ERcalcistorin/protein disulfide isomerase (PDI). Sequence determination and expression of a cDNA clone encoding a calcium storage protein with PDI activity from endoplasmic reticulum of the sea urchin egg.

Following the purification of a 58-kDa calsequestrin-like protein from the endoplasmic reticulum (ER) of sea urchin eggs (Oberdorf, J.A., Lebeche, D., Head, J. F., and Kaminer, B. (1988) J. Biol. Chem. 263, 6806-6809) and its characterization as a high capacity, low affinity calcium-binding protein (Lebeche, D., and Kaminer, B. (1992) Biochem. J. 287, 741-747) we isolated and sequenced a cDNA encoding for this protein. The deduced 496 amino acids contain a 17-residue NH2-terminal signal peptide, a KDEL COOH-terminal ER retention signal and two thioredoxin-like active site domains, -CGHC-, identical with those in protein disulfide isomerase (PDI). The sea urchin egg protein shares a 55% sequence identity with mammalian PDI and its PDI activity is 30% of the activity of rabbit liver PDI. The corresponding mRNA was found in oocytes, mature eggs, embryos, and differentiated tissues of the sea urchin in varying amounts. COS-7 cells transfected with the cDNA, expressed a 58-kDa protein immunoreactive to antibodies against the sea urchin egg protein. This molecule appears to have a dual function of calcium storage and PDI activity within the ER. We hence redesignate it ERcalcistorin/PDI (ECaSt/PDI), a protein that is distinct from calsequestrin and calreticulin.