Theradial spoke is a multi-unitprotein structure found in theaxonemes ofeukaryoticcilia andflagella.[1] Although experiments have determined the importance of the radial spoke in the proper function of theseorganelles, its structure and mode of action remain poorly understood.
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 seventeen proteins,[3] five in the head and twelve in the stalk. The spoke stalk binds to the A-tubule of eachmicrotubule 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,mutant organisms lacking properly functioning radial spokes have flagella and cilia that are immotile. Radial spokes also influence the cilium "waveform"; that is, the exact bending pattern the cilium repeats.
How the radial spoke carries out this function is poorly understood. Radial spokes are believed to interact with both the central pair microtubules and thedynein arms, perhaps in a way that maintains the rhythmic activation of thedynein motors. For example, one of the radial spoke subunits, RSP3, is an anchor protein predicted to hold another protein calledprotein kinase A (PKA). PKA would theoretically then be able to activate/inactivate the adjacent dynein arms via itskinase activity.
Human axonemal radial spoke subunits:RSPH1,RSPH3,RSPH4A,RSPH6A,RSPH9,RSPH10B,RSPH10B2,RSPH14.[4]