Radial spoke

Radial spoke

The radial spoke is a multi-unit organelles, its structure and mode of action remain poorly understood.

Cellular location and structure

The radial spoke shown in an axoneme cross-section

Radial spokes are T-shaped structures present inside the axoneme. Each spoke consists of a "head" and a "stalk," while each of these sub-structures is itself made up of many protein subunits.[2] In all, the radial spoke is known to contain at least 17 different proteins,[3] with 5 located in the head and at least 12 making up the stalk. The spoke stalk binds to the A-tubule of each microtubule outer doublet, and the spoke head faces in towards the center of the axoneme (see illustration at right).


The radial spoke is known to play a role in the mechanical movement of the flagellum/cilium. For example,

How the radial spoke carries out this function is poorly understood. Radial spokes are believed to interact with both the central pair microtubules and the dynein arms, perhaps in a way that maintains the rhythmic activation of the dynein motors. For example, one of the radial spoke subunits, RSP3, is an anchor protein predicted to hold another protein called protein kinase A (PKA). PKA would theoretically then be able to activate/inactivate the adjacent dynein arms via its kinase activity.

However, the identities and functions of the many radial spoke subunits are just beginning to be elucidated.


  1. ^ Gerald Karp (19 October 2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley and Sons. pp. 342–.  
  2. ^ Yang P, Diener DR, Yang C, et al. (March 2006). "Radial spoke proteins of Chlamydomonas flagella". J. Cell. Sci. 119 (Pt 6): 1165–74.  
  3. ^ Yang P, Diener DR, Rosenbaum JL, Sale WS (June 2001). "Localization of calmodulin and dynein light chain LC8 in flagellar radial spokes". J. Cell Biol. 153 (6): 1315–26.