The carboxyl-terminal Lys-Asp-Glu-Leu (KDEL), or a closely-related sequence, is 
important for ER localization of both lumenal as well as type II membrane 
proteins. This sequence functions as a retrieval signal at post-ER 
compartment(s), but the exact compartment(s) where the retrieval occurs remains 
unresolved. With an affinity-purified antibody against the carboxyl-terminal 
sequence of the mammalian KDEL receptor, we have investigated its subcellular 
localization using immunogold labeling on thawed cryosections of different 
tissues, such as mouse spermatids and rat pancreas, as well as HeLa, Vero, NRK, 
and mouse L cells. We show that rab1 is an excellent marker of the intermediate 
compartment, and we use this marker, as well as budding profiles of the mouse 
hepatitis virus (MHV) in cells infected with this virus, to identify this 
compartment. Our results demonstrate that the KDEL receptor is concentrated in 
the intermediate compartment, as well as in the Golgi stack. Lower but 
significant labeling was detected in the rough ER. In general, only small 
amounts of the receptor were detected on the trans side of the Golgi stack, 
including the trans-Golgi network (TGN) of normal cells and tissues. However, 
some stress conditions, such as infection with vaccinia virus or vesicular 
stomatitis virus, as well as 20 degrees C or 43 degrees C treatment, resulted in 
a significant shift of the distribution towards the trans-TGN side of the Golgi 
stack. This shift could be quantified in HeLa cells stably expressing a TGN 
marker. No significant labeling was detected in structures distal to the TGN 
under all conditions tested. After GTP gamma S treatment of permeabilized cells, 
the receptor was detected in the beta-COP-containing buds/vesicles that 
accumulate after this treatment, suggesting that these vesicles may transport 
the receptor between compartments. We propose that retrieval of KDEL-containing 
proteins occurs at multiple post-ER compartments up to the TGN along the 
exocytotic pathway, and that within this pathway, the amounts of the receptor in 
different compartments varies according to physiological conditions.