Identification of calmodulin in Acetabularia: its distribution and physiological significance

In order to test whether calmodulin is present in Acetabularia, material was isolated from this alga, using the chlorpromazine affinity method. This resulted in the purification of a protein with the correct mobility on SDS–polyacrylamide gels. This protein reacted with anti-calmodulin in dot blots, using an immunogold, silver-enhanced method, so we conclude that calmodulin is present in the alga, although at a very low concentration (200–450 pg per gram wet weight). The localization of calmodulin was studied, using fluphenazine fluorescence. The pattern is described; the chief feature being the concentration of fluorescence towards the tip during both the slow and the rapid growth phases. Fluorescence is also observed at the insertion points of hairs on the stalk and decreases in intensity between hairs of successive order. The apical fluorescence vanishes when cap formation begins. The localization of calmodulin parallels that of calcium studied by means of chlorotetra-cycline and aequorin. Expecting inhibitors of calmodulin to produce physiological effects, we studied the growth of whole and anucleate Acetabularia in the presence of trifluoperazine (TFP). TFP severely inhibited growth and cap formation, as did lanthanum. A circadian rhythm of sensitivity to these inhibitors was found. The hypothesis is put forward that calcium and calmodulin are important during a critical photosensitive phase of the circadian cycle.