Truncation of the mRNA cap-binding protein eIF4E is specific for the non-invasive implantation in pigs.

The mechanism of implantation is species specific (pig: epitheliochorial, bovine: synepitheliochorial, mouse: hemochorial). Recently, we have shown that proteolytical cleavage of the prototypical 25 kDa mRNA cap-binding protein eIF4E (eukaryotic initiation factor 4E) produces a stable variant with a molecular mass of approximately 23 kDa in porcine endometrium at the time of implantation. Here, we investigate if an eIF4E truncation also takes place in the endometrium of species with other implantation forms. Thus, eIF4E and its repressor protein 4E-BP1 were investigated in porcine, murine and bovine endometrium during the time of implantation. Our results show that eIF4E truncation is specific for the porcine implantation. In bovine and mouse uterine tissue, no cleavage of eIF4E was observed. Whereas no difference of bovine 4E-BP1 was found, in murine samples, increased phosphorylation during implantation was observed. However, porcine samples exhibit an opposite behaviour, the abundance and mainly the phosphorylation of 4E-BP1 decrease. We propose that the translation initiation in the endometrium is differently regulated by the two eIF4E forms with regard to different 4E-BP1 abundance and phosphorylation as well as different eIF4E/4E-BP1 binding dynamic depending on the type of implantation.