Calreticulin is a candidate for a calsequestrin-like function in Ca²⁺-storage compartments (calciosomes) of liver and brain

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 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 K(d) 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.