Control of Flagellum Length by a Grow-and-Lock Model

Abstract

Several cell types such as photoreceptors and spermatozoa possess very stable cilia and flagella, a feature also encountered in numerous protists. We tested an original model for the control of flagellum length in such cells, using Trypanosoma brucei as an experimental system. The growand- lock model proposes that the flagellum elongates at a linear rate and that a locking event takes place in a timely defined manner preventing further elongation or shortening. The model implies that modulation of (1) the growth rate or of (2) the timing of the locking event should impact on flagellum length, and (3) that a mature flagellum should display unique characteristics. Here, we provide experimental evidence supporting each of these three postulates. First, reducing the frequency of intraflagellar transport, the machinery responsible for construction, slows down the growth rate and results in the construction of much shorter flagella. Second, we show that the locking event is initiated at a defined stage of the cell cycle and subsequently leads to inhibition of elongation. Third, flagellum maturation is associated to a molecular marker termed FLAM8. Altogether, these results provide support for the grow-and-lock model as a new paradigm for the control of organelle length.