Trypanosome morphogenesis involves recruitment of a pleckstrin homology domain protein by an orphan kinesin to the microtubule quartet

ABSTRACT Kinesins are motor proteins found in all eukaryotic lineages that move along microtubule tracks to mediate numerous cellular processes such as mitosis and intracellular transport of cargo. In trypanosomatids, the kinesin protein superfamily has undergone a prominent expansion, giving these protists one of the most diverse kinesin repertoires. This has led to the emergence of two trypanosomatid-restricted groups of kinesins. Here, we characterize in Trypanosoma brucei TbKifX2, a hitherto orphaned kinesin that belongs to one of these groups. Representing a rare instance, TbKifX2 tightly interacts with TbPH1, a kinesin-like protein with an inactive motor domain. TbPH1 is named after a pleckstrin homology (PH) domain present within its carboxy-terminal tail. TbKifX2 recruits TbPH1 to the microtubule quartet (MtQ), a characteristic but poorly understood cytoskeletal structure that is part of the multipartite flagellum attachment zone (FAZ) and extends from the basal body to the anterior of the cell body. The proximal proteome of TbPH1 is comprised of four proteins that localize to the FAZ, consistent with the notion that the TbKifX2/TbPH1 complex are the first identified proteins to bind the MtQ along its whole length. Simultaneous ablation of both TbKifX2 and TbPH1 leads to the formation of prominent protrusions from the cell posterior. Thus, these two trypanosomatid-restricted proteins, which specifically localize to the MtQ in a microtubule-rich cell, appear to be contributors to morphogenesis in T. brucei. IMPORTANCETrypanosomatids are a group of unicellular parasites that infect a wide range of hosts from land plants to animals. They are also eukaryotes that have been shaped by prolonged independent evolution since this domain of life has radiated from a common ancestor almost 2 billion years ago. Thus, any resulting unique biological properties can be potentially exploited for treatment of infectious diseases caused by trypanosomatids. The cytoskeleton of trypanosomatids represents an ancient organelle that has undergone such modification. Here, we show that two trypanosomatid-specific proteins named TbPH1 and TbKifX2 form a complex that localizes to the microtubule quartet, a cytoskeletal structure characteristic to trypanosomatids. Ablation of these proteins in Trypansoma brucei leads to distinct morphological defects, making them not only intrinsically interesting topics of study, but potential therapeutic targets as well.