Target molecules of calmodulin on microtubules of Tetrahymena cilia

In the course of an attempt to isolate the calmodulin-binding proteins (CaMBPs) from cilia of Tetrahymena, it was found that some CaMBPs tend to interact with axonemal microtubules. The present study demonstrates this interaction by cosedimentation experiments using in vitro polymerized Tetrahymena axonemal microtubules and Tetrahymena CaMBPs purified from axonemes by calmodulin affinity column chromatography. Analysis by the [125I]calmodulin overlay method showed that at least three CaMBPs (Mr69, 45, and 37 kDa) cosediment with microtubules. Furthermore, without any addition of exogenous CaMBPs, microtubules purified after three cycles of temperature-dependent polymerization and depolymerization included the above CaMBPs and additional CaMBPs (Mr30, 26, and 22 kDa) which could not cosediment with microtubules. From the results, we have classified these microtubule-associated CaMBPs into two groups: (i) CaMBPs which interact with microtubules only during polymerization (30, 26, and 22 kDa), and (ii) CaMBPs which interact not only with microtubules during polymerization, but also with polymerized microtubules (69, 45, and 37 kDa). These results suggest that the microtubule-associated CaMBPs, especially those of the latter group, are located on the surface of ciliary microtubules, and may become the target molecules of calmodulin at Ca2+-triggered ciliary reversal.