ABSTRACT: The unicellular eukaryote Trypanosoma brucei relies heavily on posttranscriptional regulatory mechanisms, as pol II transcription is polycistronic. The parasite has six isoforms of the cap-binding translation initiation factor eIF4E, and five eiF4Gs (PMID: 29077018), which potentially allow for differential mRNA target selection in order to fine-tune translation. EIF4E3 and EIF4E4 appear to be general initiation factors; we are investigating the EIF4E proteins that are presumed to have more specialized functions, i.e., EIF4E1, EIF4E2, EIF4E5, and EIF4E6. EIF4E1 interacts with 4E-interacting protein (4EIP) and not with any EIF4G; they functionally resemble mammalian 4E-HP and GIGYF2. 4EIP suppresses translation and provokes mRNA degradation. 4EIP and EIF4E1 are dispensable in slender “bloodstream forms” (BSFs), which multiply in mammals, but 4EIP is required for translation suppression in the growth-arrested “stumpy” BSF (PMID: 30124912). Meanwhile EIF4E1, but not 4EIP, is required for survival of “procyclic forms” (PCFs), which grow in Tsetse. Tethered EIF4E1 is suppressive only when 4EIP is present. We are investigating whether it can, without an EIF4G, activate translation in BSFs/PCFs, as well as how target mRNAs are selected in absence and presence of 4EIP, based on the proteins it associates with. Like EIF4E1, EIF4E2 does not interact with any EIF4G protein, but was found in association with a homolog of the histone mRNA stem-loop-binding protein, called SLBP2, in PCFs (PMID: 29288414). The protein binding partners in BSFs have not been addressed this far. EIF4E3, EIF4E4, EIF4E5, and EIF4E6 all stimulate expression when tethered. They interact with different EIF4G homologs, and each is essential in at least one life-cycle stage, indicating that each serves a distinct role. We have now evidence that EIF4E6 interacts specifically, not only with EIF4G5, but also with a previously characterized stimulatory complex containing MKT1, PBP1, and LSM12, which we aimed to confirm by this quantitative mass spectrometry approach. The complex is recruited to mRNAs via sequence-specific RNA-binding proteins (PMID: 24470144), offering a novel mechanism for specific translation activation by the EIF4E6-EIF4G5 complex. EIF4E5 on the other hand associates with either EIF4G1 or EIF4G2 to exert its functions. It is dispensable in the BSF, but essential in PCFs, where knock-down results in a motility-related phenotype. Furthermore, previous studies identified several 14-3-3 homologs to be associated with EIF4E5 in complex with either EIF4G1 or EIF4G2 in PCFs (PMID: 24962368). In the course of this study, PTP-tagged versions of EIF4E1+/- 4EIP (BSF, PCF), EIF4E2, EIF4E5, and EIF4E6 (BSF only) were pulled down and bound proteins were analyzed by quantitative mass spectrometry, with EIF4E3-PTP (BSF) and GFP-PTP (BSF, PCF) serving as controls.
Thomas Ruppert 
