Functional dissection of structural domains in the receptor for colony-stimulating factor-1

Receptor tyrosine kinases (RTKs) transduce external signals to the interior of the cell via a cytoplasmic kinase domain. We demonstrated previously that ligand-induced kinase activation of the colony-stimulating factor-1 receptor (CSF-1R) occurs via receptor oligomerization without propagation of conformational changes through the transmembrane (TM) domain (Lee, A. W., and Nienhuis, A. W. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 7270-7274). We have now examined the role of the different subdomains in the metabolic and signaling properties of CSF-1R. Two types of chimeric receptors have been utilized, Glyfms A, with the extracellular and TM domains of glycophorin A (GpA) and the cytoplasmic domain of CSF-1R, and Glyfms B, where only the extracellular domain originates from GpA. Glyfms A was found to exhibit a higher basal level of in vitro kinase activity, an increased associated phosphatidylinositol (PtdIns) 3-kinase activity and to support enhanced cellular mitogenesis, compared with wild-type CSF-1R or to Glyfms B. The constitutive activation of Glyfms A is consistent with the hypothesis that the TM domain may play a role in receptor oligomerization. Cross-linking with anti-GpA antibodies activated the kinase function of Glyfms B leading to an increase in PtdIns 3-kinase association and to the transmission of a mitogenic signal. Our results indicate that an activated kinase domain contains the major determinant for coupling with PtdIns 3-kinase, independent of extracellular and TM sequences and of ligand binding. Both chimeric receptors underwent internalization in the presence of anti-GpA antibodies but were not degraded, including the tyrosine-phosphorylated and kinase-active population. These results suggest that structural determinants in the extracellular domain must be important for targeting internalized receptors for lysosomal degradation.