|The animal cell|
The endoplasmic reticulum (ER) is a type of nuclear envelope. Endoplasmic reticulum occurs in most types of eukaryotic cells, including the most primitive Giardia, but is absent from red blood cells and spermatozoa. There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis. The rough endoplasmic reticulum is especially prominent in cells such as hepatocytes whereas active smooth endoplasmic reticulum lacks ribosomes and functions in lipid metabolism, carbohydrate metabolism, and detoxification and is especially abundant in mammalian liver and gonad cells. The lacey membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter, Albert Claude, Brody Meskers and Ernest F. Fullam, using electron microscopy.
- Rough endoplasmic reticulum 1.1
Smooth endoplasmic reticulum 1.2
- Sarcoplasmic reticulum 1.2.1
- Protein transport 2.1
- See also 3
- References 4
- External links 5
The general structure of the endoplasmic reticulum is a network of membranes called cisternae. These sac-like structures are held together by the cytoskeleton. The phospholipid membrane encloses a space, the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol. The functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids, but varies between ER and cell type and cell function. The quantity of RER and SER in a cell can slowly interchange from one type to the other, depending on the changing metabolic activities of the cell. Transformation can include embedding of new proteins in membrane as well as structural changes. Changes in protein content may occur without noticeable structural changes.
Rough endoplasmic reticulum
The surface of the rough endoplasmic reticulum (often abbreviated RER or Rough ER) is studded with protein-manufacturing mRNA of a protein destined for the secretory pathway. The first 5-30 amino acids polymerized encode a signal peptide, a molecular message that is recognized and bound by a signal recognition particle (SRP). Translation pauses and the ribosome complex binds to the RER translocon where translation continues with the nascent protein forming into the RER lumen and/or membrane. The protein is processed in the ER lumen by an enzyme (a signal peptidase), which removes the signal peptide. Ribosomes at this point may be released back into the cytosol; however, non-translating ribosomes are also known to stay associated with translocons.
The membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, and continuous with, the outer layer of the
- Lipid and protein composition of Endoplasmic reticulum in OPM database
- Animations of the various cell functions referenced here
- Soltys, B.J., Falah, M.S. and Gupta, R.S. (1996) Identification of endoplasmic reticulum in the primitive eukaryote Giardia lamblia using cryoelectron microscopy and antibody to Bip. J. Cell Science 109: 1909-1917.
- Porter KR, Claude A, Fullam EF (March 1945). "A study of tissue culture cells by electron microscopy". J Exp Med. 81 (3): 233–246.
- Görlich D, Prehn S, Hartmann E, Kalies KU, Rapoport TA. (Oct 1992). "A mammalian homolog of SEC61p and SECYp is associated with ribosomes and nascent polypeptides during translocation.". Cell 71 (3): 489–503.
- Lodish, Harvey; et al. (2003). Molecular Cell Biology (5th ed.). W. H. Freeman. pp. 659–666.
- Seiser, R. M. (2000). "The Fate of Membrane-bound Ribosomes Following the Termination of Protein Synthesis". Journal of Biological Chemistry 275 (43): 33820–33827.
- Shibata, Yoko; Voeltz, Gia K.; Rapoport, Tom A. (2006). "Rough Sheets and Smooth Tubules".
- Endoplasmic reticulum. (n.d.). McGraw-Hill Encyclopedia of Science and Technology. Retrieved September 13, 2006, from Answers.com Web site: http://www.answers.com/topic/endoplasmic-reticulum
- Levine T (September 2004). "Short-range intracellular trafficking of small molecules across endoplasmic reticulum junctions". Trends Cell Biol. 14 (9): 483–90.
- Levine T, Loewen C (August 2006). "Inter-organelle membrane contact sites: through a glass, darkly". Curr. Opin. Cell Biol. 18 (4): 371–8.
- "Functions of Smooth ER". University of Minnesota Duluth.
- Maxfield FR, Wüstner D (October 2002). "Intracellular cholesterol transport". J. Clin. Invest. 110 (7): 891–8.
- Toyoshima C, Nakasako M, Nomura H, Ogawa H (2000). "Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution". Nature 405 (6787): 647–55.
