It takes balance to make a flagellum.

Simple questions in cell biology seldom have simple answers, but on page 405, Marshall and Rosenbaum provide evidence for a strikingly straightforward mechanism for controlling the sizes of organelles. Determination of flagellar length in Chlamydomonas provides a tractable experimental system for studying organelle size regulation, and Marshall and Rosenbaum found that an equilibrium between assembly and disassembly of tubulin at the flagellar tip can explain most aspects of flagellar length control.

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. ▪