The nuclear trafficking of extracellular fibroblast growth factor (FGF)-1 correlates with the perinuclear association of the FGF receptor-1alpha isoforms but not the FGF receptor-1beta isoforms.
The alternatively spliced fibroblast growth factor receptor (FGFR)-1 isoforms, FGFR-1alpha and FGFR-1beta, are characterized by the presence of either three or two Ig-like loops in the extracellular domain and are differentially expressed during embryonic development and tumor progression. We have previously shown that in cells irreversibly committed to DNA synthesis by FGF-1, approximately 15% of cell surface FGFR-1 traffics to a perinuclear locale as a structurally intact and functional tyrosine kinase (Prudovsky, I., Savion, N., Zhan, X., Friesel, R., Xu, J., Hou, J., McKeehan, W. L., and Maciag, T. (1994) J. Biol. Chem. 269, 31720-31724). In order to define the structural requirement for association of FGFR-1 with the nucleus, the expression and trafficking of FGFR-1 in FGFR-1alpha and FGFR-1beta L6 myoblast transfectants was studied. Although FGFR-1alpha was expressed as p145 and p125 forms, FGFR-1beta was expressed as p120 and p100 forms in the L6 myoblast transfectants. Tunicamycin and N-glyconase experiments suggest that these forms of FGFR-1alpha and FGFR-1beta are the result of differential glycosylation. However, only the p145 form of FGFR-1alpha and the p120 form of FGFR-1beta were able to bind FGF-1 and activate tyrosine phosphorylation. Pulse-chase analysis of FGFR-1 biosynthesis suggests that the p125 and p100 proteins are the precursor forms of p145 FGFR-1alpha and p120 FGFR-1beta, respectively. Because ligand-chase analysis demonstrated that FGFR-1beta L6 myoblast transfectants exhibited a reduced efficiency of nuclear translocation of exogenous FGF-1 when compared with FGFR-1alpha transfectants, the intracellular trafficking of the FGFR-1alpha and FGFR-1beta isoforms was studied using an in vitro kinase assay to amplify immunoprecipitated FGFR-1. Indeed, the appearance of the FGFR-1alpha but not FGFR-1beta isoform in the nuclear fraction of L6 myoblast transfectants suggests that the distal Ig-like loop in FGFR-1alpha mediates the differential nuclear association of FGFR-1alpha as a structurally intact and functional tyrosine kinase. Further, the FGFR-1beta L6 myoblast transfectants but not the FGFR-1alpha myoblast transfectants exhibited a pronounced morphologic change in response to exogenous FGF-1. Because this phenotype change involves the induction of a rounded cellular shape, it is possible that the FGFR-1alpha and FGFR-1beta may ultimately exhibit differential trafficking to adhesion sites.
Prudovsky
I A
IA
Department of Molecular Biology, Holland Laboratory, American Red Cross, Rockville, Maryland 20855, USA.
Savion
N
N
LaVallee
T M
TM
Maciag
T
T
D017398:Alternative Splicing; D000818:Animals; D001345:Autoradiography; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D016220:Fibroblast Growth Factor 1 / Q000378:metabolism*; D015870:Gene Expression; D006031:Glycosylation; D015151:Immunoblotting; D007700:Kinetics; D008024:Ligands; D008715:Methionine / Q000378:metabolism; D008970:Molecular Weight; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism; D051496:Receptor, Fibroblast Growth Factor, Type 1; D017468:Receptors, Fibroblast Growth Factor / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism; D015398:Signal Transduction; D013462:Sulfur Radioisotopes; D014162:Transfection; D014415:Tunicamycin / Q000494:pharmacology
D008024:Ligands; D017468:Receptors, Fibroblast Growth Factor; D011994:Recombinant Proteins; D013462:Sulfur Radioisotopes; D016220:Fibroblast Growth Factor 1; D014415:Tunicamycin; D008715:Methionine; D020794:Receptor Protein-Tyrosine Kinases; D051496:Receptor, Fibroblast Growth Factor, Type 1