ERp28, a human endoplasmic-reticulum-lumenal protein, is a member of the protein disulfide isomerase family but lacks a CXXC thioredoxin-box motif.

We report on the isolation, sequence and a putative role of a human endoplasmic-reticulum-lumenal protein, ERp28. The protein has the C-terminal retention signal KEEL and localizes to the endoplasmic reticulum (ER) as seen by subcellular fractionation and immunofluorescence studies. The protein has significant sequence similarity to members of the protein disulfide isomerase (PDI) family, although it lacks the thioredoxin box (CGHC) motif. We propose, on the basis of sequence analysis, a model of the domain structure of PDI, representing a significant extension of previously proposed models. Our results are in partial agreement with recently published NMR data [Kemmink, J., Darby, J., Dijkstra, K., Nilges, M. & Creighton, T. E. (1997) Curr. Biol. 7, 239-245] and indicate that PDI contains, in addition to the two thioredoxin folds described in previous models, two thioredoxin folds within the domains previously defined as b and b'. The thioredoxin domain of ERp28 shares a higher degree of similarity with the corresponding active and inactive domains of PDI than with other members of the PDI family, indicating that ERp28 developed from an ancient form of PDI or a PDI precursor. In contrast to Ig-heavy-chain-binding protein, human ERp28 is not induced by metabolic stress (tunicamycin). In in vitro experiments, ERp28 and calnexin precipitate with overexpressed, wild-type hepatitis B small surface antigen and with a mutated ER-retained form. This indicates that ERp28, as calnexin, may be involved in the processing of secretory proteins within the ER.