Major splice variants and multiple polyadenylation site utilization in mRNAs encoding human translation initiation factors eIF4E1 and eIF4E3 regulate the translational regulators?

Abstract

Alternative polyadenylation is an important and pervasive mechanism that generates heterogeneous 3'-termini of mRNA and is considered an important regulator of gene expression. We performed bioinformatics analyses of ESTs and the 3'-UTRs of the main transcript splice variants of the translational initiation factor eIF4E1 and its family members, eIF4E2 and eIF4E3. This systematic analysis led to the prediction of new polyadenylation signals. All identified polyadenylation sites were subsequently verified by 3'RACE of transcripts isolated from human lymphoblastic cell lines. This led to the observation that multiple simultaneous polyadenylation site utilization occurs in single cell population. Importantly, we described the use of new polyadenylation site in the eIF4E1 mRNA, which lacked any known polyadenylation signal. The proportion of eIF4E1 transcripts derived from the first two polyadenylation sites in eIF4E1 mRNA achieved 15% in a wide range of cell lines. This result demonstrates the ubiquitous presence of ARE-lacking transcripts, which escape HuR/Auf1-mediated control, the main mechanism of eIF4E1 gene expression regulation. We found many EST clones documenting the significant production of transcript variants 2-4 of eIF4E2 gene that encode proteins with C-termini that were distinct from the mainly studied prototypical isoform A. Similarly, eIF4E3 mRNAs are produced as two main variants with the same very long 3'-UTR with potential for heavy post-transcriptional regulation. We identified sparsely documented transcript variant 1 of eIF4E3 gene in human placenta. eIF4E3 truncated transcript variants were found mainly in brain. We propose to elucidate the minor splice variants of eIF4E2 and eIF4E3 in great detail because they might produce proteins with modified features that fulfill different cellular roles from their major counterparts.

eIF4E3 forms an active eIF4F complex during stresses (eIF4FS) targeting mTOR and re-programs the translatome

“…Curiously, the sedimentation of the faster migrating band observed on the eIF4E3 blots (*) was unaltered by Torin1 treatment, indicating that it was not recruited into an eIF4F complex. This behaviour suggests that it might be the N-terminally truncated isoform B that was reported not to interact with eIF4G (the eIF4E3 polyclonal antibody used in these studies was raised against the C-terminal region of the protein) ( 52 , 53 ). Therefore, upon mTOR inhibition the sedimentation profiles of eIF4E1 and eIF4E3 are inversed (Figure 2F and G ).…”

Signal Transduction in Ribosome Biogenesis: A Recipe to Avoid Disaster

“…While the p70 S6K1 variant was found to be highly expressed in non-small cell lung cancer (NSCLC), the splice variants of aminoacyl-tRNA synthetases were observed under stress conditions and likely represent part of the cellular stress response [149,150]. Likewise, Mrvovà et al reported the identification of diverse alternatively spliced forms of eIF4E family members in acute lymphoblastic leukemia cell lines [151]. These alternatively spliced mRNAs were shown to either contain different polyadenylation sites, affecting the stability of the mRNA, or encode diverse C-termini, potentially altering the function of the protein.…”

Integrative identification of hub genes in development of atrial fibrillation related stroke

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