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 calreticulin-deficient mouse embryonic fibroblasts to examine the function of calreticulin 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 inhibited Ca²⁺ release in response to bradykinin, yet they release Ca²⁺ upon direct activation with the inositol 1,4,5-trisphosphate (InsP₃). These cells fail to produce a measurable level of InsP₃ upon stimulation with bradykinin, 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 bradykinin receptor, which affects its ability to initiate InsP₃-dependent Ca²⁺ release in calreticulin-deficient cells. We concluded that the C domain of calreticulin plays a role in Ca²⁺ storage and that the N domain may participate in its chaperone functions.