Gene fusion and overlapping reading frames in the mammalian genes for 4E-BP3 and MASK

4E-BP3 is a member of the eukaryotic initiation factor (eIF) 4F-binding protein family of translational repressors. eIF4E-binding proteins (4E-BPs) inhibit translation initiation by sequestering eIF4E, the cap-binding protein, from eIF4G thus preventing ribosome recruitment to the mRNA. Previous analysis of 4E-BP3 expression uncovered an 8.5-kb mRNA variant of unknown origin. To study this splice variant, we determined the structure of the genomic locus encoding human 4E-BP3 (EIF4EBP3). EIF4EBP3 is located on human chromosome 5q31.3 and comprises three exons (A, B, and C) and two introns. Exon B contains the region of the open reading frame responsible for eIF4E binding. GenBank(TM) searches revealed multiple expressed sequence tags originating from the alternative splicing of exon B with unidentified upstream exons. Further studies revealed that the 8.5-kb transcript arises from the fusion of EIF4EBP3 with the mammalian homologue of Drosophila MASK ( multiple ankyrin repeats, single KH domain), which is crucial for photoreceptor differentiation, cell survival, and proliferation. Surprisingly, the open reading frame of the MASK-BP3 transcript is different from that of 4E-BP3, which indicates that exon B is translated using an alternative reading frame. A gene fusion similar to that of MASK and EIF4EBP3 has been reported only once in mammals for the UEV1-Kua transcript. The use of an alternative reading frame is also very rare, having been described for two loci, INK4a/ARF and XLalphas/ALEX. The simultaneous exploitation of both mechanisms underscores the flexibility of mammalian genomes and has important implications for the functional analysis of 4E-BP3 and MASK. Interestingly, both eIF4E and MASK are downstream effectors of the Ras/MAPK pathway, which provides a rationale for the MASK-BP3 fusion in mammals.