Characterization of the budding compartment of mouse hepatitis virus: evidence that transport from the RER to the Golgi complex requires only one vesicular transport step

Coronaviruses have been shown to bud into a smooth membrane compartment that is physically contiguous with the cis face of the Golgi complex (Tooze et al., 1984 (Tooze et al., , 1988 Tooze and Tooze, 1985) . In the case of the avian infectious bronchitis virus the M (previously referred to as El) protein of the virus, when expressed by itself, localizes to the region of the ER/cis-Golgi boundary (Machamer et al., 1990) . For another coronavirus, mouse hepatitis virus (MHV),~ which has been studied extensively at the EM level (Tooze et al., 1984 (Tooze et al., , 1988 Tooze and Tooze, 1985) , the situation is more complex. In this case the M protein of the virus, when expressed by itself localizes to late Golgi elements including the trans-Golgi network (TGN; Krijnse-Locker et al., 1992a) . Our present working model to rationalize the difference in localization of the M protein of MHV in infected cells (the bulk in pre-Golgi structures) and when expressed independently (the bulk in late Golgi) is that in infected ceUs the M protein is retained at the site of budding by an interaction with the second membrane protein, the spike (S) protein, as well as possibly with the viral nucleocapsid (Griffiths and Rottier, 1992) .