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 Ca(2+)-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 Ca(2+) influx. Under conditions where it did not affect the spatially complex elevations in free cytosolic Ca(2+) concentration (Ca(2+)) due to InsP(3)-induced Ca(2+) release, overexpressed CRT decreased by 46% the Ca(2+)-gated Cl(-) current due to Ca(2+) influx. Deletion mutants revealed that CRT requires its high capacity Ca(2+)-binding domain to reduce the elevations of Ca(2+) due to Ca(2+) influx. This functional domain was also required for CRT to attenuate the InsP(3)-induced decline in the free Ca(2+) concentration within the ER lumen (Ca(2+)), as monitored with a "chameleon" indicator. Our data suggest that by buffering Ca(2+) near resting levels, CRT may prevent InsP(3) from depleting the intracellular stores sufficiently to activate Ca(2+) influx.