Figure 1.

Structure of cilia/flagella. (A) Cilia/flagella have a cylindrical architecture composed of axoneme, plasma membrane, and basal body. (B) Axoneme has a microtubule-based structure formed of nine microtubule doublets at the periphery and two microtubule singlets at the center, which are associated with protein complexes such as radial spokes, inner dynein arm, outer dynein arm, and nexin linkers. There is a mechanical feedback from the bending on the regulation of dynein activity, which switches the activity of dyneins on the opposite side of the central pair microtubules.¹⁵ (C) In the presence of nexin proteins which cross-link the neighboring doublets, microtubules are not free to slide and active forces generated by dynein molecular motors bend the cilium/flagellum. Nexin linkers play an important role to convert the action of dynein motors in microtubule bending,^16,17 and radial spokes ensure that dyneins work together in a highly coordinated manner to generate a regular wave pattern.¹⁸