Novel roles for the flagellum in cell morphogenesis and cytokinesis of trypanosomes.

Flagella and cilia are elaborate cytoskeletal structures conserved from protists to mammals, where they fulfil functions related to motility or sensitivity. Here we demonstrate novel roles for the flagellum in the control of cell size, shape, polarity and division of the protozoan Trypanosoma brucei. To investigate the function of the flagellum, its formation was perturbed by inducible RNA interference silencing of com ponents required for intraflagellar transport, a dynamic process necessary for flagellum assembly. First, we show that down-regulation of intraflagellar transport leads to assembly of a shorter flagellum. Strikingly, cells with a shorter flagellum are smaller, with a direct correlation between flagellum length and cell size. Detailed morphogenetic analysis reveals that the tip of the new flagellum defines the point where cytokinesis is initiated. Secondly, when new flagellum formation is completely blocked, non-flagellated cells are very short, lose their normal shape and polarity, and fail to undergo cytokinesis. We show that flagellum elongation controls formation of cytoskeletal structures (present in the cell body) that act as molecular organizers of the cell.