The origin of the autophagosomal membrane
Macroautophagy is initiated by the formation of the phagophore (also called the isolation membrane). This membrane can both selectively and non-selectively engulf cytosolic components, grow and close around the sequestered components and then deliver them to a degradative organelle, the lysosome. Where this membrane comes from and how it grows is not well understood. Since the discovery of autophagy in the 1950s the source of the membrane has been investigated, debated and re-investigated, with the consensus view oscillating between a de novo assembly mechanism or formation from the membranes of the endoplasmic reticulum (ER) or the Golgi. In recent months, new information has emerged that both the ER and mitochondria may provide a membrane source, enlightening some older findings and revealing how complex the initiation of autophagy may be in mammalian cells.