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

To document the localization of the KDEL-R more clearly, the immunogold labeling of the KDEL-R in MHV-infected mouse L ceils was quantitated as described in the Materials and Methods (Table I) . The specific density of the label (gold per micrometer of membrane) was highest in the IC/MHV budding compartment. The Golgi stack had slightly less but comparable amounts of gold particles. The gold density of Figure 2 . Labeling of HeLa cells for the KDEL-R. These cells were allowed to internalize two different sizes of BSA-gold particles before fixation. First, a 16-rtm particle was chased overnight into late endosomes and lysosomes (L), and second, a 4-nm particle (arrows) was internalized for 5 min into early endosornes (E). The membranes of the endocytic compartments are invariably devoid of label for the KDEL-R. (A and B) Two typical but different labeling patterns. In A, the label is predominantly associated with cisternae on one side of the Golgi stack (G), whereas in B, the stack is essentially free of label, while the peripheral elements label (arrowheads). In A, the clustering of the label is caused by the use of an intermediate pig anti-rabbit antibody step that amplifies the signal from the bound antibody. M, mitochondrium; N, nucleus. Bars, 100 nm. the IC and the Golgi stack is severalfold higher than the average of the rough ER system (rough ER and nuclear envelope). No label was detected on the plasma membrane in this sampling analysis. These quantitative results further illustrate that the receptor is highly enriched in both the IC and the Golgi stack with significant amounts in the rough ER system.