Cortical alveoli of Paramecium: a vast submembranous calcium storage compartment

The plasma membrane of Paramecium is underlain by a continuous layer of membrane vesicles known as cortical alveoli, whose function was unknown but whose organization had suggested some resemblance with muscle sarcoplasmic reticulum. The occurrence of antimonate precipitates within the alveoli first indicated to us that they may indeed correspond to a vast calcium storage site. To analyze the possible involvement of this compartment in calcium sequestration more directly, we have developed a new fractionation method, involving a Percoll gradient, that allows rapid purification of the surface layer (cortex) of Paramecium in good yield and purity and in which the alveoli retain their in vivo topological orientation. This fraction pumped calcium very actively in a closed membrane compartment, with strict dependence on ATP and Mg2+. The pumping activity was affected by anti-calmodulin drugs but no Triton-soluble calmodulin binding protein could be identified, using gel overlay procedures. The high affinity of the pump for calcium (Km = 0.5 microM) suggests that it plays an important role in the normal physiological environment of the cytosol. This may be related to at least three calcium-regulated processes that take place in the immediate vicinity of alveoli: trichocyst exocytosis, ciliary beating and cytoskeletal elements dynamics during division.