ERP60 DOES NOT SUBSTITUTE FOR PROTEIN DISULFIDE-ISOMERASE AS THE BETA-SUBUNIT OF PROLYL 4-HYDROXYLASE

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyses the formation of 4-hydro xyproline in collagens. The vertebrate enzymes are alpha(2) beta(2) te tramers while the Caenorhabditis elegans enzyme is an alpha beta dimer . The beta-subunit is identical to protein disulphide isomerase (PDI), a multifunctional endoplasmic reticulum luminal polypeptide. ERp60 is a PDI isoform that was initially misidentified as a phosphatidylinosi tol-specific phospholipase C. We report here on the cloning and expres sion of the human and Drosophila ERp60 polypeptides. The overall amino acid sequence identity and similarity between the processed human ERp 60 and PDI polypeptides are 29% and 56% respectively, and those betwee n the Drosophila ERp60 and human PDI polypeptides 29% and 55%. The two ERp60 polypeptides were found to be similar to human PDI within almos t all their domains, the only exception being the extreme C-terminal r egion. Nevertheless, when the human or Drosophila ERp60 was expressed in insect cells together with an alpha-subunit of human prolyl 4-hydro xylase, no tetramer was formed and no prolyl 4-hydroxylase activity wa s generated in the cells. Additional experiments with hybrid polypepti des in which the C-terminal regions had been exchanged between the hum an ERp60 and PDI polypeptides demonstrated that the differences in the C-terminal region are not the only reason for the lack of prolyl 4-hy droxylase tetramer formation by ERp60.