Heavy chain binding protein (BiP/GRP78) and endoplasmin are exported from the endoplasmic reticulum in rat exocrine pancreatic cells, similar to protein disulfide-isomerase.

Previously we found that in rat exocrine pancreatic cells, protein disulfide-isomerase (PDI), one of the major resident proteins in the lumen of the endoplasmic reticulum (ER) of many cells, is localized not only in the ER but also in the Golgi apparatus, secretory granules, plasma membranes, and even in the glandular lumens, despite possessing the ER retention signal KDEL (Lys-Asp-Glu-Leu) at the carboxyl terminus. In this report, we examined whether other ER luminal proteins bearing the KDEL signal at their C-termini, such as BiP/GRP78 and endoplasmin/GRP94 are also exported from the ER. We prepared two kinds of affinity-purified polyclonal antibodies; one against a synthetic peptide with 12 amino acids which is identical to the carboxyl terminus of BiP and another against purified endoplasmin. Immunoblot analysis using these two antibodies showed that BiP and endoplasmin exist in both the plasma membrane and the microsomal fractions, similar to the intracellular distribution of PDI in rat exocrine pancreas. The ratios of the amount of the three proteins in the two fractions, however, were variable, suggesting that the KDEL-bearing proteins such as PDI, BiP, and endoplasmin are exported from the ER with different efficiencies. Postembedding protein A-immunogold electron microscopy revealed that endoplasmin was exported from the ER and secreted to the extracellular space. The secretion of PDI in rat pancreatic lobules was inhibited by Brefeldin A (BFA) and by guanidino acid esters (FOY-305), which are known to be the inhibitors of the intracellular transport. Taken together with the previous immunogold electron microscopic analyses by Akagi et al. (1988), it is strongly suggested that in rat exocrine pancreatic cells PDI and the other KDEL-bearing proteins found in the extracellular space were not artificially released by cell damage during incubation but were secreted via the normal secretory pathway.