The rubella virus E1 glycoprotein is arrested in a novel post-ER, pre- Golgi compartment

The collective ultrastructural and biochemical findings suggested that unassembled E1 subunits accumulate at a pre-Golgi site which is in continuity with the RER but morphologically does not resemble either RER or Golgi complex. To determine if ER markers were present at the site of E1 arrest, we used antibodies to various resident proteins of this organelle in double immunofluorescence experiments. A polyclonal antiserum that recognizes four RER membrane proteins (33) gave a reticular cytoplasmic staining pattern typical of the RER; however, no staining of the El-containing compartment was evident (Fig. 6, A and B) . Similarly, a polyclonal antibody to a 160-kD ER membrane protein (obtained from D. Meyer) and another antiserum to three other integral membrane proteins of the RER (15) did not label the site of E1 accumulation (not shown). The fact that very little reticular ER staining for E1 was evident (Fig. 6 A) suggests that E1 rapidly leaves the RER after translocation. In contrast to that of RER membrane proteins, the distribution of two KDEL-containing content proteins, protein disulfide isomerase (PDI), and BiP, partially overlapped with that of E1 (Fig. 6, C-F) . However, these proteins were clearly not concentrated in this compartment.