Sensing the mechanical state of the axoneme and integration of Ca2+ signaling by outer arm dynein

Abstract

Axonemal dyneins have been demonstrated to monitor the mechanical state of the axoneme and must also alter activity in response to various signaling pathways. The central pair/radial spoke systems are clearly involved in controlling inner dynein arm function; however, the mechanisms by which the outer dynein arm transduces regulatory signals appear quite distinct at the molecular level. In Chlamydomonas, these regulatory components include thioredoxins involved in response to redox changes, molecules that tether the gamma heavy-chain motor unit to the A-tubule of the outer doublet and a Ca(2+)-binding protein that controls the structure of the gamma heavy-chain N-terminal domain. Together, these studies now suggest that the gamma heavy chain acts as a key regulatory node for controlling outer arm function in response to alterations in curvature and ligand binding. Furthermore, they allow us to propose a testable molecular mechanism by which altered Ca(2+) levels might lead to a change in ciliary waveform by controlling whether one heavy chain of outer arm dynein acts as a microtubule translocase or as an ATP-dependent brake that limits the amount of interdoublet sliding.

Fig. 1. Interaction Map for Outer Arm Dynein

This diagram illustrates the interactions that are known to occur within outer arm dynein; only associations for which there is direct experimental evidence in Chlamydomonas are illustrated. The color code is: blue, A and B tubules; yellow, HCs; light green, Lis1/NudC; dark green, redox-sensitive LCs, docking complex component DC3 and unidentified soluble substrates; magenta, IC/LC complex, red, LC4 Ca²⁺-sensor; cyan, LC1; orange, docking complex and Oda5 proteins required for arm assembly; pink, Oda5-associated adenylate kinase. The Ca²⁺-sensitive association of LC4 with IC1 is shown in red. Although indicated by single circles, LCs 6, 8, 9 and 10 are actually homodimers. This figure was prepared using Cytoscape v.2.6.3 (http://www.cytoscape.org) (Shannon et al. 2003).

Fig. 2. Models for Mechano-sensing and Ca²⁺ Signaling by Outer Arm Dynein

This model depicts the γ HC of outer arm dynein and one possible scenario for its response to nucleotide hydrolysis under low and high Ca²⁺ conditions. The diagrams are oriented with the distal or + end of the microtubules to the left. The HC regions are color-coded as follows: blue, motor unit; red, linker, black, N-terminal domain; purple, microtubule-binding domain and coiled coil domain. The γ HC-associated LC1 and LC4 proteins and the ICs and docking complex (DC) are also illustrated; the view is from the outside of the axoneme looking inwards. In this version, we have assumed that both copies of LC1 are bound simultaneously to the A-tubule and act as a loose tether. Thus, during a cycle of ATP binding and hydrolysis, the γ HC motor unit moves with respect to the A-tubule. Note that the N-terminal domain alters its orientation during this process. In contrast, under high Ca²⁺ conditions, LC4 releases one site on the γ HC N-terminal region and instead binds IC1 leading to a triple-tethered state. We suggest that this third interaction with the A-tubule disallows motion of the N-terminal region and consequently inhibits this HC from acting as a motor. Indeed under these conditions, the γ HC may act as an ATP-dependent brake to inhibit sliding.