Targeting trypanosomatid translation factor interactions

Trypanosomatids are eukaryotic parasites that migrate between insect vectors and mammalian hosts. They cause a number of serious diseases with major impacts on human health on a global scale. One example is Leishmania, which causes Leishmaniasis, a disease that affects approximately 12 million people in Asia, Middle East, Africa, South America and Southern Europe. Trans-splicing of trypanosomatid polycistronic transcripts produces polyadenylated monocistronic mRNAs that are modified at their 5’ends to form the cap4 structure (m7Gpppm36,6,2’Apm2’Apm2’Cpm23,2’U). Two trypanosomatid translation initiation factor 4E isomers (eIF4E3 and eIF4E4) have extended N-terminal regions that include PAM2 motifs; these regions can interact with PABC domains in the poly(A)-binding proteins PABP1. Using NMR, X-ray crystallography and biophysical quantitation of biomolecular interactions, we show that tight binding between Leishmania eIF4E4 and PABP1, mediated largely by hydrophobic interactions, underpins a novel architecture in the translation factor complex bridging the 5’ and 3’ ends of the monocistronic mRNAs. We demonstrate that this complex can also accommodate Leishmania eIF4G3 which, in contrast to the standard eukaryotic initiation complex paradigm, is found to bind to eIF4E4, but not to PABP1. Exceptionally, therefore, by binding to two protein ligands and to the mRNA cap4 structure, the trypanosomatid N-terminally extended form of eIF4E acts as a core molecular scaffold for the mRNA-cap-binding complex. Finally, our data indicate that the eIF4E4 N-terminal extension is an intrinsically disordered region that transitions to a partly folded form upon binding to PABP1