The presence of this enzyme captured our attention because it belongs to the phosphagen kinases, enzymes that reversibly dephosphorylate ATP and phosphorylate a specific substrate, forming an ATP buffer system for the temporally rapid but limited regeneration of ATP when required (reviewed in ). (sp.). Therefore understanding the factors that allow for the successful infection of the insect Curculigoside vector remains a relevant research interest. The fly is more than a passive vector as the parasites undergo specific development programmes before becoming infective again. These can be relatively simple as in the case of that is mostly found in the mouthparts and alternates between two major forms. However, this is much more sophisticated in the subgroup (and that proliferates first in the gut and then reaches the proboscis before attaching to the surface of the labrum and releasing infective cells in the hypopharynx . To survive in all these different environments, trypanosomes adapt their shape and morphology, their metabolism and their surface Akt2 protein composition . Trypanosomes are members of the Excavata, one of the major early Curculigoside branching eukaryotic lineages. Their morphology is characterised by an elongated shape and the presence of a flagellum attached to the cell body. The flagellum is a multifunctional organelle, with known roles in motility, morphogenesis and attachment to the epithelium of the salivary glands (review in ). It has also been proposed to function in environmental sensing [8C11]. Biophysical analysis demonstrated that flagellum disposition and motility are particularly well suited to swimming in a crowded environment among erythrocytes, such as encountered for the bloodstream form of the parasite . Recently, forward flagellum-mediated motility has been shown to be crucial for parasites to migrate anteriorly from the posterior midgut to the anterior midgut of the tsetse . These data raise the question of the control of flagellum motility and how it could be modulated to fit the different environments encountered by during its cyclical development. A proteomics analysis of intact flagella purified from cultured procyclic trypanosomes revealed the presence of the enzyme arginine kinase (AK) in the membrane and matrix fraction . The enzyme shows rapid turnover in both mature and growing flagella but RNAi silencing suggests it is not required for growth in culture . The flagellar location has also been confirmed upon epitope tagging and overexpression in cultured procyclic . The presence of this enzyme captured our attention because it belongs to the phosphagen kinases, enzymes that reversibly dephosphorylate ATP and phosphorylate a specific substrate, forming an ATP buffer system for the temporally rapid but limited regeneration of ATP when required (reviewed in ). In both spermatozoa and protists, flagellar beat frequency relies on the presence of ATP. In the flagellum of spermatozoa, phosphagen kinases are thought to act as energy shuttles to transfer phosphates from their point of synthesis to the beating flagellum where ATP consumption occurs [16,17]. While evidence is not extensive regarding the same function in flagellated protists, exposure of cilia to ADP decreases beat frequency, but is restored upon addition of phosphorylated arginine . This argues that although direct evidence is lacking for phosphagen kinases as a energy shuttle in flagellated protists, generation and access to phosphagens could be important in maintaining the beat frequency of spermatozoa and protist flagella. Here we investigated the possible role of arginine kinase during parasite development in the tsetse fly. We present phylogenetic data revealing that trypanosome genes were likely acquired by horizontal gene transfer from an invertebrate host and that separate duplication events led to the emergence of flagellar forms of AK in and possibly in in mildly affects parasite motility but leads to lowered infectivity and competitiveness in tsetse, supporting a contribution to infection. To our knowledge, this is the first evidence regarding the possible relevance of Curculigoside flagellar AK in Curculigoside the life cycle of copies in and AnTat 1.1 (wild-type or WT) cell line . Procyclic trypomastigotes of the WT cell line and its derivatives were cultured in SDM-79  supplemented with 10% foetal bovine.