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Nat Commun. 2019 Mar 8;10(1):1143. doi: 10.1038/s41467-019-09051-x.

Inner lumen proteins stabilize doublet microtubules in cilia and flagella.

Author information

1
Department of Cell Biology and Anatomy, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan.
2
Department of Physics and Structural Biology Research Center, Chikusa-ku, Nagoya University, Nagoya, 464-8602, Japan.
3
Graduate School of Biostudies, Kyoto University, Kyoto, 606-8502, Japan.
4
Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama, 226-8503, Japan.
5
WPI Nano Life Science Institute, Kanazawa University, Kakuma, Kanazawa, 920-1192, Japan.
6
Department of Cell Biology and Anatomy, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo, 113-0033, Japan. mkikkawa@m.u-tokyo.ac.jp.

Abstract

Motile cilia are microtubule-based organelles that play important roles in most eukaryotes. Although axonemal microtubules are sufficiently stable to withstand their beating motion, it remains unknown how they are stabilized while serving as tracks for axonemal dyneins. To address this question, we have identified two uncharacterized proteins, FAP45 and FAP52, as microtubule inner proteins (MIPs) in Chlamydomonas. These proteins are conserved among eukaryotes with motile cilia. Using cryo-electron tomography (cryo-ET) and high-speed atomic force microscopy (HS-AFM), we show that lack of these proteins leads to a loss of inner protrusions in B-tubules and less stable microtubules. These protrusions are located near the inner junctions of doublet microtubules and lack of both FAP52 and a known inner junction protein FAP20 results in detachment of the B-tubule from the A-tubule, as well as flagellar shortening. These results demonstrate that FAP45 and FAP52 bind to the inside of microtubules and stabilize ciliary axonemes.

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