SUBSTRATE SPECIFICITIES AND IDENTIFICATION OF A PUTATIVE BINDING-SITEFOR PI3K IN THE CARBOXY TAIL OF THE MURINE FLT3 RECEPTOR TYROSINE KINASE

Flt3 is a receptor tyrosine kinase (RTK) structurally related to the C SF-1R encoded by the c-fms locus, Kit and the PDGFR which is restricte d in its expression to hematopoietic precursor populations and several distinct cell types within the central nervous system. Although the l igand for Flt3 has recently been identified, the developmental functio n of Flt3 within these tissues has not yet been described. In order to examine the signalling properties of this receptor, we previously con structed a chimeric molecule containing the extracellular domain of CS F-1R fused to the transmembrane and cytoplasmic domain of mouse Flt3 ( FF3). The ability of the FF3 to directly associate with or tyrosine ph osphorylate specific cytoplasmic signalling molecules in vivo was exam ined. GAP, Vav, She, and to a lessor extent PLC gamma become tyrosine- phosphorylated but no in vivo association with the receptor was detect able. FF3 associates with PI3K activity and the SH2 domains of p85 and Grb-2. Phosphopeptide competition experiments suggest that the PI3K b inding site is located outside of the kinase insert in the carboxy tai l of the receptor.