![Clara cell lung - TEM](https://upload.wikimedia.org/wikipedia/commons/c/c2/Clara_cell_lung_-_TEM.jpg)
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most
eukaryotic cells
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
and forms an interconnected network of flattened, membrane-enclosed sacs known as
cisterna
A cisterna (plural cisternae) is a flattened membrane vesicle found in the endoplasmic reticulum and Golgi apparatus. Cisternae are an integral part of the packaging and modification processes of proteins occurring in the Golgi.
Function
Protein ...
e (in the RER), and tubular structures in the SER. The membranes of the ER are continuous with the outer
nuclear membrane
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membrane ...
. The endoplasmic reticulum is not found in
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s, or
spermatozoa
A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, ...
.
The two types of ER share many of the same
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s and engage in certain common activities such as the synthesis of certain
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
s and
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
. Different types of
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
contain different ratios of the two types of ER depending on the activities of the cell. RER is found mainly toward the nucleus of cell and SER towards the cell membrane or plasma membrane of cell.
The outer (
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
ic) face of the RER is studded with
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
s that are the sites of
protein synthesis
Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside Cell (biology), cells, homeostasis, balancing the loss of cellular proteins (via Proteolysis, degradation or Protein targeting, export) through the product ...
. The RER is especially prominent in cells such as
hepatocyte
A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass.
These cells are involved in:
* Protein synthesis
* Protein storage
* Transformation of carbohydrates
* Synthesis of cholesterol, ...
s. The SER lacks ribosomes and functions in
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
synthesis but not
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
, the production of
steroid hormone
A steroid hormone is a steroid that acts as a hormone. Steroid hormones can be grouped into two classes: corticosteroids (typically made in the adrenal cortex, hence ''cortico-'') and sex steroids (typically made in the gonads or placenta). Wi ...
s, and
detoxification
Detoxification or detoxication (detox for short) is the physiological or medicinal removal of toxic substances from a living organism, including the human body, which is mainly carried out by the liver. Additionally, it can refer to the period of ...
.
The SER is especially abundant in mammalian
liver
The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
and
gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
cells.
The ER was observed with
light microscope
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...
by Garnier in 1897, who coined the term ''ergastoplasm''.
With
electron microscopy
An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...
, the lacy membranes of the endoplasmic reticulum were first seen in 1945 by
Keith R. Porter
Keith Roberts Porter (June 11, 1912 – May 2, 1997) was a Canadian-American cell biologist. He created pioneering biology techniques and research using electron microscopy of cells. Porter also contributed to the development of other experiment ...
,
Albert Claude
Albert Claude (; 24 August 1899 – 22 May 1983) was a Belgian- American cell biologist and medical doctor who shared the Nobel Prize in Physiology or Medicine in 1974 with Christian de Duve and George Emil Palade. His elementary education star ...
, and Ernest F. Fullam.
Later, the word ''
reticulum
Reticulum is a small, faint constellation in the southern sky. Its name is Latin for a small net, or reticle—a net of crosshairs at the focus of a telescope eyepiece that is used to measure star positions. The constellation is best viewed b ...
'', which means "network", was applied by Porter in 1953 to describe this fabric of membranes.
Structure
![Nucleus ER golgi](https://upload.wikimedia.org/wikipedia/commons/f/f6/Nucleus_ER_golgi.svg)
The general structure of the endoplasmic reticulum is a network of membranes called
cisterna
A cisterna (plural cisternae) is a flattened membrane vesicle found in the endoplasmic reticulum and Golgi apparatus. Cisternae are an integral part of the packaging and modification processes of proteins occurring in the Golgi.
Function
Protein ...
e. These sac-like structures are held together by the
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
. The
phospholipid membrane
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
encloses the cisternal space (or lumen), which is continuous with the
perinuclear space
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membra ...
but separate from the
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
. 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 both rough and smooth endoplasmic reticulum 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
![Protein translation](https://upload.wikimedia.org/wikipedia/commons/9/94/Protein_translation.gif)
The surface of the rough endoplasmic reticulum (often abbreviated ''RER'' or ''rough ER''; also called ''granular endoplasmic reticulum'') is studded with protein-manufacturing
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
s giving it a "rough" appearance (hence its name). The binding site of the ribosome on the rough endoplasmic reticulum is the
translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transport ...
. However, the ribosomes are not a stable part of this organelle's structure as they are constantly being bound and released from the membrane. A ribosome only binds to the RER once a specific protein-nucleic acid complex forms in the cytosol. This special complex forms when a free ribosome begins
translating
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transl ...
the
mRNA
In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein.
mRNA is ...
of a protein destined for the
secretory pathway 440px
Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...
