ARG(777) PLAYS A MAJOR ROLE IN THE CONFORMATION OF THE COLONY-STIMULATING FACTOR-I RECEPTOR INTRACELLULAR KINASE DOMAIN

A point mutation substituting Arg(777) by Gln was obtained in a highly conserved region of the human colony-stimulating factor-1 receptor (C SF-1R) sequence, Constitutive expression of wild-type receptors in CHO cells confers susceptibility to CSF-1 for proliferation whereas the m utated receptors exhibited a 90% reduced efficiency in proliferation. We sought to determine the alterations intervening in the CSF-1 signal transduction of the Arg(777)Gln mutated receptor. We found that ligan d binding and ligand-induced CSF-1R internalization were unaffected. C SF-1-induced receptor dimerization and autophosphorylation mere impair ed to the same extent as mitogen-activated protein kinase activation ( 90%). However, only phosphatidylinositol 3-kinase activation and ligan d-induced receptor ubiquitination were abrogated by the mutation. Thes e features probably reflect the inability of the mutated CSF-1R kinase domain to fold properly and hence to autophosphorylate and/or to asso ciate correctly with transduction proteins. These data may indicate a role for the conserved regions of the RTK kinase domains in the stabil ization of the intracellular domain conformation.