Volume 271, Number 24,
Issue of June 14, 1996
pp. 14198-14205
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
The Nuclear Trafficking of Extracellular Fibroblast Growth Factor
(FGF)-1 Correlates with the Perinuclear Association of the FGF
Receptor-1
Isoforms but Not the FGF Receptor-1
Isoforms
(Received for publication, August 28, 1995, and in revised form, April 2, 1996)
Igor A.
Prudovsky
,
Naphtali
Savion
,
Theresa M.
LaVallee
and
Thomas
Maciag
From the Department of Molecular Biology, Holland Laboratory,
American Red Cross, Rockville, Maryland 20855
The alternatively spliced fibroblast growth
factor receptor (FGFR)-1 isoforms, FGFR-1
and FGFR-1
, 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-1 and FGFR-1 L6 myoblast
transfectants was studied. Although FGFR-1 was expressed as p145 and
p125 forms, FGFR-1 was expressed as p120 and p100 forms in the L6
myoblast transfectants. Tunicamycin and N-glyconase
experiments suggest that these forms of FGFR-1 and FGFR-1 are the
result of differential glycosylation. However, only the p145 form of
FGFR-1 and the p120 form of FGFR-1 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-1 and p120 FGFR-1, respectively. Because
ligand-chase analysis demonstrated that FGFR-1 L6 myoblast
transfectants exhibited a reduced efficiency of nuclear translocation
of exogenous FGF-1 when compared with FGFR-1 transfectants, the
intracellular trafficking of the FGFR-1 and FGFR-1 isoforms
was studied using an in vitro kinase assay to amplify
immunoprecipitated FGFR-1. Indeed, the appearance of the FGFR-1 but
not FGFR-1 isoform in the nuclear fraction of L6 myoblast
transfectants suggests that the distal Ig-like loop in FGFR-1
mediates the differential nuclear association of FGFR-1 as a
structurally intact and functional tyrosine kinase. Further, the
FGFR-1 L6 myoblast transfectants but not the FGFR-1 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-1 and FGFR-1 may ultimately exhibit differential trafficking
to adhesion sites.
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