Axonemal dyneins form inner and outer arms, cause sliding among nine microtubule doublets to generate bending motion of eukaryotic flagella/cilia. However, it is not clear how the linear motion of dynein is integrated into well-coordinated bending. To understand the mechanism of flagellar/ciliary bending motion, we reconstructed 3D structure of inner and outer dynein arms at various nucleotide states using the technique of electron cryo-tomography and single particle averaging. Our averaged tomogram visualized three heavy chains of outer arms dyneins from Chlamydomonas stack vertically, while eight inner arm dyneins make a horizontal array (Ishikawa et al. (2007) JMB; Bui et al. (2008) JCB) (figure). We also found that the arrangement of inner dyneins and other linkers is not symmetrical among nine microtubule doublets (Bui et al. (2009) JCB). By further image analysis we revealed the shift of the ATPase head of dynein toward the tip of flagella during Pi release. The orientation of the coild-coil stalk is constant. This shift can winch adjacent microtubule. Interestingly apo and nucleotide-bound forms of dynein coexist and they make clusters in flagella, which could explain torsion for bending.View Large Image | View Hi-Res Image | Download PowerPoint Slide