UC San Diego

The communication between the 5’ 7-methylguanosine cap structure and the 3’ poly(A) tail of eukaryotic mRNAs plays a crucial role in translation regulation. Scientists hypothesized that mRNAs interact with the cap-binding protein eIF4E, the poly(A)-binding protein PABP1, and eIF4GI through interactions with the 5’ cap structure and the 3’ poly(A) tail. These interactions bring the 5’ end and the 3’ end of the mRNA into proximity known as the eukaryotic “closed-loop” model. The formation of this “closed-loop” structure allows the physical communication between the 5’ end and the 3’ end of the mRNA, and it is thought to enhance translation initiation. Although the “closed-loop” model is a widely accepted hypothesis, there is still conflicting evidence both supporting and opposing this model.

To further elucidate this model, we tested the formation of the “pseudo-closed-loop” structure with human proteins in vitro using the electrophoretic mobility shift assay (EMSA). However, instead of forming the single “pseudo-closed-loop” structure, our results show that two non-interacting complexes are formed in vitro: the 5’cap•eIF4E•eIF4GI complex and the poly(A)-tail•PABP1•eIF4GI complex. The formation of these two separate complexes suggests that the binding of eIF4GI to eIF4E and PABP1 is competitive and mutually exclusive. Overall, we hypothesize that the formation of the “pseudo-closed-loop” structure may not occur in all mRNA transcripts or all contexts of eukaryotic translation initiation.