Abstract
Cilia and flagella arc important organelles for cell motility and sensing, and defects of these nanomachines have been linked to several human diseases. Microtubules (MTs) in cilia and flagella provide the core scaffold for anchoring the dynein motors and many other associated proteins. The nine peripheral MT doublets consist of a full MT (the A-tubule) and a partial MT (the B-tubule). They are hyperstable in comparison to the dynamically instable cytoplasmic MTs, but the molecular foundation of this stability is not well understood. Therefore, we used cryoelectron tomography of demembranated Chlamydomonas flagella to study their ultrastructure at molecular resolution. The resulting tomograms were averaged using the software program PEET and visualized via the programs IMOD and CHIMERA. The structure of the MT doublet was visualized at higher resolution than in previous studies. The internal proteins MIP1,2, and 3, as well as the inner B-tubule junction and the doublet-specific B-tubule density ("beak"-structure) were identified at unprecedented detail. A clear understanding of the MT doublet ultrastructure is necessary to better understand its function and eventually target and repair MT defects