Arnaud E, Touriol C, Boutonnet C, Gensac M C, Vagner S, Prats H, Prats A C
INSERM U397, Endocrinologie et Communication Cellulaire, Institut Louis Bugnard, C.H.U. Rangueil, 31403 Toulouse Cedex 04, France.
Mol Cell Biol. 1999 Jan;19(1):505-14. doi: 10.1128/MCB.19.1.505.
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.
人成纤维细胞生长因子2（FGF-2）的四种同工型源于在三个CUG起始密码子和一个AUG起始密码子处的选择性翻译起始。在此，我们鉴定了一种新的34 kDa FGF-2同工型，其表达起始于第五个起始密码子。通过使用N端定向抗体在HeLa细胞中鉴定出了这种34 kDa FGF-2。通过定点诱变确定其起始密码子为位于FGF-2 mRNA 5'端86个核苷酸（nt）处的CUG密码子。体外翻译和使用双顺反子RNA的COS-7细胞转染均表明，34 kDa FGF-2仅以帽依赖性方式表达。这与18、22、22.5和24 kDa的其他FGF-2同工型的表达形成对比，后者由内部核糖体进入位点（IRES）控制。令人惊讶的是，在存在FGF-2 mRNA的5823 nt长的3'非翻译区的情况下，其他FGF-2同工型的表达在体外部分变为帽依赖性。因此，FGF-2 mRNA可以通过帽依赖性和IRES驱动的机制进行翻译，这两种机制之间的平衡调节着不同FGF-2同工型的比例。还研究了新FGF-2的功能。我们发现，与其他同工型相比，34 kDa FGF-2能使NIH 3T3细胞在低血清条件下存活。对34 kDa FGF-2 N端区域一个新的富含精氨酸的核定位序列（NLS）进行了鉴定，发现它与人免疫缺陷病毒1型Rev蛋白的NLS相似。这些数据表明，34 kDa FGF-2的功能是由核靶标介导的。
