Behavior of multiple dyneins in the axoneme. (A) Schematic diagram of the motion of cytoplasmic dyneins revealed by in vitro motility assays of yeast dynein and axonemal dynein dimers from mouse respiratory cilia in vivo analyzed by cryo-electron tomography. Left: cytoplasmic dynein (homodimer). Two heads can be 16 nm or longer apart [47,48]. They are rarely at the same position. Center and right: axonemal dynein. The distance between the two heads are either 0 nm or 8 nm . (B) Distribution of heterogeneous structures of ODAs forming an array on MTD in the presence of ADP.Vi, revealed by cryo-electron tomography and image classification of Chlamydomonas flagella . Upper panels: image classification of ODAs in the tomogram. Red: ODA in the ADP.Vi form. Blue: ODA in the apo form. Middle panels: schematic diagram of isolated dyneins in the presence and absence of ADP.Vi. With 1-mM ADP.Vi, the ADP.Vi form dominates. Bottom panels: ODA in flagella. Even in the presence of ADP.Vi, many ODAs remain in the apo form. Interestingly, two conformations form cluster as seen in the top panels.