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

Our recent data argue strongly in favor of the one vesicular step model whereby the IC is a functional domain of the ER (Krijnse-Locker et al., 1994) . In the latter study, we used cells infected with mouse hepatitis virus (MHV) as a model system, and we showed that this virus both buds and acquires the first O-linked sugar, N-acetyl-galactosamine, on its M membrane protein in the IC. Furthermore, conditions that blocked vesicular transport, such as 14°C in vivo or GTP3,S in vitro, had no effect on transport from the rough ER to the IC, although these conditions, as expected, prevented the M protein from acquiring Golgi sugar modifications. Moreover, the compartment where MHV buds was enriched in p58, rab2, as well as in the KDEL protein PDI, which is often considered to be exclusively found in the rough ER from immunofluorescence studies. The latter finding was subsequently strengthened by the localization to the budding compartment of a novel HDEL calcium-binding protein, ERC 55, that was identified in HeLa cells (Weis et al., 1994) . In all of our studies using markers of the IC, as well as using PDI and ERC 55, the labeling consistently extended to one cisterna on the cis side of the Golgi stack, although we emphasize that because of the three-dimensional complexity of the Golgi stack, this cisterna is not apparent in all sections through the stack. We have proposed therefore that this cisterna is part of the IC, and that the bona fide Golgi complex would start at the next, second cisterna (Krijnse-Locker et al., 1994; Griffiths and Rottier, 1992 ; see also Ellisman, 1985a, 1985b) .