α- and β-Tubulin Lattice of the Axonemal Microtubule Doublet and Binding Proteins Revealed by Single Particle Cryo-Electron Microscopy and Tomography

Microtubule doublet (MTD) is the main skeleton of cilia/flagella. Many proteins, such as dyneins and radial spokes, bind to MTD, and generate or regulate force. While the structure of the reconstituted microtubule has been solved at atomic resolution, nature of the axonemal MTD is still unclear. There are a few hypotheses of the lattice arrangement of its alpha- and beta-tubulins, but it has not been described how dyneins and radial spokes bind to MTD. In this study, we analyzed the three-dimensional structure of Tetrahymena MTD at similar to 19 angstrom resolution by single particle cryo-electron microscopy. To identify alpha- and beta-tubulins, we combined image analysis of MTD with specific kinesin decoration. This work reveals that alpha-and beta-tubulins form a B-lattice arrangement in the entire MTD with a seam at the outer junction. We revealed the unique way in which inner arm dyneins, radial spokes, and proteins inside MTD bind and bridge protofilaments.