Microdomain arrangement of the SERCA-type Ca2+ pump (Ca2+-ATPase) in subplasmalemmal calcium stores of Paramecium cells

We localized SERCA pumps to the inner region of alveolar sac membranes, fac ing 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 ana lysis, these represented oligomeric arrangements of similar to 8-nm large I MP subunits, suggesting formation of SERCA aggregates (as known from sarcop lasmic reticulum). We found not only monomers of typical molecular size (si milar to 106 kD) but also oligomeric forms on Western blots (using anti-SER CA antibodies, also against endogenous SERCA from alveolar sacs) and on ele ctrophoresis gel-autoradiographs of P-32-labeled phosphoenzyme intermediate s. Selective enrichment of SERCA-pump molecules in the inner alveolar sac m embrane region may eliminate Ca2+ after centripetal spread observed during exocytosis activation, while the plasmalemmal Ca2+ pump may maintain or ree stablish [Ca2+] in the narrow subplasmalemmal space between the outer alveo lar sac membrane region and the cell membrane. We show for the first time t he microzonal arrangement of SERCA molecules in a Ca2+ store of a secretory system, an intensely discussed issue in stimulus-secretion coupling resear ch.