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

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.