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

The previous experiments indicated that newly synthesized E1 leaves the RER and accumulates in the smooth membrane . Palmitylated E1 is sensitive to endo H. Cells were labeled with [3H]palmitate for 6 h and E1 was immunoprecipitated as above and subjected to SDS-PAGE and fluorography. Samples were divided into two halves and incubated with (+) or without (-) endo H. Radiolabeled protein standards (kD) are ineluded for reference. tubular network, but it was not clear if this compartment is on the main line or represents a site of diversion of protein traffic from the exocytic pathway. To determine if proteins can enter and leave these structures and reach the Golgi complex, CHOE1 cells were infected with the ts045 strain of VSV. At 39.5~ the G protein of ts045 does not enter the Golgi complex due to a temperature-sensitive transport defect, but when shifted to 32~ the protein rapidly exits the ER and is transported through the Golgi to the cell surface (24, 28) . When CHOE1 cells were kept at 39.5~ VSV G protein was present in both the PER and the E1 tubular network (Fig. 10, A and B ). This indicates that although unable to enter the Golgi complex, G protein had access to the site of E1 arrest at the restrictive temperature. To follow the movement of G protein out of the ER system, infected cells were first incubated at 39.5~ and subsequently shifted to 32~ with cycloheximide present. By 5 min after shift to the permissive temperature, G protein could be seen within vesicular structures surrounding the tubular network and throughout the cytoplasm (Figs. 10, C and D) , G protein was seen in similar vesicular structures when infected cells were held at 15~ following G protein buildup at 39.5~ (Fig. 10 L) . Continued incubation at 32~ allowed complete egress of G from the El-containing compartment and movement of the G-containing vesicles to the Golgi region (Fig. 10, E-H) .