3D structure of the axoneme and radial spokes. (A) Axoneme from C. reinhardtii flagella seen from the proximal end (the minus end of the microtubule). RS, radial spoke (in a blue dotted line); ODA, outer dynein arm; IDA, inner dynein arm; MTD, microtubule doublet. The central pair apparatus is shown in light green. The inset highlights one microtubule doublet, radial spoke, and central pair apparatus seen from the side (left, the proximal end). The head, neck, and stalk subdomains of the radial spoke are indicated. Modified from Bui and Ishikawa (2013). (B) 3D structure of the 96-nm periodic unit from C. reinhardtii flagella (EMDataBank accession no. EMD-2117). Proximal end is left. The color code is based on previous studies (Heuser et al., 2009; Pigino et al., 2011; Bui et al., 2012). Dark blue, radial spoke; light blue, outer dynein arm; red, inner dynein arm; green, dynein regulatory complex; yellow, intermediate and light chains of dynein f; orange, microtubule; gray, unidentified. Radial spoke proteins located in Oda et al. (2014) and dynein species (α, β, and γ are part of the outer dynein arm, and a–f form the inner dynein arm) based on Bui et al. (2012) are indicated.