NOVEL CAP INTERACTING PROTEIN EIF4E-IP2 AS A TRANSLATION REGULATOR

Translation of most cellular mRNAs in eukaryotes proceeds through a cap-dependent pathway, in which the cap-binding complex, eIF4F, anchors at the 5` cap and recruits 40S ribosomal subunits, along with several initiation factors. Upon exposure to stress, cap-dependent translation is mostly inhibited, except for a restricted set of proteins that continue to be synthesized, mostly through cap-independent mechanisms. Digenetic parasites provide an excellent model system for studying mRNA fate and translatability during stress, since the switch between vector and host requires that the parasites develop resistance mechanisms to a multitude of stresses. Trypanosomatid genomes encode six paralogs of the eukaryote cap-binding protein eIF4E, each playing a targeted role during specific stresses. Expression of the different cap-binding proteins, their phosphorylation status and their cap-binding activities are altered along the growth-curve of promastigotes, and during differentiation. Proteins that interact with the different cap-binding protein paralogs are also involved in directing their activity, forming a regulatory network. LeishIF4E-IP1 binds specifically to the LeishIF4E-1 isoform and reduces its capacity to bind the cap-structure; the tertiary structure of their complex has been recently completed. This interaction is observed mainly in promastigotes, and can therefore explain why LeishIF4E-1 continues to function in amastigotes, unlike the other isoforms. We recently identified a second LeishIF4E-interacting protein, Leish4E-IP2. In promastigotes, Leish4E-IP2 interacts with several LeishIF4Es, but is hardly pulled down from m⁷GTP affinity columns, suggesting that it could actually inhibit the ability of its associated LeishIF4E to bind the cap structure. Overexpressed 4E-IP2 limits the cap binding activity of several cap binding proteins, such as LeishIF4E-1, LeishIF4E-3 and LeishIF4E-4. Global translation is repressed in cells that overexpress Leish4E-IP2. This also accounts for faster differentiation to amastigote like cells. Thus, Leish4E-IP2 could play a key role in translation regulation during the progression of parasites through their growth cycle and differentiation.