Endoplasmic reticulum
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2008-07-16T23:19:28Z
TimVickers
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[[Image:Clara cell lung - TEM.jpg|thumb|315px|Micrograph of rough endoplasmic reticulum network around the [[Cell nucleus|nucleus]] (shown in lower right-hand side of the picture). Dark small circles in the network are [[mitochondria]].]]
The '''endoplasmic reticulum''', or '''ER''', is an [[organelle]] found in all [[eukaryote|eukaryotic cells]] that is an interconnected network of [[tubules]], [[vesicle (biology)|vesicles]] and [[cisterna]]e. The lacey membranes of the endoplasmic reticulum were first seen by [[Keith R. Porter]], [[Albert Claude]], and Ernest F. Fullam in 1945.<ref name="FirstER">{{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 }}</ref>
These structures are responsible for several specialized functions: [[Protein biosynthesis|protein translation]], folding and transport of proteins to be used in the [[cell membrane]] (e.g. [[transmembrane receptor]]s and other [[integral membrane protein]]s), or to be secreted ([[exocytosis|exocytosed]]) from the cell (e.g. digestive [[enzyme]]s); sequestration of [[calcium]]; and production and storage of [[glycogen]], [[steroid]]s, and other [[macromolecules]].<ref>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</ref> The endoplasmic reticulum is part of the [[endomembrane system]]. The basic structure and composition of the ER membrane is similar to the [[plasma membrane]].
== Structure ==
[[Image:nucleus ER golgi.jpg|thumb|365px|'''1''' [[Cell nucleus|Nucleus]]
'''2''' [[Nuclear pore]]
'''3''' Rough endoplasmic reticulum (rER)
'''4''' Smooth endoplasmic reticulum (sER)
'''5''' [[Ribosome]] on the rough ER
'''6''' [[Protein]]s that are transported
'''7''' Transport [[vesicle]]
'''8''' [[Golgi apparatus]]
'''9''' 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 [[cisterna]]e (sac-like structures) held together by the [[cytoskeleton]]. The [[phospholipid membrane]] encloses 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 [[ribosomes]] giving it a "rough" appearance (hence its name).<ref name="campbell">Campbell, Neil A. (1996) ''Biology Fourth Edition.'' Benjamin/Cummings Publishing, pp. 120-121 ISBN 0-8053-1940-9</ref> 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.<ref name="lodish">Lodish, Harvey, et al. (2003) ''Molecular Cell Biology 5th Edition''. W. H. Freeman, pp. 659-666 ISBN 0716743663</ref> The free ribosome begins producing the [[polypeptide]] until a cytosolic [[signal recognition particle]] recognizes the [[pre-piece]] of 5-15 [[hydrophobic]] [[amino acid|AA]]s 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.<ref>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</ref> [[COP II]] brings vesicles to the golgi and [[COP I]] brings the membrane back. The rough endoplasmic reticulum works in concert with the [[Golgi complex]] to [[protein targeting|target new proteins]] to their proper destinations.
The RER is key in producing
*[[lysosome|lysosomal]] enzymes with a [[Mannose-6-phosphate]] marker added in the cis-Golgi network
*[[Secretion|Secreted]] proteins, either secreted constitutively with no tag, or regulated secretion involving [[clathrin]] and paired basic amino acids in the [[signal peptide]].
*[[integral membrane proteins]] that stay imbedded in the membrane as vesicles exit and bind to new membranes. [[Rab (G-protein)|Rab]] proteins are key in targeting the membrane, [[SNAP-25|SNAP]] and [[SNARE]] proteins are key in the fusion event.
*initial [[glycosylation]] as 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, [[glycosyltransferase]] recognizes the AA sequence [[Asparagine|N]]X[[serine|S]] or [[Asparagine|N]]X[[threonine|T]] (with the S/T residue phosphorylated) and adds a 14 sugar backbone (2 ''N''-acetylglucosamine, 9 branching [[mannose]], and 3 [[glucose]] at the end) to the side chain [[nitrogen]] of Asn.
