Figure 3. Models for the formation of virus-induced double-membrane vesicles.

The protrusion and detachment model (a) proposes that part of the endoplasmic reticulum (ER) cisterna starts to bend, pinches off and then seals to form a double-membrane vesicle (DMV). Interactions between the lumenal domains of viral membrane proteins (coloured yellow in the ER membrane) could mediate the tight apposition of the two bilayers and induce curvature. In the double-budding model (b), a single-membrane vesicle buds into the lumen of the ER and then buds out again, and the membrane proteins could mediate inward as well as outward budding. Figure adapted, with permission, from Ref. 20 © (1999) American Society for Microbiology.