Kinetoplastid‐specific X2‐family kinesins interact with a kinesin‐like pleckstrin homology domain protein that localizes to the trypanosomal microtubule quartet

Kinesins are motor proteins found in all eukaryotic lineages that move along microtubules to mediate cellular processes such as mitosis and intracellular transport. In trypanosomatids, the kinesin superfamily has undergone a prominent expansion, resulting in one of the most diverse kinesin repertoires that includes the two kinetoplastid‐restricted families X1 and X2. Here, we characterize in Trypanosoma brucei TbKifX2A, an orphaned X2 kinesin. TbKifX2A tightly interacts with TbPH1, a kinesin‐like protein with a likely inactive motor domain, a rarely reported occurrence. Both TbKifX2A and TbPH1 localize to the microtubule quartet (MtQ), a characteristic but poorly understood cytoskeletal structure that wraps around the flagellar pocket as it extends to the cell body anterior. The proximal proteome of TbPH1 revealed two other interacting proteins, the flagellar pocket protein FP45 and intriguingly another X2 kinesin, TbKifX2C. Simultaneous ablation of TbKifX2A/TbPH1 results in the depletion of FP45 and TbKifX2C and also an expansion of the flagellar pocket, among other morphological defects. TbKifX2A is the first motor protein to be localized to the MtQ. The observation that TbKifX2C also associates with the MtQ suggests that the X2 kinesin family may have co‐evolved with the MtQ, both kinetoplastid‐specific traits. Kinesins are motor proteins that deliver cargo along microtubule tracks. In Trypanosoma brucei, TbPH1, a putative inactive kinesin with a pleckstrin homology domain, interacts with kinetoplastid‐specific kinesins TbKifX2A and TbKifX2C. TbPH1 and TbKifX2A localize to a discrete, trypanosome‐specific cytoskeletal structure, the microtubule quartet. Depletion of TbPH1/TbKifX2A results in flagellar pocket expansion and a variety of other morphological defects.