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

In our previous study, we showed that the KDEL-containing protein disulfide isomerase (PDI) labeled not only the rough ER, but also the IC, extending to one cisterna on one side of the Golgi stack (Krijnse-Locker et al., 1994) . As shown in Fig. 6 , the PDI labeling also colocalized with rabl in an uninfected L cell treated with SLO, both extending to one Golgi cisterna. This point was documented further by using antibodies against a spectrum of KDEL proteins (Nguyen Van et al., 1989; Peter et al., 1992) , including the calcium binding proteins CaBP1, CaBP2, and CaBP3 (calreticulin), as well as a generic anti-KDEL peptide antibody. The localization of these proteins in infected (not shown) and uninfected cells (Fig. 7) was indistinguishable from that for PDI with a strong labeling of the rough ER, nuclear envelope, and the IC, extending in some sections to one, and occasionally two Golgi cisternae (not shown; see below). Fig. 7 shows representative examples of the double labeling of KDEL-R and either the anti-KDEL peptide antibody (Fig. 7 , A and B) or anti CaBP1 (Fig. 7 C) in uninfected ceils permeabilized with SLO. Whereas the Golgi stack is, for the most part, unlabeled for the KDEL proteins, significant levels of labeling are usually found in one and occasionally two cisternae, as well as in typical IC membranes on one side of the stack (Fig. 7, A-C) . These membrane structures also la- Figure 5 . Cryosections of an uninfected L cell permeabilized ~vith SLO. (A) Double labeling for rabl (small arrowheads, 10 nm gold) and p58 (arrows, 5 nm gold). Note the extensive nature of the IC extending to one side of the Golgi stack (G). The large arrow shows a continuity between a narrow, presumably tubular domain with a larger, more electron-dense vesicular profile of the IC. The large arrowheads show coated buds that are most likely to be COP. (B) Single labeled for rabl, a continuity between the stack and a curved, cisternal element labeled for rabl is shown. Bars, 100 nm. Figure 6 . Colocalization of rabl and PDI in an SLO-treated, uninfected L cell. Rabl (arrowheads, 10 nm gold) colocalizes with PDI (arrows, 5 nm gold) in membrane structures close to the cis side of the Golgi stack, including one cisterna. N, nucleus; C, putative clathrin bud.