Cannibalism, although forbidden in most human societies, is part of the everyday life of a cell. To clear cellular debris and defective organelles or mobilize nutrients in times of deprivation, cells perform a self-eating process called autophagy. In this process, cellular material is surrounded by an isolation membrane that expands to form an enclosed double-membrane vesicle called the autophagosome. This structure fuses with lysosomes where the contents are degraded. The regulation of autophagy in physiological and pathological situations is an area of great interest. Although many aspects of the membrane biology associated with this process (reviewed in Simonsen and Tooze, (2009)) have been elucidated at a molecular level, the source of the isolation membrane is still a point of contention. Does it form de novo or is derived from a cellular organelle like the ER?Axe et al. (see also Comment from Simonsen and Stenmark), showed that cup-shaped structures emerging from the ER (termed ‘omegasomes’ due to their shape) act as platforms for the formation of autophagosomes. Now, a study in Nature Cell Biology (Hayashi-Nishino et al.) adds further support for the ER as the source of this membrane. In cultured cells, the authors found a close association between the ER and the IM and that this ER-IM shared the same markers as Axe et al’s omegasomes. They then used electron tomography to zoom in for a 3D view of the ER-IM structure. This technique showed that a portion of the ER forms a cradle that surrounds the IM and the two structures appear connected. Elucidating the molecular basis of this interaction will be an important next step.
Hayashi-Nishino et al. (2009) Nat Cell Biol. 11, 1433 - 1437
Simonsen and Tooze (2009) J. Cell Biol. 186(6):773-782
Axe et al. (2009) J. Cell Biol. 182(4):685-701
Simonsen and Stenmark (2008) J. Cell Biol. 182(4):621-622