Transient dimerization in the early secretory pathway characterizes the FGF-Receptors biosynthesis

The tyrosine kinase Fibroblast Growth Factor Receptors (FGFRs) are processed though different stages of maturation/glycosylation occurring in the various compartments of the secretory pathway. In this study, we have determined a novel event that characterizes the biosynthesis of FGFR3. We show that a limited fraction of the immature mannose-rich wild-type receptor undergoes transient dimerization in the early phase of the secretory pathway. The mature FGFR3 glycomers however, pursues the cell surface as monomers. The constitutive kinase activity associated to the K650E FGFR3 mutant that is retained in the endoplasmic reticulum (ER), increases dimerization efficiency and accordingly, abrogation of the kinase activity restores the “basal” dimerization level observed for the wild-type FGFR3. Furthermore, the K650E mutant dimerizes and trans-phosphorylates the immature wild-type FGFR3 supporting the hypothesis that dimerization of mutant and wild-type FGFR3 causes the retention of the latter in the ER. This may explain the genetic dominance of the K650E allele in the TDII disease. Fluorescence-based protein fragment complementation assay confirms FGFR3 dimerization in vivo and indicates FGFR3 as a cargo of the mannose-specific lectin ERGIC-53. Altogether these data highlight transient dimerization as a step in the FGFR3 biosynthesis and we hypothesize that this event could be part of a ‘pre-assembly quality control’ of the receptor, since the receptor function on the cell surface is exerted through the homo-dimerization upon the interaction with the specific FGF ligand. Furthermore, we show for the first time the presence of ERGIC53/FGFR3 complexes, suggesting a role for ERGIC53 in the transport of the FGFR3 from the ER to the Golgi. Finally, we observed that FGFR2, another member of the FGFR family, also shows a ‘basal’ level of dimerization of the immature mannose-rich isoforms, suggesting that intracellular dimerization could be a mechanism shared by all the members of the FGFRs family.