Cell specific transformation by c-fms activating loop mutations is attributable to constitutive receptor degradation

Expression of a receptor for human macrophage-colony stimulating factor (M- CSF or CSF-1), containing a point mutation which changes an aspartate to a valine at position 802 of the activating loop of the kinase domain, potentl y transforms the haemopoietic cell line FDC-P1 Set prevents Rat-2 fibroblas t transformation, In order to understand this apparent paradox, aspartate 8 02 was changed by cassette mutagenesis to each of the other 19 amino acids, AU hydrophobic amino acid substitutions were transforming when tested in F DC-P1 cells yet inactivating when tested in Rat-2 fibroblasts. These same a mino acid substitutions also activated receptor degradation, strongly sugge sting a causal relationship between receptor degradation and inactivation i n fibroblasts, Point mutations or small deletions of Y708 within the kinase insert region of the mutant D802V receptor partly inhibited receptor degra dation, The more stable D802V receptor derivatives were able to transform b oth FDC-P1 cells and Rat-2 fibroblasts, so establishing that the cell speci fic effect of the c-fmsD802V activating loop mutation is attributable to re ceptor degradation which accompanies kinase activation and prevents the tra nsformation of Rat-2 hut not of FDC-P1 cells.