A tangle of pockets, tubes, and sac-like membrane structures runs through the cells of humans, animals, plants, and fungi: the endoplasmic reticulum (ER). In the ER, proteins are manufactured, folded into their three-dimensional structure and modified, lipids and hormones are produced, and calcium concentrations in the cell are controlled. In addition, the ER forms the basis for the cellular transport system, feeds misfolded proteins to intracellular disposal and renders toxins that have entered the cell harmless. In view of its multiple tasks, the ER is constantly being remodeled. A process called ER-phagy (roughly “self-digestion of the ER”) is responsible for ER degradation. Involved is a group of signal-receiving proteins—receptors--that are responsible for the membrane curvatures of the ER and thus for its multiple forms in the cell. In ER-phagy, the receptors accumulate at specific sites on the ER and increase membrane curvature to such an extent that, as a consequence, part of the ER is strangulated and broken down into its component parts by cellular recycling structures (autophagosomes).
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