.
The first 5–30
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s polymerized encode a
signal peptide
A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16-30 amino acids long) present at the N-ter ...
, a molecular message that is recognized and bound by a
signal recognition particle
The signal recognition particle (SRP) is an abundant, cytosolic, universally conserved ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane ...
(SRP). Translation pauses and the ribosome complex binds to the RER
translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transport ...
where translation continues with the
nascent
Nascent may refer to:
* '' Nascent'', a 2005 Australian dance film with choreography by Garry Stewart
* '' Nascent (film)'', a 2016 Central African short documentary film by Lindsay Branham and Jon Kasbe
See also
*
*
* Nascent hydrogen, disc ...
(new) protein forming into the RER lumen and/or membrane. The protein is processed in the ER lumen by an enzyme (a signal
peptidase
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the fo ...
), 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
nuclear envelope
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membrane ...
.
The double membrane sheets are stacked and connected through several right- or left-handed helical ramps, the "Terasaki ramps", giving rise to a structure resembling a
parking garage
A multistorey car park (British and Singapore English) or parking garage (American English), also called a multistory, parking building, parking structure, parkade (mainly Canadian), parking ramp, parking deck or indoor parking, is a build ...
.
Although there is no continuous membrane between the endoplasmic reticulum and the
Golgi apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...
, membrane-bound
transport vesicles
In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion ( exocytosis), uptake ( endocytosis) and transport of ma ...
shuttle proteins between these two compartments. Vesicles are surrounded by
coating proteins called COPI and COPII.
COPII
The Coat Protein Complex II, or COPII, is a group of proteins that facilitate the formation of vesicles to transport proteins from the endoplasmic reticulum to the Golgi apparatus or endoplasmic-reticulum–Golgi intermediate compartment. This ...
targets vesicles to the Golgi apparatus and
COPI
COPI is a coatomer, a protein complex that coats vesicles transporting proteins from the ''cis'' end of the Golgi complex
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most ...
marks them to be brought back to the rough endoplasmic reticulum. The rough endoplasmic reticulum works in concert with the
Golgi complex
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles insi ...
to
target new proteins to their proper destinations. The second method of transport out of the endoplasmic reticulum involves areas called
membrane contact site Membrane contact sites (MCS) are close appositions between two organelles. Ultrastructural studies typically reveal an intermembrane distance in the order of the size of a single protein, as small as 10 nm or wider, with no clear upper limit. ...
s, where the membranes of the endoplasmic reticulum and other organelles are held closely together, allowing the transfer of lipids and other small molecules.
The rough endoplasmic reticulum is key in multiple functions:
* Manufacture of
lysosomal
A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...
enzymes with a
mannose-6-phosphate
Mannose-6-phosphate (M6P) is a molecule bound by lectin in the immune system. M6P is converted to fructose 6-phosphate by mannose phosphate isomerase.
M6P is a key targeting signal for acid hydrolase precursor proteins that are destined for transp ...
marker added in the ''cis''-Golgi network.
* Manufacture of
secreted 440px
Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...
proteins, either secreted constitutively with no tag or secreted in a regulatory manner involving
clathrin
Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. Whe ...
and paired basic amino acids in the
signal peptide
A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16-30 amino acids long) present at the N-ter ...
.
*
Integral membrane proteins
An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a signi ...
that stay embedded in the membrane as vesicles exit and bind to new membranes.
Rab
Rab âːb( dlm, Arba, la, Arba, it, Arbe, german: Arbey) is an island in the northern Dalmatia region in Croatia, located just off the northern Croatian coast in the Adriatic Sea.
The island is long, has an area of and 9,328 inhabitants (2 ...
proteins are key in targeting the membrane;
SNAP
Snap or SNAP may refer to:
Arts and entertainment
* ''Snap'', the original release title for the 2013 film ''Enter the Dangerous Mind''
*''Snap'' (TV series), a CITV programme
* ''The Stanly News and Press'', a newspaper in Albemarle, North Carol ...
and
SNARE
SNARE proteins – " SNAP REceptor" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells,
and some numbers in plants. The primary role of SNARE proteins is to mediate vesicle fu ...
proteins are key in the fusion event.
* Initial
glycosylation
Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not al ...
as assembly continues. This is N-linked (O-linking occurs in the Golgi).
