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

The second model we tested was vaccinia virus infection. Whereas the early stages of the virus acquire their membranes from the IC to form the intracellular mature virus (IMV), at a late stage in the infection, the IMV becomes enwrapped b y a cisterna that originates from the TGN (Schmelz et al., 1994) . In sections of cells infected for >8 h with vaccinia, these wrapping membranes can easily be identified. The use of a vaccinia recombinant expressing the M protein of MHV gave us a second marker for the TGN since, after a cycloheximide chase, the bulk of this protein in this system is localized to the trans-Golgi/TGN under these conditions (Krijnse-Locker et al., 1992) . Sections were made of L cells infected for 8 h with the M protein recombinant vaccinia virus. These were double labeled for the KDEL-R and for the M protein. The two proteins exten-sively colocalized and this colocalization extended to the membranes enwrapping the virions (not shown; see below). Thus, in two different viral systems, the KDEL-R appeared to be found in extremely high amounts in the TGN. This result was clearly at variance with the very low levels detected on the trans side of the stack, both in the tissues and the uninfected HeLa and NRK cells. Since the KDEL-R labeled the whole stack in the MHV-infected L cells, these data collectively suggested that, after viral infection, the distribution of the KDEL-R shifts from the cis side towards the TGN.