- Endoplasmic reticulum
The endoplasmic reticulum (Greek endo = "within" (prefix), plásma = "formed entity", Latin reticulum = "little net"), or ER, is an
organellefound in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945.cite journal | author=Porter KR, Claude A, Fullam EF | date=1945 Mar | title=A study of tissue culture cells by electron microscopy | journal=J Exp Med. | volume=81 | pages=233–246 | doi=10.1084/jem.81.3.233 ]
These structures are responsible for several specialized functions: protein translation, folding and transport of proteins to be used in the
cell membrane(e.g. transmembrane receptors and other integral membrane proteins), or to be secreted (exocytosed) from the cell (e.g. digestive enzymes); sequestration of calcium; and production and storage of glycogen, steroids, and other macromolecules. [Spurger, L. (2002). "Endoplasmic reticulum: Structure and function". University of Texas Medical Branch. Retrieved September 13, 2006, from http://cellbio.utmb.edu/cellbio/rer1.htm] The endoplasmic reticulum is part of the endomembrane system. The basic structure and composition of the ER membrane is similar to the plasma membrane.
Nuclear pore3 Rough endoplasmic reticulum (RER) 4 Smooth endoplasmic reticulum (SER) 5 Ribosomeon the rough ER 6 Proteins that are transported 7 Transport vesicle8 Golgi apparatus9 Cis face of the Golgi apparatus 10 Trans face of the Golgi apparatus 11 Cisternae of the Golgi apparatus]
The general structure of the endoplasmic reticulum is an extensive membrane network of
cisternae (sac-like structures) held together by the cytoskeleton. The phospholipid membraneencloses a space, the cisternal space (or lumen), from the cytosol. The functions of the endoplasmic reticulum vary greatly depending on the exact type of endoplasmic reticulum and the type of cell in which it resides. The three varieties are called " rough endoplasmic reticulum", " smooth endoplasmic reticulum", and " sarcoplasmic reticulum".
The quantity of RER and SER in a cell can quickly interchange from one type to the other, depending on changing metabolic needs: one type will undergo numerous changes including new proteins embedded in the membranes in order to transform. Also, massive changes in the protein content can occur without any noticeable structural changes, depending on the enzymatic needs of the cell (as per the functions listed below).
Rough endoplasmic reticulum
The surface of the rough endoplasmic reticulum is studded with protein-manufacturing
ribosomesgiving it a "rough" appearance (hence its name).Campbell, Neil A. (1996) "Biology Fourth Edition." Benjamin/Cummings Publishing, pp. 120-121 ISBN 0-8053-1940-9] But it should be noted that these ribosomes are not resident of the endoplasmic reticulum initially. The ribosomes only bind to the ER once it begins to synthesize a protein destined for sorting.Lodish, Harvey, et al. (2003) "Molecular Cell Biology 5th Edition". W. H. Freeman, pp. 659-666 ISBN 0716743663] The free ribosome begins producing the polypeptideuntil a cytosolic signal recognition particlerecognizes the pre-pieceof 5-15 hydrophobic amino acids preceded by a positively charged (basic) amino acid. This makes it easy for the complex to loop the sequence through the hydrophobic membrane. The pre-piece is then cleaved off.
The membrane of the rough endoplasmic reticulum is continuous with the outer layer of the
nuclear envelope. Although there is no continuous membrane between the rough ER and the Golgi apparatus, membrane bound vesicles shuttle proteins between these two compartments. [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] COP IIbrings vesicles to the golgi and COP Ibrings the membrane back. The rough endoplasmic reticulum works in concert with the Golgi complexto target new proteins to their proper destinations.
The RER is key in producing
*lysosomal enzymes with a
Mannose-6-phosphatemarker added in the cis-Golgi network
*Secreted proteins, either secreted constitutively with no tag, or regulated secretion involving
clathrinand paired basic amino acids in the signal peptide.
integral membrane proteinsthat stay imbedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane, SNAP and SNAREproteins are key in the fusion event.
glycosylationas assembly continues. This is either N-linked or O-linked (O-linked may likely occur in the golgi).
**N-linked glycosylation: if the protein is properly folded,
glycosyltransferaserecognizes the AA sequence NXS or NXT (with the S/T residue phosphorylated) and adds a 14 sugar backbone (2 "N"-acetylglucosamine, 9 branching mannose, and 3 glucoseat the end) to the side chain nitrogenof Asn.
mooth endoplasmic reticulum
The smooth endoplasmic reticulum has functions in several metabolic processes, including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and
steroid metabolism. [cite journal |author=Maxfield FR, Wüstner D |title=Intracellular cholesterol transport |journal=J. Clin. Invest. |volume=110 |issue=7 |pages=891–8 |year=2002 |month=October |pmid=12370264 |pmc=151159 |doi=10.1172/JCI16500 |url=http://dx.doi.org/10.1172/JCI16500] It is connected to the nuclear envelope. Smooth endoplasmic reticulum is found in a variety of cell types (both animal and plant) and it serves different functions in each. The Smooth ER also contains the enzyme Glucose-6-phosphatasewhich converts glucose-6-phosphateto glucose, a step in gluconeogenesis. The Smooth ER consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of rough endoplasmic reticulum. The network of smooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and the products of these enzymes. The smooth endoplasmic reticulum is known for its storage of calciumions in muscle cells.
The "sarcoplasmic reticulum" (Greek sarx = "flesh") is a special type of smooth ER found in smooth and
striated muscle. The only structural difference between this organelle and the smooth endoplasmic reticulum is the medley of protein they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: the smooth ER synthesizes molecules and the sarcoplasmic reticulum stores and pumps calcium ions. The sarcoplasmic reticulum contains large stores of calcium, which it sequesters and then releases when the cell is depolarized.cite journal |author=Toyoshima C, Nakasako M, Nomura H, Ogawa H |title=Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 A resolution |journal=Nature |volume=405 |issue=6787 |pages=647–55 |year=2000 |pmid=10864315 |doi=10.1038/35015017] This has the effect of triggering muscle contraction.
The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called
Correct folding of newly-made proteins is made possible by several endoplasmic reticulum chaperone proteins, including
protein disulfide isomerase(PDI), ERp29, the Hsp70family member Grp78, calnexin, calreticulin, and the peptidylpropyl isomerase family.Only properly-folded proteins are transported from the rough ER to the Golgi complex.
Transport of proteins
Secretory proteins, mostly
glycoproteins, are moved across the endoplasmic reticulum membrane. Proteins that are transported by the endoplasmic reticulum and from there throughout the cell are marked with an address tag called a signal sequence. The N-terminus (one end) of a polypeptidechain (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 cytoskeletontoward their destination.
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 endoplasmic reticulum resident proteins are retained in the endoplasmic reticulum 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, variation on KDEL does occur and other sequences can also give rise to endoplasmic reticulum retention. It is not known if such variation can lead to sub-endoplasmic reticulum 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.
*Insertion of proteins into the endoplasmic reticulum membrane:
Integral proteins must be inserted into the endoplasmic reticulum membrane after they are synthesized. Insertion into the endoplasmic reticulum membrane requires the correct topogenic sequences.
Glycosylationinvolves the attachment of oligosaccharides.
*Disulfide bond formation and rearrangement: Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.
*Drug Metabolism: The smooth ER is the site at which some drugs are modified by microsomal enzymes which include the
* [http://multimedia.mcb.harvard.edu/media.html Animations of the various cell functions referenced here]
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