KDEL receptor expression is not coordinatedly up-regulated with ER stress-induced reticuloplasmin expression in HeLa cells.

Perturbation of the normal functioning of the endoplasmic reticulum (ER) increases the expression of lumenal proteins, such as grp78, and calreticulin. These proteins are retained within the compartment by a salvage mechanism involving the recognition of a C-terminal tetra-peptide sequence by the KDEL receptor. We have investigated whether disrupting normal ER function concomitantly increases the expression of the mRNAs encoding the two mammalian isoforms of the receptor, erd2.1 and erd2.2. Inhibition of N-linked glycosylation of proteins by tunicamycin had no effect upon the levels of the single mRNA species recognized by the erd2.1 probe, or the multiple transcripts detected with the erd2.2 cDNA probe. ER Ca2+ store depletion by thapsigargin did not increase erd2.1 mRNA, but actually caused a decrease in erd2.2 mRNA. Both thapsigargin, and tunicamycin, increased calreticulin secretion from the cells, although this might be due to more than simply saturation of KDEL receptor binding.