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 en doplasmic-reticulum-lumenal protein, ERp28. The protein has the C-term inal retention signal KEEL and localizes to the endoplasmic reticulum (ER) as seen by subcellular fractionation and immunofluorescence studi es. The protein has significant sequence similarity to members of the protein disulfide isomerase (PDI) family, although it lacks the thiore doxin box (CGHC) motif. We propose, on the basis of sequence analysis, a model of the domain structure of PDT, representing a significant ex tension of previously proposed models. Our results are in partial agre ement with recently published NMR data [Kemmink, J., Darby, J., Dijkst ra, K., Nilges, M. & Creighton, T. E. (1997) Curr. Biol. 7, 239-245] a nd indicate that PDI contains, in addition to the two thioredoxin fold s described in previous models, two thioredoxin folds within the domai ns previously defined as b and b'. The thioredoxin domain of ERp28 sha res a higher degree of similarity with the corresponding active and in active domains of PDI than with other members of the PDI family, indic ating that ERp28 developed from an ancient form of PDI or a PDI precur sor. In contrast to Ig-heavy-chain-binding protein, human ERp28 is not induced by metabolic stress (tunicamycin). In in vitro experiments, E Rp28 and calnexin precipitate with overexpressed, wild-type hepatitis B small surface antigen and with a mutated ER-retained form. This indi cates that ERp28, as calnexin, may be involved in the processing of se cretory proteins within the ER.