Heterogeneous Nuclear Ribonucleoprotein A1 Is a Novel Internal Ribosome Entry Sitetrans-Acting Factor That Modulates Alternative Initiation of Translation of the Fibroblast Growth Factor 2 mRNA

Abstract Fibroblast growth factor 2 (FGF-2) is expressed in isoforms of different molecular masses from one mRNA species by alternative start of translation. The higher molecular mass isoforms (FGF-221 and 23) contain an arginine-rich N-terminus organized in RG-motifs followed by the 18 kDa FGF-2 (FGF-218) core which is common to all isoforms. Both isoforms localize differentially to the nucleus. Here, we analyzed the nuclear localization of FGF-221. Surprisingly, the lack of one RG-motif in FGF-221 resulted in the nucleolar distribution characteristic of FGF-218. We have previously shown that 23 kDa FGF-2 (FGF-223) interacts specifically with the survival of motoneuron (SMN) protein, an assembly protein for small nuclear ribonucleoprotein particles. For this assembly, Sm-proteins methylated by protein arginine methyltransferase 5 (PRMT5) are required. In our study, we aimed to analyze whether FGF-223 is also a substrate for symmetrical methylation by PRMT5. We could confirm that both proteins exist in a common complex. Moreover, PRMT5 methylates FGF-223 in vitro, whereas mutated inactive PRMT5 does not. FGF-223 is therefore a new substrate of PRMT5. With regard to function, inhibition of methyltransferase activity in HEK293T cells leads to cytoplasmic enrichment of FGF-2, indicating the importance of arginine methylation for shuttling of FGF-223 to the nucleus.

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Alternative translation of human fibroblast growth factor 2 mRNA occurs by internal entry of ribosomes.

Molecular and Cellular Biology ◽

10.1128/mcb.15.1.35 ◽

1995 ◽

Vol 15 (1) ◽

pp. 35-44 ◽

Cited By ~ 218

Author(s):

S Vagner ◽

M C Gensac ◽

A Maret ◽

F Bayard ◽

F Amalric ◽

...

Keyword(s):

Growth Factor ◽

Fibroblast Growth Factor ◽

Translation Initiation ◽

Internal Ribosome Entry Site ◽

Human Fibroblast ◽

Untranslated Region ◽

Fibroblast Growth Factor 2 ◽

Fibroblast Growth ◽

Entry Site ◽

Internal Ribosome Entry

Alternative initiations of translation of the human fibroblast growth factor 2 (FGF-2) mRNA, at three CUG start codons and one AUG start codon, result in the synthesis of four isoforms of FGF-2. This process has important consequences on the fate of FGF-2: the CUG-initiated products are nuclear and their constitutive expression is able to induce cell immortalization, whereas the AUG-initiated product, mostly cytoplasmic, can generate cell transformation. Thus, the different isoforms probably have distinct targets in the cell. We show here that translation initiation of the FGF-2 mRNA breaks the rule of the cap-dependent ribosome scanning mechanism. First, translation of the FGF-2 mRNA was shown to be cap independent in vitro. This cap-independent translation required a sequence located between nucleotides (nt) 192 and 256 from the 5' end of the 318-nt-long 5' untranslated region. Second, expression of bicistronic vectors in COS-7 cells indicated that the FGF-2 mRNA is translated through a process of internal ribosome entry mediated by the mRNA leader sequence. By introducing additional AUG codons into the RNA leader sequence, we localized an internal ribosome entry site to between nt 154 and 318 of the 5' untranslated region, just upstream of the first CUG. The presence of an internal ribosome entry site in the FGF-2 mRNA suggests that the process of internal translation initiation, by controlling the expression of a growth factor, could have a crucial role in the control of cell proliferation and differentiation.

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