===Smooth endoplasmic reticulum===
The smooth endoplasmic reticulum has functions in several metabolic processes, including synthesis of lipids and steroids, metabolism of carbohydrates and calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and [[steroid metabolism]].<ref>{{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}}</ref> 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-phosphatase]] which converts [[glucose-6-phosphate]] to 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 [[calcium]] ions in [[muscle cell]]s.
===Sarcoplasmic reticulum===
The ''sarcoplasmic reticulum'' is a special type of smooth ER found in [[smooth muscle|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.<ref name="sarcoplasmic">{{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}}</ref> This has the effect of triggering [[muscle contraction]].
==Functions==
The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called [[cisternae]].
Correct folding of newly-made proteins is made possible by several endoplasmic reticulum [[Chaperone (protein)|chaperone]] proteins, including [[protein disulfide isomerase]] (PDI), ERp29, the [[Hsp70]] family 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 peptide|signal sequence]]. The N-terminus (one end) of a [[polypeptide]] chain (i.e., a protein) contains a few [[amino acid]]s 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 [[vesicle (biology)|vesicle]]s and moved along the [[cytoskeleton]] toward 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 [[Amino acid#Table of standard amino acid abbreviations and basic properties|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.
===Other functions===
*'''Insertion of proteins into the endoplasmic reticulum membrane''': [[Integral protein]]s must be inserted into the endoplasmic reticulum membrane after they are synthesized. Insertion into the endoplasmic reticulum membrane requires the correct [[topogenic sequence]]s.
*'''Glycosylation''': [[Glycosylation]] involves the attachment of [[oligosaccharide]]s.
*'''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 [[cytochrome P450]] enzymes.
==See also==
* [[ERAD]]
* [[Protein targeting]]
* [[Secretory pathway]]
==References==
<references/>
==External links==
* {{UMichOPM|localization|localization|Endoplasmic%20reticulum%20membrane}}
* [http://multimedia.mcb.harvard.edu/media.html Animations of the various cell functions referenced here]
{{organelles}}
[[Category:Organelles]]
[[ar:شبكة إندوبلازمية]]
[[bn:এন্ডোপ্লাজমিক রেটিকুলাম]]
[[bs:Endoplazmatski retikulum]]
[[bg:Ендоплазмен ретикулум]]
[[ca:Reticle endoplasmàtic]]
[[cs:Endoplazmatické retikulum]]
[[cy:Reticwlwm endoplasmig]]
[[da:Endoplasmatisk reticulum]]
[[de:Endoplasmatisches Retikulum]]
[[es:Retículo endoplasmático]]
[[fr:Réticulum endoplasmique]]
[[gl:Retículo endoplasmático]]
[[ko:소포체]]
[[hr:Endoplazmatski retikulum]]
[[is:Frymisnet]]
[[it:Reticolo endoplasmatico]]
[[he:רשתית תוך-פלזמית]]
[[lv:Endoplazmatiskais tīkls]]
[[lb:Endoplasmatescht Retikulum]]
[[lt:Endoplazminis tinklas]]
[[mk:Ендоплазматичен ретикулум]]
[[ms:Jalinan endoplasma]]
[[nl:Endoplasmatisch reticulum]]
[[ja:小胞体]]
[[no:Endoplasmatisk retikulum]]
[[oc:Reticulum endoplasmic]]
[[pl:Retikulum endoplazmatyczne]]
[[pt:Retículo endoplasmático]]
[[ru:Эндоплазматический ретикулум]]
[[sk:Endoplazmatické retikulum]]
[[sl:Endoplazemski retikulum]]
[[sr:Храпави ендоплазматични ретикулум]]
[[fi:Endoplasmakalvosto]]
[[sv:Endoplasmatiska nätverket]]
[[th:เอนโดพลาสมิกเรติคูลัม]]
[[vi:Mạng lưới nội chất]]
[[tr:Endoplazmik retikulum]]
[[uk:Ендоплазматичний ретикулум]]
[[zh:内质网]]