4.7:
Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is an interconnected series of membranous sacs and tubules that is continuous with the outer nuclear membrane. One portion of the ER, the rough ER, gets its name from the ribosomes that are attached to it. When viewed microscopically, the ribosomes give this portion of ER a rough appearance.
Once bound, the ribosomes release newly translated proteins into the lumen, a space enclosed by the ER membrane, where proteins undergo structural modifications, such as folding into their tertiary structures. The proteins are also inspected for proper translation and folding by chaperone proteins. If they pass this quality control, they're packaged into vesicles and released towards the Golgi apparatus.
However, if they fail, they're degraded in the cytosol and the amino acids are recycled. The other portion of the ER is called the smooth ER, as it lacks bound ribosomes. This region synthesizes carbohydrates and lipids, which are also packaged for delivery to the Golgi apparatus.
In most eukaryotic cells, the smooth ER also stores calcium ions for use as second messengers. For example, in muscle cells called myocytes, where the smooth ER is known as sarcoplasmic reticulum, the stored calcium ions are used to contract muscles.
4.7:
Endoplasmic Reticulum
The Endoplasmic Reticulum (ER) in eukaryotic cells is a substantial network of interconnected membranes with diverse functions, from calcium storage to biomolecule synthesis. A primary component of the endomembrane system, the ER manufactures phospholipids critical for membrane function throughout the cell. Additionally, the two distinct regions of the ER specialize in the manufacture of specific lipids and proteins.
The rough ER is characterized by the presence of microscopically-visible ribosomes on its surface. As a ribosome begins translation of an mRNA in the cytosol, the presence of a signal sequence directs the ribosome to the surface of the rough ER. A receptor in the membrane of the ER recognizes this sequence and facilitates the entry of the growing polypeptide into the ER lumen through a transmembrane protein complex. With the assistance of chaperones, nascent proteins fold and undergo other functional modifications, including glycosylation, disulfide bond formation, and oligomerization. Properly folded and modified proteins are then packaged into vesicles to be shipped to the Golgi apparatus and other locations in the cell. Chaperones identify improperly folded proteins and facilitate degradation in the cytosol by proteasomes.
Lacking ribosomes, the smooth ER is the cellular location of lipid and steroid synthesis, cellular detoxification, carbohydrate metabolism and storage of calcium ions. Cells that specialize in the secretion of hormones tend to be abundant in smooth ER. Likewise, the detoxifying cells of the liver are rich with smooth ER. Smooth ER is also the cellular storage site of otherwise toxic calcium ions; this stored calcium can then be rapidly released as a signaling molecule, stimulating cellular functions including muscle cell contraction and vesicular release. The storage and rapid reuptake of calcium ions in the ER are facilitated by resident calcium-binding proteins.
Suggested Reading
Mancias, Joseph D., and Jonathan Goldberg. "Exiting the endoplasmic reticulum." Traffic 6, no. 4 (2005): 278-285. [Source]
Nagai, Kiyoshi, Chris Oubridge, Andreas Kuglstatter, Elena Menichelli, Catherine Isel, and Luca Jovine. "Structure, function and evolution of the signal recognition particle." The EMBO Journal 22, no. 14 (2003): 3479-3485. [Source]
Stathopulos, Peter B., Min-duk Seo, Masahiro Enomoto, Fernando J. Amador, Noboru Ishiyama, and Mitsuhiko Ikura. "Themes and variations in ER/SR calcium release channels: structure and function." Physiology 27, no. 6 (2012): 331-342. [Source]