Calreticulin modulates capacitative Ca2+ influx by controlling the extent of inositol 1,4,5-trisphosphate-induced Ca2+ store depletion

Calreticulin (CRT) is a highly conserved Ca2+-binding protein that resides in the lumen of the endoplasmic reticulum (ER). We overexpressed CRT in Xenopus oocytes to determine how it could modulate inositol 1,4-5-trisphosphate (InsP(3))-induced Ca2+ influx. Under conditions where it did not affect the spatially complex elevations in free cytosolic Ca2+ concentration ([Ca2+](i)) due to InsP(3)-induced Ca2+ release, overexpressed CRT decreased by 46% the Ca2+-gated Cl- current due to Ca2+ influx. Deletion mutants revealed that CRT requires its high capacity Ca2+-binding domain to reduce the elevations of [Ca2+](i), due to Ca2+ influx. This functional domain was also required for CRT to attenuate the InsP(3)-induced decline in the free Ca2+ concentration within the ER lumen ([Ca2+](ER)), as monitored with a "chameleon" indicator. Our data suggest that by buffering [Ca2+](ER) near resting levels, CRT may prevent InsP(3) from depleting the intracellular stores sufficiently to activate Ca2+ influx.