Molecular cloning of ERp29, a novel and widely expressed resident of the endoplasmic reticulum

Abstract

We have isolated a full-length cDNA clone for a novel 29 kDa protein that is highly expressed in rat enamel cells. The clone encodes a 259-residue protein, here named ERp29, with structural features (signal peptide and a variant endoplasmic reticulum-retention motif, KEEL) that indicate it is a reticuloplasmin. ERp29 has limited homology with protein disulfide isomerase and its cognates, but lacks their characteristic thioredoxin-like catalytic moiety and calcium-binding motifs. ERp29 mRNA was expressed in all rat tissues tested, and a homologous transcript 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 reticuloplasmins. We conclude that ERp29 is a new, highly conserved member of the reticuloplasmin family which is widely expressed. The apparent lack of both calcium binding properties and stress responsiveness distinguish ERp29 from all major reticuloplasmins characterised to date.

windbeutel,a gene required for dorsoventral patterning inDrosophila,encodes a protein that has homologies to vertebrate proteins of the endoplasmic reticulum

“…Similar structures are predicted for the rat ERp29 protein (Demmer et al 1997), leading to suggestions of the ends of the protein being folded as globular domains. This suggestion is supported by comparisons of the rat and human proteins (90% identical), where the nonconserved amino acids are clustered in the signal peptide and carboxy-terminal parts (Demmer et al 1997).…”

“…They are proposed to function as molecular chaperons during protein assembly and degradation (Nigam et al 1994) and as calcium buffers (Hubbard 1996). ERp29 was shown to be unresponsive to cellular stresses that induce known heat shock proteins that are related to members of the reticuloplasmin family, and it is not a calcium-binding protein (Demmer et al 1997). These results suggest that ERp29 and its Drosophila and human homologs form a new group of reticuloplasmins with a conserved, but presently unknown, function.…”

“…Database similarity searches revealed that the putative protein encoded by wind is homologous to the rat endoplasmic reticulum protein ERp29 precursor (Demmer et al 1997). The two proteins share 30% overall identities and 40% homology, taking conserved changes into account (Fig.…”

“…6). The rat protein has been proposed to reside in the ER because it was isolated from the microsomal fraction of the cells (Demmer et al 1997), and it also contains a variant of the ER retention signal (KEEL, Pelham 1990). Significantly, Wind, like the other two proteins, contains two of those variant ER retention signals, located in comparable positions in the carboxyl terminus of the molecule (Figs.…”

“…Wind contains a thioredoxin-like motif between amino acids 24-27 [Cys-Thr-Gly-Cys] as well as a single Cys residue in amino acid position 149. The single Cys is also conserved in the rat sequence, ERp29, which displays significant homology (25% identities over a stretch of 103-122 residues) to two different protein disulfide isomerases, but does not contain any intact thioredoxin-like motif (Demmer et al 1997). The Figure 6.…”