Flagellar targeting of an arginine kinase requires the conserved Lipidated Intraflagellar Transport (LIFT) pathway in Trypanosoma brucei

Both intraflagellar transport (IFT) and lipidated intraflagellar transport (LIFT) pathways are essential for cilia/flagella biogenesis, motility and sensory functions. In the LIFT pathway, lipidated cargoes are transported into the cilia through the coordinated actions of cargo carrier proteins such as Unc119 or PDE6δ, as well as small GTPases Arl13b and Arl3 in the cilium. Our previous studies revealed a single Arl13b ortholog in the evolutionarily divergent Trypanosoma brucei. TbArl13 catalyses two TbArl3 homologs, TbArl3A and TbArl3C, suggesting the presence of a conserved LIFT pathway in these protozoan parasites. Only a single homolog to the cargo carrier protein Unc119 was identified in T. brucei genome, but its function in lipidated protein transport has not been characterized. In this study, we exploited the proximity-based biotinylation approach to identify binding partners of TbUnc119. We showed that TbUnc119 binds to a flagellar arginine kinase TbAK3 in a myristoylation-dependent manner and is responsible for its targeting and enrichment in the flagellum. Interestingly, only TbArl3A, but not TbArl3C interacts with TbUnc119 in a GTP-dependant manner, suggesting functional specialization of Arl3-GTPases in T. brucei. This study establishes the function of TbUnc119 as a myristoylated cargo carrier and supports the presence of a conserved LIFT pathway in T. brucei.