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

The intermediate compartment was originally defined as a system of vacuoles located between the ER and cis-Golgi which newly synthesized viral proteins could enter at 15~ (52) . Although the intermediate compartment is still poorly defined, a number of properties and functions have been ascribed to it including; (a) palmitylation of viral membrane proteins (6); (b) budding of mouse hepatitus virus coronavirus (61) ; and (c) site of localization of certain membrane proteins (53, 54) . Originally, it was proposed that the intermediate compartment was the site where retrieval of KDELbearing proteins occurred. However, more recently this process has been suggested to occur in the Golgi complex (29) . An intermediate compartment would be expected to have physical properties and resident proteins distinct from the PER and cis-Golgi, and results obtained by Schweizer and colleagues demonstrate that such is the case (56) . The structure described in this paper shares some of the characteristics attributed to the intermediate compartment, such as, (a) palmitylation which occurs either proximal to or at the site of E1 accumulation; (b) it is not affected by agents which disrupt the Golgi or ER; and (c) it lacks RER and Golgi resident membrane proteins. It differs, however, in that transport of E1 is arrested proximal to the 15~ compartment as defined by transport of VSV G protein. This raises the question of how the compartment we have partially characterized and the intermediate compartment interact. The presence of physical connections between them seems unlikely. Rather, it seems more likely that a round of vesicular transport is required for passage of proteins from the site of E1 arrest to the pre-Golgi vacuoles that constitute the 15~ intermediate compartment.