** N-linked glycosylation: If the protein is properly folded,
oligosaccharyltransferase
Oligosaccharyltransferase or OST () is a membrane protein protein complex, complex that transfers a 14-sugar oligosaccharide from dolichol to nascent protein. It is a type of glycosyltransferase. The sugar Glc3Man9GlcNAc2 (where Glc=Glucose, Man= ...
recognizes the AA sequence
NX
S or
NX
T (with the S/T residue phosphorylated) and adds a 14-sugar backbone (2-''N''-acetylglucosamine, 9-branching
mannose
Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...
, and 3-
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
at the end) to the side-chain
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
of Asn.
Smooth endoplasmic reticulum
![0313 Endoplasmic Reticulum c labeled](https://upload.wikimedia.org/wikipedia/commons/a/a9/0313_Endoplasmic_Reticulum_c_labeled.png)
In most cells the smooth endoplasmic reticulum (abbreviated SER) is scarce. Instead there are areas where the ER is partly smooth and partly rough, this area is called the transitional ER. The transitional ER gets its name because it contains ER exit sites. These are areas where the transport vesicles that contain lipids and proteins made in the ER, detach from the ER and start moving to the
Golgi apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...
. Specialized cells can have a lot of smooth endoplasmic reticulum and in these cells the smooth ER has many functions.
It synthesizes
lipids
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
,
phospholipids
Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
, and
steroids
A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter membrane fluidity; and a ...
. Cells which secrete these products, such as those in the
testes
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...
,
ovaries
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
, and
sebaceous gland
A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest number ...
s have an abundance of smooth endoplasmic reticulum. It also carries out the metabolism of carbohydrates, detoxification of natural metabolism products and of alcohol and drugs, attachment of receptors on cell membrane proteins, and
steroid metabolism. In muscle cells, it regulates
calcium ion
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
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
The enzyme glucose 6-phosphatase (EC 3.1.3.9, G6Pase; systematic name D-glucose-6-phosphate phosphohydrolase) catalyzes the hydrolysis of glucose 6-phosphate, resulting in the creation of a phosphate group and free glucose:
: D-glucose 6-phos ...
, which converts
glucose-6-phosphate
Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose sugar phosphorylated at the hydroxy group on carbon 6. This dianion is very common in cells as the majority of glucose entering a cell will become phosphorylated in this wa ...
to glucose, a step in
gluconeogenesis
Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...
. It is connected to the
nuclear envelope
The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material.
The nuclear envelope consists of two lipid bilayer membrane ...
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.
Sarcoplasmic reticulum
![Blausen 0801 SkeletalMuscle](https://upload.wikimedia.org/wikipedia/commons/6/6f/Blausen_0801_SkeletalMuscle.png)
The sarcoplasmic reticulum (SR), from the Greek σάρξ ''sarx'' ("flesh"), is smooth ER found in
muscle cell
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...
s. The only structural difference between this organelle and the smooth endoplasmic reticulum is the composition of proteins they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: The endoplasmic reticulum synthesizes molecules, while the sarcoplasmic reticulum stores calcium ions and pumps them out into the sarcoplasm when the muscle fiber is stimulated.
After their release from the sarcoplasmic reticulum, calcium ions interact with contractile proteins that utilize ATP to shorten the muscle fiber. The sarcoplasmic reticulum plays a major role in
excitation-contraction coupling
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as w ...
.
Functions
The endoplasmic reticulum serves many general functions, including the folding of protein molecules in sacs called
cisterna
A cisterna (plural cisternae) is a flattened membrane vesicle found in the endoplasmic reticulum and Golgi apparatus. Cisternae are an integral part of the packaging and modification processes of proteins occurring in the Golgi.
Function
Protein ...
e and the transport of synthesized proteins in
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
to the
Golgi apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...
. Rough endoplasmic reticulum is also involved in protein synthesis. Correct folding of newly made proteins is made possible by several endoplasmic reticulum
chaperone proteins, including
protein disulfide isomerase
Protein disulfide isomerase (), or PDI, is an enzyme in the endoplasmic reticulum (ER) in eukaryotes and the periplasm of bacteria that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as the ...
(PDI), ERp29, the
Hsp70
The 70 kilodalton heat shock proteins (Hsp70s or DnaK) are a family of conserved ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms. Intracellularly localized Hsp70s are an importa ...
family member
BiP/Grp78,
calnexin
Calnexin (CNX) is 67kDaintegral protein (that appears variously as a 90kDa, 80kDa, or 75kDa band on western blotting depending on the source of the antibody) of the endoplasmic reticulum (ER). It consists of a large (50 kDa) N-terminal calcium- bi ...
