Trypanosomatid protozoa are unusual eukaryotes that are well known for having unusual ways of controlling their gene expression. The lack of a refined mode of transcriptional control in these organisms is compensated by several post-transcriptional control mechanisms, such as control of mRNA turnover and selection of mRNA for translation, that may modulate protein synthesis in response to several environmental conditions found in different hosts. In other eukaryotes, selection of mRNA for translation is mediated by the complex eIF4F, a heterotrimeric protein complex composed by the subunits eIF4E, eIF4G, and eIF4A, where the eIF4E binds to the 5'-cap structure of mature mRNAs. In this review, we present and discuss the characteristics of six trypanosomatid eIF4E homologs and their associated proteins that form multiple eIF4F complexes. The existence of multiple eIF4F complexes in trypanosomatids evokes exquisite mechanisms for differential mRNA recognition for translation.

Members of the eIF4E mRNA cap-binding family are involved in translation and the modulation of transcript availability in other systems as part of a three-component complex including eIF4G and eIF4A. The kinetoplastids possess four described eIF4E and five eIF4G homologs. We have identified two new eIF4E family proteins in Trypanosoma brucei, and define distinct complexes associated with the fifth member, TbEIF4E5. The cytosolic TbEIF4E5 protein binds cap 0 in vitro. TbEIF4E5 was found in association with two of the five TbEIF4Gs. TbIF4EG1 bound TbEIF4E5, a 47.5-kDa protein with two RNA-binding domains, and either the regulatory protein 14-3-3 II or a 117.5-kDa protein with guanylyltransferase and methyltransferase domains in a potentially dynamic interaction. The TbEIF4G2/TbEIF4E5 complex was associated with a 17.9-kDa hypothetical protein and both 14-3-3 variants I and II. Knockdown of TbEIF4E5 resulted in the loss of productive cell movement, as evidenced by the inability of the cells to remain in suspension in liquid culture and the loss of social motility on semisolid plating medium, as well as a minor reduction of translation. Cells appeared lethargic, as opposed to compromised in flagellar function per se. The minimal use of transcriptional control in kinetoplastids requires these organisms to implement downstream mechanisms to regulate gene expression, and the TbEIF4E5/TbEIF4G1/117.5-kDa complex in particular may be a key player in that process. We suggest that a pathway involved in cell motility is affected, directly or indirectly, by one of the TbEIF4E5 complexes.

Trypanosomes lack the transcriptional control characteristic of the majority of eukaryotes that is mediated by gene-specific promoters in a one-gene-one-promoter arrangement. Rather, their genomes are transcribed in large polycistrons with no obvious functional linkage. Posttranscriptional regulation of gene expression must thus play a larger role in these organisms. The eIF4E homolog TbEIF4E6 binds mRNA cap analogs in vitro and is part of a complex in vivo that may fulfill such a role. Knockdown of TbEIF4E6 tagged with protein A-tobacco etch virus protease cleavage site-protein C to approximately 15% of the normal expression level resulted in viable cells that displayed a set of phenotypes linked to detachment of the flagellum from the length of the cell body, if not outright flagellum loss. While these cells appeared and behaved as normal under stationary liquid culture conditions, standard centrifugation resulted in a marked increase in flagellar detachment. Furthermore, the ability of TbEIF4E6-depleted cells to engage in social motility was reduced. The TbEIF4E6 protein forms a cytosolic complex containing a triad of proteins, including the eIF4G homolog TbEIF4G5 and a hypothetical protein of 70.3 kDa, referred to as TbG5-IP. The TbG5-IP analysis revealed two domains with predicted secondary structures conserved in mRNA capping enzymes: nucleoside triphosphate hydrolase and guanylyltransferase. These complex members have the potential for RNA interaction, either via the 5' cap structure for TbEIF4E6 and TbG5-IP or through RNA-binding domains in TbEIF4G5. The associated proteins provide a signpost for future studies to determine how this complex affects capped RNA molecules.