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No beat without FAP46

The microtubules in motile cilia and flagella of eukaryotic cells are arranged in a ‘9+2’ axoneme; two central single microtubules that are surrounded by a ring containing nine microtubule doublets. The central microtubule pair is associated with at least seven individual projections that contain distinct protein complexes. Previous work has indicated that each of these complexes has unique roles in cilia motility, but their precise molecular composition and the mechanisms by which the projections contribute to dynein-driven microtubule sliding have remained unclear. Here, Elizabeth Smith, George Witman and co-workers (p. 3904) isolate and characterise a new Chlamydomonas mutant that lacks the C1d projection on the central microtubule pair. They show that FAP46 encodes a protein that associates with FAP54, FAP74 and FAP221 in the C1d complex and that complex formation depends on FAP46. Furthermore, FAP46-null mutants have impaired motility: more than half the cells lacking FAP46 are immobile, whereas the remaining cells are only able to swim with slow and shaky movements. Furthermore, the loss of FAP46 leads to a substantially decreased microtubule-sliding velocity. Together, these results support the hypothesis that the C1d projection on the central microtubule pair has important roles in mediating normal flagellar movement.