CaBP2 is a rat homolog of ERp72 with proteindisulfide isomerase activity

Ca‐binding protein 2 (CaBP2) has been described previously as an intracisternal calcium‐binding microsomal glycoprotein [1]. We report now the primary sequence of this protein as deduced from the corresponding cDNA. The protein possesses a C‐terminal ‐KEEL retention sequence and three repeats of the thioredoxin‐like motive ‐EFYAPNCGHCK‐, and represents the rat homolog of ERp72 [2]. In contrast to earlier reports on ERp72, CaBP2 possesses significant proteindisulfide isomerase activity. Furthermore, in contrast to ERp72, CaBP2 is a glycoporotein containing O‐linked glycans. The amount of CaBP2 in H‐35 Reuber hepatoma cells increases in parallel with that of immunoglobin heavy‐chain‐binding protein under conditions which lead to impaired glycosylation, while the amount of calreticulin, another KDEL‐containing glycoprotein, remains almost unchanged.