MOLECULAR-CLONING OF ERP29, A NOVEL AND WIDELY EXPRESSED RESIDENT OF THE ENDOPLASMIC-RETICULUM

We have isolated a full-length cDNA clone for a novel 29 kDa protein t hat is highly expressed in rat enamel cells. The clone encodes a 259-r esidue protein, here named ERp29, with structural features (signal pep tide and a variant endoplasmic reticulum-retention motif, KEEL) that i ndicate it is a reticuloplasmin. ERp29 has limited homology with prote in disulfide isomerase and its cognates, but lacks their characteristi c thioredoxin-like catalytic moiety and calcium-binding motifs. ERp29 mRNA was expressed in all rat tissues tested, and a homologous transcr ipt was detected in other animal livers (primate, ruminant, marsupial) . In human hepatoma cells, ERp29 mRNA expression was not increased by stresses (tunicamycin, calcium ionophore) that induced other reticulop lasmins. We conclude that ERp29 is a new, highly conserved member of t he reticuloplasmin family which is widely expressed. The apparent lack of both calcium binding properties and stress responsiveness distingu ish ERp29 from all major reticuloplasmins characterised to date.