Structural features of the colony-stimulating factor 1 receptor that affect its association with phosphatidylinositol 3-kinase.

The colony-stimulating factor 1 receptor (CSF-1R), immunoprecipitated with either anti-phosphotyrosine or anti-receptor antibodies from lysates of ligand-stimulated cells, is associated with a phosphatidylinositol (PtdIns) 3-kinase activity. The ligand-independent transforming efficiencies of human CSF-1R mutants containing certain amino acid substitutions at codon 301 in their extracellular domains correlated directly with their levels of associated lipid kinase activity. A tyrosine kinase defective CSF-1R mutant (CSF-1R[met616]), containing a mutated ATP binding site, lacked associated PtdIns 3-kinase activity in immune complexes recovered from CSF-1-stimulated cells. However, CSF-1R[met616] associated with PtdIns 3-kinase when phosphorylated in trans in CSF-1-stimulated cells coexpressing an enzymatically competent CSF-1R tyrosine kinase. Another CSF-1R mutant, (CSF-1R[delta KI]), lacking 67 amino acids from its intracellular 'kinase insert' domain, exhibited a partially impaired ligand-dependent mitogenic response and a significant reduction in its associated PtdIns 3-kinase activity. Ligand-stimulated CSF-1R[delta KI] molecules contained levels of phosphotyrosine almost equivalent to wild-type receptors, but were phosphorylated at different sites in vitro. Therefore, the association of CSF-1R with active PtdIns 3-kinase required the receptor tyrosine kinase activity, was triggered by receptor phosphorylation on tyrosine and, in this series of mutants, correlated with their mitogenic potential. Although the receptor KI domain strongly contributes to the association with PtdIns 3-kinase, this region is not strictly essential for the interaction.