ARTICLE

Microdomain Arrangement of the SERCA-type Ca²⁺ Pump (Ca²⁺-ATPase) in Subplasmalemmal Calcium Stores of Paramecium Cells

Correspondence to: H. Plattner, Dept. of Biology, Univ. of Konstanz, D-78434 Konstanz, Germany.

We localized SERCA pumps to the inner region of alveolar sac membranes, facing the cell interior, by combining ultrastructural and biochemical methods. Immunogold labeling largely predominated in the inner alveolar sac region which displayed aggregates of intramembrane particles (IMPs). On image analysis, these represented oligomeric arrangements of ~8-nm large IMP subunits, suggesting formation of SERCA aggregates (as known from sarcoplasmic reticulum). We found not only monomers of typical molecular size (~106 kD) but also oligomeric forms on Western blots (using anti-SERCA antibodies, also against endogenous SERCA from alveolar sacs) and on electrophoresis gelautoradiographs of ³²P-labeled phosphoenzyme intermediates. Selective enrichment of SERCA-pump molecules in the inner alveolar sac membrane region may eliminate Ca²⁺ after centripetal spread observed during exocytosis activation, while the plasmalemmal Ca²⁺ pump may maintain or reestablish [Ca²⁺] in the narrow subplasmalemmal space between the outer alveolar sac membrane region and the cell membrane. We show for the first time the microzonal arrangement of SERCA molecules in a Ca²⁺ store of a secretory system, an intensely discussed issue in stimulus–secretion coupling research. (J Histochem Cytochem 47:841–853, 1999)

Key Words: ATPase, Ca²⁺, calcium, exocytosis, microdomains, Paramecium, secretion, SERCA

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