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Information about Endoplasmic Reticulum

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The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is 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.[1] 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

Enlarge picture
1 Nucleus     2 Nuclear pore     3 Rough endoplasmic reticulum (rER)     4 Smooth endoplasmic reticulum (sER)     5 Ribosome on the rough ER     6 Proteins 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 cisternae (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.

Rough endoplasmic reticulum

The surface of the rough endoplasmic reticulum is studded with protein-manufacturing ribosomes giving it a "rough" appearance (hence its name).[2] 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. [3] The free ribosome begins producing the polypeptide until a cytosolic signal recognition particle recognizes the pre-piece of 5-15 hydrophobic AAs 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.[4] 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 target new proteins to their proper destinations.

The RER is key in producing
  • lysosomal enzymes with a Mannose-6-phosphate marker added in the cis-golgi network
  • secretory proteins, constituitively secreted with no tag or regulated secretion involving clathrin and paired basic amino acids in the pro-piece.
  • integral membrane proteins that stay imbedded 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 either N-linked or O-linked (O-linked may likely occur in the golgi).
  • N-linked glycosylation: if the protein is properly folded, glycosyl transferase recognizes 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 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, metabolism of carbohydrates and calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins. 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 cells.

Sarcoplasmic reticulum

The sarcoplasmic reticulum 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 depolarised.[5] 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 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 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. 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.

Other functions

  • 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.
  • Glycosylation: Glycosylation involves the attachment of oligosaccharides.
  • Disulfide bond formation and rearrangement: Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.
  • Drug Detoxification: The smooth ER is the site at which some drugs are detoxified.

History

The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945.[6]

See also

References

1. ^ Spurger, L. (2002). Endoplasmic reticulum: Structure and function. University of Texas Medical Branch. Retrieved September 13, 2006, from [1]
2. ^ Campbell, Neil A. (1996) Biology Fourth Edition. Benjamin/Cummings Publishing, pp. 120-121 ISBN 0-8053-1940-9
3. ^ Lodish, Harvey, et al. (2003) Molecular Cell Biology 5th Edition. W. H. Freeman, pp. 659-666 ISBN 0716743663
4. ^ Endoplasmic reticulum. (n.d.). McGraw-Hill Encyclopedia of Science and Technology. Retrieved September 13, 2006, from Answers.com Web site: [2]
5. ^ 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. PMID 10864315. 
6. ^ KR. Porter, A. Claude, and EF. Fullam. (March, 1945) A study of tissue culture cells by electron microscopy. J Exp Med 81:233-246. Retrieved on Jan 06, 2007. ([3]

External links

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Organelles of the cell
Acrosome - Cell wall - Cell membrane - Chloroplast - Cilium/Flagellum - Centrosome - Cytoplasm - Endoplasmic reticulum - Endosome - Golgi apparatus - Lysosome - Melanosome - Mitochondrion - Myofibril - Nucleus - Nucleolus - Parenthesome - Peroxisome - Plastid - Ribosome - Vacuole - Vesicle
In cell biology, an organelle is a specialized subunit within a cell, having a specific function, and separately enclosed within its own lipid membrane.

The name organelle
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A tubule is a very small tube or fistular structure.

In anatomy, a tubule is a minute tube lined with glandular epithelium.
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vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. If there is only one lipid bilayer, they are called unilamellar vesicles; otherwise they are called multilamellar.
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For the town in Lazio, see Cisterna di Latina.


A cisterna (plural cisternae) comprises a flattened membrane disk which makes up the Golgi apparatus.
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Protein synthesis is the creation of proteins using DNA and RNA. Biological and artificial methods for creation of proteins differ significantly.
  • For biological protein synthesis, see protein biosynthesis.
  • For artificial protein synthesis, see peptide synthesis.

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Transmembrane receptors are integral membrane proteins, which reside and operate typically within a cell's plasma membrane, but also in the membranes of some subcellular compartments and organelles.
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An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents.
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Exocytosis (ek-soh-sy-TOH-sis) is the process by which a cell directs secretory vesicles to the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become
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Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions.[1] In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products.
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Calcium (IPA: /ˈkalsiəm/) is the chemical element in the periodic table that has the symbol Ca and atomic number 20. It has an atomic mass of 40.078.
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Glycogen is a polysaccharide of glucose (Glc) which functions as the primary short term energy storage in animal cells. It is made primarily by the liver and the muscles, but can also be made by the brain, uterus, and the vagina.
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macromolecule implies large molecule. In the context of science and engineering, the term may be applied to conventional polymers and biopolymers (such as DNA) as well as non-polymeric molecules with large molecular mass such as lipids or macrocycles.
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endomembrane system is the system of internal membranes within eukaryotic cells that divide the cell into functional and structural compartments, or organelles. Prokaryotes do not have an endomembrane system and thus lack most organelles.
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For the town in Lazio, see Cisterna di Latina.


A cisterna (plural cisternae) comprises a flattened membrane disk which makes up the Golgi apparatus.
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cytoskeleton is a cellular "scaffolding" or "skeleton" contained, as all other organelles, within the cytoplasm. It is contained in all eukaryotic cells and recent research has shown it can be present in prokaryotic cells too.
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The cytosol (cf. cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a portion of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways and glycolysis.
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A ribosome is a small, dense, functional structure found in most known cells that assemble proteins and polypeptides used in cell division. It catalyses the assembly of individual amino acids into polypeptide chains by reading messenger RNAs and binding amino acids that are
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Peptides (from the Greek πεπτίδια, "small digestibles") are short polymers formed from the linking, in a defined order, of α-amino acids.
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Identifiers
Symbol SRP14

Entrez 6727
HUGO 11299
OMIM 600708

RefSeq NM_003134
UniProt P37108
Other data

Locus Chr. 15 q22

signal recognition particle 54kDa


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amino acid is a molecule that contains both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent.
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The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells.
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Protein targeting or protein sorting is the mechanisms by which a cell transports proteins to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an organelle, any of several interior membranes, the cell's outer membrane, or its
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Lysosomes are organelles that contain digestive enzymes (acid hydrolases). They digest excess or worn out organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell.
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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 targeting signal for acid hydrolase precursor proteins that are destined for transport to lysosomes.
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Identifiers
Symbol CLTB

Entrez 1212
HUGO 2091
OMIM 118970

RefSeq NM_001834
UniProt P09497
Other data

Locus Chr. 4 q

clathrin, heavy polypeptide (Hc)


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