Morphological changes in the radial spokes of two Chlamydomonas mutants expressing various truncated RSP3

Abstract

Various cell types utilize motile flagella and cilia to propel through the surrounding fluid or move the fluid over their surface. The machinery for movement is built around a 9 + 2 axoneme. The axonemes of flagella and cilia are microtubule‐based complexes that are constructed from hundreds of different polypeptides. Some of these polypeptides are arranged into macromolecular complexes and include the dynein motors, radial spokes (RS), and the central pair of microtubules or apparatus (CP). The T‐shaped radial spokes regulate flagellar beating. The CP positioning and orientation of the RS propagate different bends along the axoneme, and this is associated with stroke force (Satir and Christensen, 2007). It was hypothesized that the radial spoke protein 3 (RSP3) is a multi‐modular scaffold that anchors RIIa and DPY‐30 domains that tether multiple effector mechanisms for the spoke complex; a single scaffold will provide the rigidity necessary for mechanical feedback (Sivadas and Yang, 2011). Using the Chlamydomonas radial spoke complex model, truncation mutants of RSP3, missing the two RIIa‐containing RSPs along with subsets of spoke stalk proteins and mutants missing two DPY‐30 RSPs and the bulbous spokehead modules were generated. The morphological test of the hypothesis that RSP3 forms the scaffold of the radial spoke to which other RSPs bind was done at the ultrastructural level.