Basal Body Protein TbSAF1 Is Required for Microtubule Quartet Anchorage to the Basal Bodies in Trypanosoma brucei

Sperm flagellar protein 1 (Spef1, also known as CLAMP) is a microtubule-associated protein involved in various microtubule-related functions from ciliary motility to polarized cell movement and planar cell polarity. In Trypanosoma brucei, the causative agent of trypanosomiasis, a single Spef1 ortholog (TbSpef1) is associated with a microtubule quartet (MtQ), which is in close association with several single-copied organelles and is required for their coordinated biogenesis during the cell cycle. Here, we investigated the interaction network of TbSpef1 using BioID, a proximity-dependent protein-protein interaction screening method. Characterization of selected candidates provided a molecular description of TbSpef1-MtQ interactions with nearby cytoskeletal structures. Of particular interest, we identified a new basal body protein TbSAF1, which is required for TbSpef1-MtQ anchorage to the basal bodies. The results demonstrate that MtQ-basal body anchorage is critical for the spatial organization of cytoskeletal organelles, as well as the morphology of the membrane-bound flagellar pocket where endocytosis takes place in this parasite.