Structural and functional diversity of the cap-binding complexes in Leishmania

Exposure to alternating environmental conditions is an integral part of the life cycle in Leishmania, a trypanosomatid parasite that migrates between invertebrate vectors and mammalian hosts. A developmental program of gene expression promotes the parasite adaptation to the different conditions, whereby translation plays a key regulatory role. We describe a highly complex and unique network that regulates translation initiation, demonstrating unusual protein-protein interactions. There are six eIF4E paralogs (LIF4Es) and five eIF4G candidates (LIF4Gs), forming discrete complexes that vary in their stage-specific activities. It is intriguing that LIF4E1 does not pair with any LIF4G, since the latter usually serves as a scaffold protein for the translation initiation complex. Here we report on a parasite-specific interaction between LIF4E1 and LIF3a, that can explain how the LIF4E1 surpasses the lack of an LIF4G binding partner and how the 40S ribosomal subunit may be recruited. Another cap-binding protein, LIF4E2, also does not pair with any LIF4G, but appears to associate with proteins that involve cell cycle control. A structural analysis of LIF4E2 highlights a defined element that affects its biological function. Finally, in search of alternative pathways for initiating translation, we focus on the LIF3. Indeed, we show that the LIF3 complex can independently bind the cap structure offering a unique pathway for translation, possibly targeting specific transcripts. Our studies make use of CRISPR-Cas9 mediated knock-out of specific genes along with testing how their overexpression affects parasite physiology. Our data highlight unique evolutionary traits practiced by this ancient and successful parasite.