Localization of the Lys, Asp, Glu, Leu tetrapeptide receptor to the Golgi complex and the intermediate compartment in mammalian cells

In every cell type we investigated in this study, there was a high concentration of the KDEL-R on the cisternae localized to the cis aspect of the Golgi stack. Under normal, steadystate conditions, our data argue that only low levels of the receptor are found in the trans cisternae and the TGN. Under no condition did we detect any significant amounts of this protein in any compartment distal to the TGN. In many, but not all of the cell types we examined, the labeling extended to membrane elements peripheral to the Golgi stack. The bulk of this labeling was localized to the IC, which we could convincingly identify in the SLOpermeabilized, MHV-infected L cells by the presence of budding virions. In these cells, the concentration of the receptor in the IC, as estimated by quantitation of the immunogold labeling, was similar to that observed in the Golgi stack. In these cells, we could also detect a significant (albeit fivefold lower than the IC/Golgi) labeling of the rough ER and a still lower level in the nuclear envelope. In these SLO-treated cells, the accessibility for the cytoplasmic epitopes of the KDEL-R for the antibody was clearly enhanced on the sections. This was evident from a comparison of the labeling of permeabilized with nonpermeabilized cells. Therefore, in the absence of the permeabilization step to remove cytosolic components, the anti-KDEL-R may have significantly less accessibility to its antigen on the section. This point should be considered in evaluating the absence of detectable labeling in the rough ER of cells such as the pancreatic acinar cells.