Function Ca2ϩ pumps, together with Ca 2ϩ release channels, form ubiquitous Ca 2ϩ regulatory systems in muscle and non-muscle cells. The sarco(endo)plasmic reticulum Ca 2ϩ -ATPases (SERCA) 1 and the plasma membrane Ca 2ϩ -ATPases have the highest affinity for Ca 2ϩ removal from the cytoplasm and, together, set resting cytoplasmic Ca 2ϩ concentrations. Three differentially expressed genes encode SERCA proteins (1). SERCA1a and -1b are expressed in fast-twitch skeletal muscle, but loss of SERCA1 function in Brody disease is sufficiently compensated to preserve life (2). SERCA2a is the cardiac/slow-twitch isoform, whereas SERCA2b, with a C-terminal extension, is expressed in smooth muscle and non-muscle tissues. It is almost certainly an essential gene. SERCA3 is expressed in a limited set of non-muscle tissues, including endothelial, epithelial, and lymphocytic cells and platelets, and its knockout is not lethal (3).SERCA enzymes are typical of the class of P-type ATPases, which form a phosphoprotein intermediate and undergo conformational changes during the course of ATP hydrolysis (4, 5). Some of the conformational states can be stabilized, either by adjustment of reaction conditions or through mutagenesis, and characterized as intermediates in the overall reaction cycle (Fig. 1A). The phosphorylated intermediate, E 1 P(Ca) 2 , can phosphorylate ADP, whereas E 2 P can only react with water. The formation of E 1 P requires that two high affinity Ca 2ϩ binding sites be occupied. The enzyme is then phosphorylated by ATP and, concomitantly, the two Ca 2ϩ ions are occluded and can no longer exchange with cytoplasmic Ca 2ϩ . The rate-limiting transition to E 2 P is accompanied by loss of Ca 2ϩ into the lumen, the affinity having fallen by 3 orders of magnitude. Hydrolysis of E 2 P and regeneration of the high affinity Ca 2ϩ binding sites (E 1 (Ca) 2 ) complete the reversible cycle. High lumenal Ca 2ϩ drives the formation of E 1 P from phosphate (P i ), and its effect on the level of E 1 P led Jencks (5, 6) to postulate a second set of Ca 2ϩ

In a search for the non-muscle equivalent of calsequestrin (the low-affinity high-capacity Ca2(+)-binding protein responsible for Ca2+ storage within the terminal cisternae of the sarcoplasmic reticulum), acidic proteins were extracted from rat liver and brain microsomal preparations and purified by column chromatography. No calsequestrin was observed in these extracts, but the N-terminal amino acid sequence of the major Ca2(+)-binding protein of the liver microsomal fraction was determined and found to correspond to that of calreticulin. This protein was found to bind approx. 50 mol of Ca2+/mol of protein, with low affinity (average Kd approx. 1.0 mM). A monoclonal antibody, C6, raised against skeletal-muscle calsequestrin cross-reacted with calreticulin in SDS/PAGE immunoblots, but polyclonal antibodies reacted with native calreticulin only weakly, or not at all, after SDS denaturation. Immuno-gold decoration of liver ultrathin cryosections with affinity-purified antibodies against liver calreticulin revealed luminal labelling of vacuolar profiles indistinguishable from calciosomes, the subcellular structures previously identified by the use of anti-calsequestrin antibodies. We conclude that calreticulin is the Ca2(+)-binding protein segregated within the calciosome lumen, previously described as being calsequestrin-like. Because of its properties and intraluminal location, calreticulin might play a critical role in Ca2+ storage and release in non-muscle cells, similar to that played by calsequestrin in the muscle sarcoplasmic reticulum.