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A molecular ruler determines the repeat length in eukaryotic cilia and flagella

Existence of cellular structures with specific size raises a fundamental question in biology: How do cells measure length? One conceptual answer to this question is by a molecular ruler, but examples of such rulers in eukaryotes are lacking. In this work, we identified a molecular ruler in eukaryotic cilia and flagella. Using cryo-electron tomography, we found that FAP59 and FAP172 form a 96–nanometer (nm)–long complex in Chlamydomonas flagella and that the absence of the complex disrupted 96-nm repeats of axonemes. Furthermore, lengthening of the FAP59/172 complex by domain duplication resulted in extension of the repeats up to 128 nm, as well as duplication of specific axonemal components. Thus, the FAP59/172 complex is the molecular ruler that determines the 96-nm repeat length and arrangements of components in cilia and flagella. A protein complex controls the length and assembly of repeat structures in eukaryotic cilia and flagella. Molecular ruler rules cilia and flagella length Cilia and flagella contain within their ultrastructure repeating structures at regularly spaced intervals. How does the cell measure length with nanometer precision? Oda et al. identify a flagella protein complex in Chlamydomonas that appears to act as a sort of molecula ruler to define repeat length. Genetic changes that would change the length of this protein led to corresponding changes in the length of repeats within the resulting flagella. Science, this issue p. 857