helped to define a highly ordered, dynamic pathway for assembly of progesterone receptor complexes. In the present study, target proteins other than progesterone receptor were used in this cell-free system to assemble complexes in vitro and to compare the composition of resulting complexes. Targets used were human estrogen receptor, human Fes protein-tyrosine kinase, human heat shock transcription factor Hsf1, and human aryl hydrocarbon receptor. The striking similarity of resulting target complexes with previously characterized progesterone receptor complexes suggest that each of these targets undergoes a common assembly pathway involving multiple chaperone components in addition to Hsp90.

Phosphorylation of the ras GTPase-activating protein (GAP) by the p93c-fes protein-tyrosine kinase in vitro and formation of GAP-fes complexes via an SH2 domain-dependent mechanism.

The protein-tyrosine kinase encoded by the human c-fes protooncogene (p93c-fes) plays a direct role in myeloid differentiation, but downstream substrates for this kinase have not been identified. Here we report that the human ras GTPase-activating protein (GAP) is a substrate for p93c-fes in vitro. Purified, recombinant GAP was readily phosphorylated on tyrosine residues by bacterially-expressed p93c-fes. Two-dimensional tryptic mapping revealed a single GAP phosphopeptide, consistent with specific phosphorylation of GAP by p93c-fes on one or several closely-spaced tyrosine residues. Autophosphorylated p93c-fes also formed a stable complex with GAP. Complex formation is likely to involve the src homology 2 (SH2) domains of GAP and autophosphorylated tyrosine residues of p93c-fes, as deletion of the fes SH2 domain did not abolish complex formation. Furthermore, immobilized recombinant fusion proteins containing either or both of the GAP SH2 domains were able to precipitate p93c-fes with an affinity equal to that observed with a monoclonal antibody against the recombinant fes protein. Fusion proteins containing the GAP N-terminal, C-terminal catalytic, or SH3 domains did not bind to p93c-fes. Interaction of the GAP SH2 domains with p93c-fes is phosphorylation-dependent, as the recombinant SH2 domain proteins were unable to bind to a kinase-defective c-fes mutant and showed reduced binding of a mutant in which one of the two tyrosine autophosphorylation sites was replaced with phenylalanine. Stimulation of c-fes autophosphorylation in vivo may induce interaction with GAP, resulting in altered p21ras function.

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.