Fig. 5.

Mechanisms proposed for DMV biogenesis. (A) Different stages in remodelling of ER membranes that would result in DMVs by an enwrapping mechanism. This process would start with the pairing of membranes, which then would start curving until this curvature becomes so pronounced that a double-membrane vase-like compartment is formed. Finally, through a fission event of the inner membrane a closed compartment is formed, while the outer membrane is still continuous with the ER. An additional fission event of the outer membrane would sever the connection with the ER and give rise to a detached DMV. (B) An alternative mechanism that would result in DMV formation is double budding. The first step in this process would be the formation of an invagination in an ER membrane, which then buds into the ER lumen through a fission event. The resulting luminal single-membrane vesicle could subsequently interact with the ER membrane to form stretches of paired membranes. Induction of additional curvature would enable the vesicle to bud out of the ER, thereby acquiring a second membrane. Notice that stages IV and V would be topologically identical in both routes and that similar membrane remodelling events are required for both mechanisms: membrane pairing (represented by red lines), positive and negative membrane curvature (green and red double arrows, respectively) and membrane fission (blue arrowheads). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)