Effect of transmembrane and kinase domain mutations on fibroblast growth factor receptor 3 chimera signaling in PC12 cells - A model for the control of receptor tyrosine kinase activation

The effect of six point mutations causing various human skeletal dysplasias , occurring in the transmembrane (TM) and kinase domains (KD) of fibroblast growth factor receptor 3, were introduced into a chimera composed of the e xtracellular domain of human platelet-derived growth factor beta and the TM and intracellular domains of hFGFR3, Stable transfectants in rat PC12 cell s showed distinct differences in the two classes of mutations. The cells co ntaining TM mutants displayed normal expression and activation but higher r esponsiveness to lower doses of ligand, The KD mutants showed significantly altered expression patterns. Normal amounts of a lower M-r receptor (p130) reflecting incomplete glycosylation, but only greatly decreased amounts of the mature (p170) form, were observed. However, the latter material showed normal ligand-dependent activation. In contrast, the p130 form, which is r egularly observed in the expression of both native and chimeric receptors, exhibits strong ligand-independent tyrosine phosphorylation, particularly w ith the K650E mutation. Expression of two of the KD mutants (K650M and K650 E), under control of an inducible metallothionein promoter, indicated that this receptor was sufficiently autoactivated to produce at least partial di fferentiation and, in the case of the K650E mutation, to induce ligand-inde pendent neurite outgrowth. A model is presented that suggests that the low IM, (p130) KD mutants can, under the right conditions, signal intracellular ly, but when they are fully glycosylated and move to the cell surface they adopt a normal, inhibited conformation, in the form of ligand-independent d imers, that neutralizes the effects of the mutations. When ligands bind, th ese dimeric receptors are activated in a normal manner. This model suggests that unliganded dimers may be a common intermediate in receptor tyrosine k inase signaling.