MECHANICAL-PROPERTIES OF CILIARY AXONEMES AND MEMBRANES AS SHOWN BY PADDLE CILIA

Cilia with a distal membrane expansion enclosing a coiled end of the a xoneme (paddle cilia or discocilia) have been commonly reported in mar ine invertebrates. We recently showed that paddle cilia in molluscan v eligers are artifacts of non-physiological conditions. Here we investi gated the possible mechanisms of formation of paddle cilia under hypot onic conditions; particularly, whether a helical conformational change of doublet microtubules induced by Ca or proton flux is responsible. Typical paddle cilia are induced by hypotonic Ca-free solutions at nor mal or low pH, showing that axonemal coiling does not require Ca influ x or proton efflux. In addition, Triton-demembranated straight axoneme s do not coil in high Ca solutions. Most decisively, complete removal of paddle ciliary membranes with detergents, but not mere permeabiliza tion, causes immediate uncoiling and straightening of the axonemes to approximately their original length before hypotonic treatment. These findings and other data show that axonemal coiling in paddles is due t o membrane tensile stress acting on an elastic axoneme. Light and elec tron microscopy of paddles show that axonemes coil uniformly toward tb e direction of the effective stroke (doublets nos 5-6), even when beat ing is inhibited by sodium azide or glutaraldehyde before hypotonic tr eatment. This indicates that axonemes possess an intrinsic asymmetry o f stiffness within the beat plane, independent of active microtubule s liding. Paddle cilia thus reveal important mechanical properties of ci liary axonemes and membranes that should be useful for understanding c iliary function.