OVEREXPRESSION OF CALRETICULIN INCREASES THE CA2+ CAPACITY OF RAPIDLY EXCHANGING CA2+ STORES AND REVEALS ASPECTS OF THEIR LUMENAL MICROENVIRONMENT AND FUNCTION

A molecularly tagged form of calreticulin (CR), a low affinity-high capacity Ca2+ binding protein that resides in the ER lumen, was transiently trans fected into HeLa cells to specifically modify the Ca2+ buffering capacity of the intracellular Ca2+ stores. Fluorescence and confocal microscope immunocytochemistry revealed the tagged protein to be expressed by over 40% of the cells and to overlap in its distribution the endogenous CR yielding a delicate cytoplasmic network, i.e., the typical pattern of ER. In contrast, no signal was observed associated with the plasmalemma (marked by ConA) and within the nucleus. One- and two-dimensional Western blots revealed the transfected to exceed the endogenous CR of similar to 3.5-fold and to maintain its Ca2+ binding ability, whereas the expression of other ER proteins was unchanged, Ca2+ homeostasis in the transfected cells was investigated by three parallel approaches: (a) Ca-45 equilibrium loading of cell populations; (b) [Ca2+](c) measurement with fura-2 followed by quantitative immunocytochemistry of single cells and iii) [Ca2+](c), measurement of cell population upon cotransfection with the Ca2+-sensitive photoprotein, aequorin, The three approaches revealed different aspects of Ca2+ homeostasis, yielding results which were largely complementary, In particular, the following conclusions were established: (a) both endogenous and transfected CR participate in Ca2+ buffering within the IP3-sensitive, rapidly exchanging, Ca2+ stores; the other pools of the cells were in contrast unaffected by CR transfection; (b) the Ca2+ capacity of the stores is not the main limiting factor of individual IP3-mediated Ca2+ release responses triggered by receptor agonists; (c) in control cells, the contribution of CR to Ca2+ buffering within the IP3-sensitive stores accounts for similar to 45 % of the total, the rest being probably contributed by the other lumenal (and also membrane) Ca2+ binding proteins; (d) the free [Ca2+] within the lumen of the IP3-sensitive stores, revealed by the degree of Ca2+ binding to the transfected CR protein, amounts to values in (or approaching) the millimolar range; and (e) Ca2+ influx across the plasmalemma activated by depletion of the stores is directly dependent on the lumenal [Ca2+].