The origin and source of autophagosomal membranes are long-standing questions. By electron microscopy, we show that the endoplasmic reticulum (ER) associates with early autophagic structures called isolation membranes (IM) or phagophores in mammalian culture cells. Overexpression of a mutant of Atg4B, which causes defects in autophagosome formation, caused accumulation of ER-IM complexes. Electron tomography revealed the ER-IM complex as a subdomain of the ER forming a cradle encircling the IM, and showed that both ER and isolation membranes are interconnected.
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