A new 34-kilodalton isoform of human fibroblast growth factor 2 is cap dependently synthesized by using a non-AUG start codon and behaves as a survival factor.
Four isoforms of human fibroblast growth factor 2 (FGF-2) result from alternative initiations of translation at three CUG start codons and one AUG start codon. Here we characterize a new 34-kDa FGF-2 isoform whose expression is initiated at a fifth initiation codon. This 34-kDa FGF-2 was identified in HeLa cells by using an N-terminal directed antibody. Its initiation codon was identified by site-directed mutagenesis as being a CUG codon located at 86 nucleotides (nt) from the FGF-2 mRNA 5' end. Both in vitro translation and COS-7 cell transfection using bicistronic RNAs demonstrated that the 34-kDa FGF-2 was exclusively expressed in a cap-dependent manner. This contrasted with the expression of the other FGF-2 isoforms of 18, 22, 22.5, and 24 kDa, which is controlled by an internal ribosome entry site (IRES). Strikingly, expression of the other FGF-2 isoforms became partly cap dependent in vitro in the presence of the 5,823-nt-long 3' untranslated region of FGF-2 mRNA. Thus, the FGF-2 mRNA can be translated both by cap-dependent and IRES-driven mechanisms, the balance between these two mechanisms modulating the ratio of the different FGF-2 isoforms. The function of the new FGF-2 was also investigated. We found that the 34-kDa FGF-2, in contrast to the other isoforms, permitted NIH 3T3 cell survival in low-serum conditions. A new arginine-rich nuclear localization sequence (NLS) in the N-terminal region of the 34-kDa FGF-2 was characterized and found to be similar to the NLS of human immunodeficiency virus type 1 Rev protein. These data suggest that the function of the 34-kDa FGF-2 is mediated by nuclear targets.
Arnaud
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E
INSERM U397, Endocrinologie et Communication Cellulaire, Institut Louis Bugnard, C.H.U. Rangueil, 31403 Toulouse Cedex 04, France.
Touriol
C
C
Boutonnet
C
C
Gensac
M C
MC
Vagner
S
S
Prats
H
H
Prats
A C
AC
D016475:3T3 Cells; D000818:Animals; D019556:COS Cells; D002470:Cell Survival; D018387:Codon, Initiator; D016222:Fibroblast Growth Factor 2 / Q000096:biosynthesis* / Q000235:genetics; D015694:Gene Products, rev / Q000235:genetics; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D019913:Nuclear Localization Signals; D010442:Peptide Chain Initiation, Translational; D014176:Protein Biosynthesis; D012315:RNA Caps; D012333:RNA, Messenger
D018387:Codon, Initiator; D015694:Gene Products, rev; D019913:Nuclear Localization Signals; D012315:RNA Caps; D012333:RNA, Messenger; D016222:Fibroblast Growth Factor 2