DELAYED ACTIVATION OF THE STORE-OPERATED CALCIUM CURRENT-INDUCED BY CALRETICULIN OVEREXPRESSION IN RBL-1 CELLS

Calreticulin (CRT) is a high-capacity, low-affinity Ca2+-binding prote in located in the lumen of the endoplasmic reticulum (ER) of all eukar yotic cells investigated so far. Its high level of conservation among different species suggests that it serves functions fundamental to cel l survival. The role originally proposed for CRT, i.e., the main Ca2buffer 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 Ca2+ storing and sig naling by evaluating its effects on the kinetics and amplitude of the store-operated Ca2+ current (I-CRAC). We show that, in the rat basophi lic leukemia cell line RBL-1, overexpression of CRT, but not of its mu tant lacking the high-capacity Ca2+-binding domain, markedly retards t he I-CRAC development, however, only when store depletion is slower th an the rate of current activation. On the contrary, when store depleti on is rapid and complete, overexpression of CRT has no effect. The pre sent results are compatible with a major Ca2+-buffering role of CRT wi thin the ER but exclude a direct, or indirect, role of this protein on the mechanism of I-CRAC activation.