It takes balance to make a flagellum.
Using a new assay to visualizetubulin turnover, the authors demonstrate that flagella are dynamic structures in which tubulin continuously assembles and disassembles at the distal end. Movement of tubulin by intraflagellar transport (IFT) is required for microtubule assembly at the flagellar tip, but IFT is not required for microtubule disassembly; when IFT is blocked, the flagella shorten and disappear, whereas inhibiting disassembly causes flagella to lengthen. Computer simulations using a steady-state model show that flagellar length could be determined by a simple balance of tubulin assembly and disassembly rates. More complex three-dimensional organelles may require additional size control mechanisms, but the new results demonstrate that a complicated “size sensor” may not be necessary. ▪