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

We have recently presented evidence that the intermediate compartment between the rough ER and the Golgi stack is essentially a specialized subdomain (or domains) of the ER from which COP vesicles destined for the Golgi complex originate (Krijnse-Locker et al., 1994) . We operationally define the first of the Golgi compartments as that where Man I functions. This notion would be consistent with the fact that this is the first N-linked oligosaccharide modification seen after release of the 15°C block (which defines the ER to Golgi boundary) (Balch, 1989 ). In the present paper, we provide additional support for our model. First, we show that rabl is an excellent marker for the IC since it precisely colocalizes with both p58 (which is a less abundant marker) in both uninfected cells and in MHV-infected cells. In many cases, the labeling for rabl extends to the first (and occasionally two) cisternae of the Golgi stack. Further, we provide additional evidence that KDEL proteins are also enriched in the IC. In most cases, these KDEL proteins are only in very low amounts in central parts of the Golgi stack. It should again be emphasized that a more definitive definition of the boundary between the IC and the bona fide Golgi complex awaits the availability of high titer antibodies against early Golgi proteins. A consequence of this problem is that it is difficult at present to estimate how much of the rabl is found in the Golgi compartments distal to the IC.