Functional specialization of calreticulin domains

Calreticulin is a Ca2+-binding chaperone in the endoplasmic reticulum (ER), and calreticulin gene knockout is embryonic lethal. Here, we used calretic ulin-deficient mouse embryonic fibroblasts to examine the function of calre ticulin as a regulator of Ca2+ homeostasis. In cells without calreticulin, the ER has a lower capacity for Ca2+ storage, although the free ER luminal Ca2+ concentration is unchanged. Calreticulin-deficient cells show inhibite d Ca2+ release in response to bradykinin, yet they release Ca2+ upon direct activation with the inositol 1,4,5-trisphosphate (InsP(3)). These cells fa il to produce a measurable level of InsP(3) upon stimulation with bradykini n, likely because the binding of bradykinin to its cell surface receptor is impaired. Bradykinin binding and bradykinin-induced Ca2+ release are both restored by expression of full-length calreticulin and the N + P domain of the protein. Expression of the P + C domain of calreticulin does not affect bradykinin-induced Ca2+ release but restores the ER Ca2+ storage capacity. Our results indicate that calreticulin may play a role in folding of the b radykinin receptor, which affects its ability to initiate InsP(3)-dependent Ca2+ release in calreticulin-deficient cells. We concluded that the C doma in of calreticulin plays a role in Ca2+ storage and that the N domain may p articipate in its chaperone functions.