The inositol 1,4,5-trisphosphate receptor (IP 3 R) is an intracellular Ca 2+ channel that is for the largest part expressed in the endoplasmic reticulum. Its precise subcellular localization is an important factor for the correct initiation and propagation of Ca 2+ signals. The relative position of the IP 3 Rs, and thus of the IP 3 -sensitive Ca 2+ stores, to mitochondria, nucleus or plasma membrane determines in many cases the physiological consequences of IP 3 -induced Ca 2+ release. Most cell types express more than one IP 3 R isoform and their subcellular distribution is cell-type dependent. Moreover, it was recently demonstrated that depending on the physiological status of the cell redistribution of IP 3 Rs and/or of IP 3 -sensitive Ca 2+ stores could occur. This indicates that the cell must be able to regulate not only IP 3 R expression but also its distribution. The various proteins potentially determining IP 3 R localization and redistribution will therefore be discussed.

Abstract. 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 sareoplasmic 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 Mg 2÷. 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 t~M) 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.