IFT raft size scales inversely with flagellar length

Benjamin D. Engel, William B. Ludington, and Wallace F. Marshall

Department of Biochemistry and Biophysics, University of California, San Francisco

The assembly and maintenance of eukaryotic flagella is regulated by intraflagellar transport (IFT), the process of bidirectional protein traffic within the flagellum. During IFT, "rafts" of IFT proteins and axonemal precursors are transported by kinesin-II to the site of flagellar assembly at the tip, and then returned to the base by cytoplasmic dynein. Current models have failed to reconcile the observed constant speed and frequency of IFT rafts with the flagellar assembly rate, which decreases with flagellar length. Using total internal reflection fluorescence (TIRF) microscopy to track protein traffic within the flagella of Chlamydomonas reinhardtii, we have determined that the size of IFT rafts scales inversely with flagellar length. Anterograde IFT rafts in short flagella are composed of more kinesin and IFT proteins than rafts in long flagella. This result is consistent with the kinetics of flagellar regeneration and provides a unifying model for flagellar length control. IFT raft size, rather than speed or frequency, modulates the rate of flagellar assembly.

e-mail address of presenting author: benjamin.engel@ucsf.edu