Calmodulin is essential for assembling links necessary for exocytotic membrane fusion in Paramecium

Calmodulin has long been suspected to be involved in calcium‐regulated exocytosis but its precise site(s) of action has not yet been identified. In Paramecium, a genetic approach to the problem is possible as in vivo‐selected mutations in the calmodulin gene that prevent the activation of some channels have been characterized. Three of these calmodulin mutants were examined for exocytotic capacity and the mutant cam1 was found to be defective for exocytosis at 35 degrees C. The loss of exocytotic capacity in cam1 cells can be restored by transformation with the wild‐type calmodulin gene, demonstrating that its exocytotic lesion is indeed due to the mutation in the calmodulin gene. The cam1 mutant displays abnormal exocytotic sites at the non‐permissive temperature: it lacks the links (‘rosettes’ of intramembranous particles in the plasma membrane and the fibrous ‘connecting material’) which normally connect plasma and trichocyst membranes. Upon shift of cam1 cells from the permissive to a non‐permissive temperature, performed sites remain functional. These results demonstrate that calmodulin is necessary for the assembly of these links at the exocytotic site. These results do not, however, exclude the possibility of calmodulin also being involved in Ca(2+)‐dependent steps of the stimulus‐exocytosis coupling.