- Medical Cell Biology 3rd/ed. Academic Press. p. 69.
- Martini, Frederick; Nath, Judi; Bartholomew, Edwin (2014). Fundamentals of Anatomy and Physiology (10th ed.).
- Xu, C; et al (2005). "Endoplasmic Reticulum Stress: Cell Life and Death Decisions". J. Clin. Invest 115 (10): 2656–2664.
- Kober L, Zehe C, Bode J (October 2012). "Development of a novel ER stress based selection system for the isolation of highly productive clones". Biotechnol. Bioeng. 109 (10): 2599–611.
- Soltys,B.J. and Gupta, R.S. (1992) Interrelationships of endoplasmic reticulum, mitochondria, intermediate filaments, and microtubules--a quadruple fluorescence labeling study.
The endoplasmic reticulum is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell. The majority of its resident proteins are retained within it through a retention motif. This motif is composed of four amino acids at the end of the protein sequence. The most common retention sequence is KDEL (lys-asp-glu-leu). However, variations of KDEL do occur, and other sequences can also give rise to endoplasmic reticulum retention. It is not known whether such variation can lead to sub-ER localizations. There are three KDEL receptors in mammalian cells, and they have a very high degree of sequence identity. The functional differences between these receptors remain to be established.
Secretory proteins, mostly glycoproteins, are moved across the endoplasmic reticulum membrane. Proteins that are transported by the endoplasmic reticulum throughout the cell are marked with an address tag called a signal sequence. The N-terminus (one end) of a polypeptide chain (i.e., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the endoplasmic reticulum are packed into transport vesicles and moved along the cytoskeleton toward their destination. In human fibroblasts, the ER is always co-distributed with microtubules and the depolymerisation of the latter cause its co-aggregation with mitochondria, which are also associated with the ER. 
The endoplasmic reticulum serves many general functions, including the folding of protein molecules in sacs called cisternae and the transport of synthesized proteins in vesicles to the Golgi apparatus. Correct folding of newly made proteins is made possible by several endoplasmic reticulum chaperone proteins, including protein disulfide isomerase (PDI), ERp29, the Hsp70 family member BiP/Grp78, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly folded proteins are transported from the rough ER to the Golgi apparatus. Disturbances in redox regulation, calcium regulation, glucose deprivation, and viral infection or the over-expression of proteins can lead to endoplasmic reticulum stress response (ER stress), a state in which the folding of proteins slows, leading to an increase in unfolded proteins. This stress is emerging as a potential cause of damage in hypoxia/ischemia, insulin resistance, and other disorders.
.excitation-contraction coupling It plays a major role in , ("flesh"), is smooth ER found in sarx The sarcoplasmic reticulum (SR), from the Greek
The smooth endoplasmic reticulum (abbreviated SER) has functions in several metabolic processes. It synthesizes lipids, phospholipids, and steroids. Cells which secrete these products, such as those in the testes, ovaries, and skin oil glands have a great amount of smooth endoplasmic reticulum. It also carries out the metabolism of carbohydrates, drug detoxification, attachment of receptors on cell membrane proteins, and steroid metabolism. In muscle cells, it regulates calcium ion concentration. Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant), and it serves different functions in each. The smooth endoplasmic reticulum also contains the enzyme glucose-6-phosphatase, which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. It is connected to the nuclear envelope and consists of tubules that are located near the cell periphery. These tubes sometimes branch forming a network that is reticular in appearance. In some cells, there are dilated areas like the sacs of rough endoplasmic reticulum. The network of smooth endoplasmic reticulum allows for an increased surface area to be devoted to the action or storage of key enzymes and the products of these enzymes.
Smooth endoplasmic reticulum
- Manufacture of lysosomal enzymes with a mannose-6-phosphate marker added in the cis-Golgi network
- Manufacture of secreted proteins, either secreted constitutively with no tag or secreted in a regulatory manner involving clathrin and paired basic amino acids in the signal peptide.
- Integral membrane proteins that stay embedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane; SNAP and SNARE proteins are key in the fusion event.
- Initial glycosylation as assembly continues. This is N-linked (O-linking occurs in the Golgi).
The rough endoplasmic reticulum is key in multiple functions: