Possible involvement of a calmodulin regulated Ca2+ -ATPase in exocytosis performance in Paramecium tetraurelia cells.

Surface membrane fractions from Paramecium tetraurelia cells contain a calmodulin-stimulated Ca2+-ATPase responding to low levels of free Ca2+ and with features characteristic of a membrane-bound ATPase. Among the different strains analyzed this enzyme was practically absent selectively from the 'non-discharge' mutant nd9-28 degrees C (from J. Beisson): if cultured at a permissive temperature (18 degrees C), this strain showed identical values of calmodulin-stimulated Ca2+-ATPase activity as wild-type cells (7S) or strains with mutations which do not affect exocytosis performance. We conclude that this calmodulin-stimulated Ca2+-activated ATPase might be a prerequisite for membrane fusion in the course of exocytosis performance.