During the initiation stage of eukaryotic mRNA translation, the eIF4G (eukaryotic initiation factor 4G) proteins act as an aggregation point for recruiting the small ribosomal subunit to an mRNA. We previously used RNAi (RNA interference) to reduce expression of endogenous eIF4GI proteins, resulting in reduced protein synthesis rates and alterations in the morphology of cells. Expression of EIF4G1 cDNAs, encoding different isoforms (f–a) which arise through selection of alternative initiation codons, rescued translation to different extents. Furthermore, overexpression of the eIF4GII paralogue in the eIF4GI-knockdown background was unable to restore translation to the same extent as eIF4GIf/e isoforms, suggesting that translation events governed by this protein are different. In the present study we show that multiple isoforms of eIF4GII exist in mammalian cells, arising from multiple promoters and alternative splicing events, and have identified a non-canonical CUG initiation codon which extends the eIF4GII N-terminus. We further show that the rescue of translation in eIF4GI/eIF4GII double-knockdown cells by our novel isoforms of eIF4GII is as robust as that observed with either eIF4GIf or eIF4GIe, and more than that observed with the original eIF4GII. As the novel eIF4GII sequence diverges from eIF4GI, these data suggest that the eIF4GII N-terminus plays an alternative role in initiation factor assembly.

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