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.
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