Calreticulin transacylase: Genesis,mechanism of action and biological applications

Our earlier investigations have identified a unique enzyme in the endoplasmic reticulum (ER) termed Acetoxy Drug: Protein Transacetylase (TAase) catalyzing the transfer of acetyl group from polyphenolic acetates (PA) to certain receptor proteins (RP). An elegant assay procedure for TAase was developed based on the inhibition of glutathione S-transferase (GST) due to acetylation by a model acetoxycoumarin, 7, 8-Diacetoxy-4-methylcoumarin (DAMC). TAase purified from various mammalian tissue microsomes to homogeneity exhibited a molecular weight (M.wt) of 55 kDa. Further, by N-terminal sequencing TAase was identified as Calreticulin (CR), a multifunctional Ca²⁺-binding protein in ER lumen. The identity of TAase with CR was evidenced by proteomics studies such as immunoreactivity with anti-CR antibody and mass spectrometry. This function of CR was termed Calreticulin transacetylase (CRTAase). CRTAase was also found to mediate the transfer of acetyl group from DAMC to RP such as NADPH Cytochrome c Reductase (CYPR) and Nitric Oxide Synthase (NOS). The autoacetylation of purified human placental CRTAase concomitant with the acetylation of RP by DAMC was observed. CRTAase activity was found to be inhibited by Ca²⁺. Our investigations on the individual domains (N, P and C) of CR from a nematode Haemonchus contortus revealed that the P-domain alone was found to possess CRTAase activity. Based on the observation that the autoacetylated CR was a stable intermediate in the CRTAase catalyzed protein acetylation by PA, a putative mechanism was proposed. Further, CRTAase was also found capable of transferring propionyl group from a propoxy derivative of polyphenol, 7,8-Dipropoxy-4-methylcoumarin (DPMC) to RP and concomitant autopropionylation of CR was encountered. Hence, CRTAase was assigned the general term Calreticulin Transacylase. Also, CRTAase was found to act upon the biological acyl group donors, acetyl CoA and propionyl CoA. CRTAase mediated modulation of specific functional proteins by way of acylation was exploited to elicit the biological applications of PA.