,
calreticulin
Calreticulin also known as calregulin, CRP55, CaBP3, calsequestrin-like protein, and endoplasmic reticulum resident protein 60 (ERp60) is a protein that in humans is encoded by the ''CALR'' gene.
Calreticulin is a multifunctional soluble prote ...
, and the peptidylprolyl isomerase family. Only properly folded proteins are transported from the rough ER to the Golgi apparatus – unfolded proteins cause an
unfolded protein response
The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum (ER) stress. It has been found to be conserved between all mammalian species, as well as yeast and worm organisms.
The UPR is activated in resp ...
as a stress response in the ER. Disturbances in
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
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.
Protein transport
Secretory proteins, mostly
glycoproteins
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
, 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
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
chain (i.e., a protein) contains a few
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s that work as an address tag, which are removed when the polypeptide reaches its destination. Nascent peptides reach the ER via the
translocon The translocon (also known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transport ...
, a membrane-embedded multiprotein complex. Proteins that are destined for places outside the endoplasmic reticulum are packed into transport
vesicle
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
s and moved along the
cytoskeleton
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
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 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 sequences are
KDEL for lumen located proteins and
KKXX for transmembrane protein. However, variations of KDEL and KKXX 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 (
1,
2 and
3) receptors in mammalian cells, and they have a very high degree of sequence identity. The functional differences between these receptors remain to be established.
Bioenergetics regulation of ER ATP supply by a CaATiER mechanism
![CaATiER model 081219](https://upload.wikimedia.org/wikipedia/commons/5/52/CaATiER_model_081219.jpg)
The endoplasmic reticulum does not harbor an ATP-regeneration machinery, and therefore requires ATP import from mitochondria. The imported ATP is vital for the ER to carry out its house keeping cellular functions, such as for protein folding and trafficking.
The ER ATP transporter, SLC35B1/AXER, was recently cloned and characterized, and the mitochondria supply ATP to the ER through a ''Ca
2+-antagonized transport into the ER'' (''CaATiER'') mechanism. The ''CaATiER'' mechanism shows sensitivity to cytosolic Ca
2+ ranging from high nM to low μM range, with the Ca
2+-sensing element yet to be identified and validated.
Clinical significance
Increased and supraphysiological ER stress in pancreatic β cells disrupts normal insulin secretion, leading to hyperinsulinemia and consequently peripheral insulin resistance associated with obesity in humans. Human clinical trials also suggested a causal link between obesity-induced increase in insulin secretion and peripheral insulin resistance.
Abnormalities in
XBP1
X-box binding protein 1, also known as XBP1, is a protein which in humans is encoded by the ''XBP1'' gene. The ''XBP1'' gene is located on chromosome 22 while a closely related pseudogene has been identified and localized to chromosome 5. The XBP ...
lead to a heightened
endoplasmic reticulum stress response and subsequently causes a higher susceptibility for inflammatory processes that may even contribute to
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
.
In the
colon, XBP1 anomalies have been linked to the inflammatory bowel diseases including
Crohn's disease
Crohn's disease is a type of inflammatory bowel disease (IBD) that may affect any segment of the gastrointestinal tract. Symptoms often include abdominal pain, diarrhea (which may be bloody if inflammation is severe), fever, abdominal distension ...
.
The
unfolded protein response
The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum (ER) stress. It has been found to be conserved between all mammalian species, as well as yeast and worm organisms.
The UPR is activated in resp ...
(UPR) is a
cellular stress response Cellular stress response is the wide range of molecular changes that cells undergo in response to environmental stressors, including extremes of temperature, exposure to toxins, and mechanical damage. Cellular stress responses can also be caused b ...
related to the endoplasmic reticulum. The UPR is activated in response to an accumulation of unfolded or misfolded
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s in the
lumen of the endoplasmic reticulum. The UPR functions to restore normal function of the cell by halting protein
translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
, degrading misfolded proteins, and activating the signaling pathways that lead to increasing the production of molecular
chaperones involved in
protein folding
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduci ...
. Sustained overactivation of the UPR has been implicated in
prion
Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...
diseases as well as several other
neurodegenerative diseases
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
and the inhibition of the UPR could become a treatment for those diseases.
References
External links
Endoplasmic ReticulumLipid and protein composition of Endoplasmic reticulumin
OPM database
Orientations of Proteins in Membranes (OPM) database provides spatial positions of membrane protein protein structure, structures with respect to the lipid bilayer. Positions of the proteins are calculated using an Implicit solvation, implicit sol ...
Animations of the various cell functions referenced here
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