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 (InsP₃)-induced Ca²⁺ influx. Under conditions where it did not affect the spatially complex elevations in free cytosolic Ca²⁺ concentration ([Ca²⁺](i)) due to InsP₃-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 InsP₃-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 InsP₃ from depleting the intracellular stores sufficiently to activate Ca²⁺ influx.