Investigating the axoneme CCDC40 protein reveals new insights in trypanosome morphogenesis and division

Cilia and flagella contribute to cell morphogenesis in multiple organisms. In the parasite Trypanosoma brucei, the flagellum is attached along the length of the cell body and acts as a guide for cell division, while its motility function is required for the completion of cytokinesis. To tease apart the contributions of flagellum length and motility to trypanosome morphogenesis, we investigated the coiled-coil containing domain 40 (CCDC40 or FAP172) protein. Iterative Ultrastructure Expansion Microscopy (iU-ExM) revealed that CCDC40 is associated to the 96-nm repeats of the trypanosome axoneme. CCDC40 depletion by RNAi leads to loss of components from the dynein regulatory complex, inner dynein arms and radial spokes, resulting in disconnected microtubule doublets and disorganised axoneme structure, abrogating motility and resulting in flagella and cell bodies 2-3 times shorter than normal. We show for the first time that this short flagellum phenotype is associated to slower tubulin incorporation and premature acquisition of the maturation marker FLAM8 but not of the locking protein CEP164C. Surprisingly, these short and immotile trypanosomes grow and divide normally. We discuss the significance of these observations for trypanosome morphogenesis and division.