The Endoplasmic Reticulum

In keeping with its name, the endoplasmic reticulum (ER) is a large, continuous, and dynamic organelle that extends throughout the whole cell. The morphology of this network varies depending on the cell type and on functional demands. Synthesis of most membrane and secreted proteins, as well as lipids, occurs at the ER. The ER is the first station along the secretory pathway, where proteins fold and assemble with assistance from ER-resident chaperones, before being dispatched further along the endomembrane system. The ER moreover provides the major Ca2+ storage in the cell, and serves as a hub for signal integration. Distinct subregions of the ER have specialized tasks. In particular, contact sites with other organelles, such as mitochondria, are fundamental for signaling and lipid transfer. Through the unfolded protein response, the size and functional capacity of the ER are adapted in order to meet physiological and developmental needs.