Delayed Activation of the Store-operated Calcium Current Induced by Calreticulin Overexpression in RBL-1 Cells

Calreticulin (CRT) is a high-capacity, low-affinity Ca(2+)-binding protein located in the lumen of the endoplasmic reticulum (ER) of all eukaryotic cells investigated so far. Its high level of conservation among different species suggests that it serves functions fundamental to cell survival. The role originally proposed for CRT, i.e., the main Ca(2+) buffer of the ER, has been obscured or even casted by its implication in processes as diverse as gene expression, protein folding, and cell adhesion. In this work we seek the role of CRT in Ca(2+) storing and signaling by evaluating its effects on the kinetics and amplitude of the store-operated Ca(2+) current (I(CRAC)). We show that, in the rat basophilic leukemia cell line RBL-1, overexpression of CRT, but not of its mutant lacking the high-capacity Ca(2+)-binding domain, markedly retards the I(CRAC) development, however, only when store depletion is slower than the rate of current activation. On the contrary, when store depletion is rapid and complete, overexpression of CRT has no effect. The present results are compatible with a major Ca(2+)-buffering role of CRT within the ER but exclude a direct, or indirect, role of this protein on the mechanism of I(CRAC) activation.