Differential distribution of calcium stores in Paramecium cells. Occurrence of a subplasmalemmal store with a calsequestrin-like protein

We have analyzed in Paramecium cells the occurrence and intracellular distribution of the high capacity/low affinity calcium-binding proteins, calsequestrin (CS) and calreticulin (CR) using antibodies against CS from rat skeletal muscle and against CR from rat liver, respectively. As revealed by Western blots, a CS-like protein isolated by affinity chromatography from Paramecium cells comigrated with CS isolated from rat skeletal muscle. The immunoreactivity of this 53 kDa protein band was blocked when the antibodies had been preadsorbed with purified rat CS. A band of identical molecular size was shown to bind ⁴⁵Ca in overlays. By immunofluorescence and immunogold labeling this CS-like protein was localized selectively to the extended subplasmalemmal calcium stores, the 'alveolar sacs', which cover almost the entire cell surface. Concomitantly the 53 kDa ⁴⁵Ca-binding band became increasingly intense in overlays as we increasingly enriched alveolar sacs. Antibodies against rat CR react with a 61 kDa band but do not cross-react with CS-like protein in Paramecium. These antibodies selectively stained intracellular reticular structures, identified bona fide as endoplasmic reticulum.