Endoplasmic reticulum

Source: Wikipedia, the free encyclopedia.
(Redirected from
Rough endoplasmic reticulum
)
Animal cell diagram
Components of a typical animal cell:
  1. Nucleolus
  2. Nucleus
  3. Ribosome (dots as part of 5)
  4. Vesicle
  5. Rough endoplasmic reticulum
  6. Golgi apparatus (or, Golgi body)
  7. Cytoskeleton
  8. Smooth endoplasmic reticulum
  9. Mitochondrion
  10. Vacuole
  11. Cytosol (fluid that contains organelles; with which, comprises cytoplasm)
  12. Lysosome
  13. Centrosome
  14. Cell membrane
mitochondria
.

The endoplasmic reticulum (ER) is a part of a transportation system of the

spermatozoa
.

The two types of ER share many of the same proteins and engage in certain common activities such as the synthesis of certain lipids and cholesterol. Different types of cells 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 (

protein synthesis. The RER is especially prominent in cells such as hepatocytes. The SER lacks ribosomes and functions in lipid synthesis but not metabolism, the production of steroid hormones, and detoxification.[1] The SER is especially abundant in mammalian liver and gonad
cells.

The ER was observed by

electron microscopy in 1945 by Keith R. Porter, Albert Claude, and Ernest F. Fullam.[4] Later, the word reticulum, which means "network", was applied by Porter in 1953 to describe this fabric of membranes.[5]

Structure

vesicle   8 Golgi apparatus
   9 Cis face of the Golgi apparatus   10 Trans face of the Golgi apparatus   11 Cisternae of the Golgi apparatus
3D rendering of endoplasmic reticulum

The general structure of the endoplasmic reticulum is a network of membranes called

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 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.[6][7]

Rough endoplasmic reticulum

secretory pathway
is synthesized and secreted into the rough endoplasmic reticulum, which appears at the upper right approximately halfway through the animation

The surface of the rough endoplasmic reticulum (often abbreviated RER or rough ER; also called granular endoplasmic reticulum) is studded with protein-manufacturing

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.[11]

The membrane of the rough endoplasmic reticulum is in the form of large double-membrane sheets that are located near, and continuous with, the outer layer of the

Golgi complex to target new proteins to their proper destinations. The second method of transport out of the endoplasmic reticulum involves areas called membrane contact sites, where the membranes of the endoplasmic reticulum and other organelles are held closely together, allowing the transfer of lipids and other small molecules.[16][17]

The rough endoplasmic reticulum is key in multiple functions:

Smooth endoplasmic reticulum

Electron micrograph showing smooth ER (arrow) in mouse tissue, at 110,510× magnification

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

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.[12] 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.[citation needed
]

Sarcoplasmic reticulum

Skeletal muscle fiber, with sarcoplasmic reticulum colored in blue
Main article: Sarcoplasmic reticulum
See also: Calcium-induced calcium release

The sarcoplasmic reticulum (SR), from the Greek σάρξ sarx ("flesh"), is smooth ER found in

excitation-contraction coupling.[26]

Functions

The endoplasmic reticulum serves many general functions, including the folding of protein molecules in sacs called

protein disulfide isomerase (PDI), ERp29, the Hsp70 family member BiP/Grp78, calnexin, calreticulin, 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 as a stress response in the ER. Disturbances in redox regulation, calcium regulation, glucose deprivation, and viral infection[27] or the over-expression of proteins[28] 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.[29]

Protein transport

Secretory proteins, mostly

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.[30]

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.[31] 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.[32]

Bioenergetics regulation of ER ATP supply by a CaATiER mechanism

Ca2+-antagonized transport into the endoplasmic reticulum (CaATiER) model

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.[33]

The ER ATP transporter, SLC35B1/AXER, was recently cloned and characterized,[34] and the mitochondria supply ATP to the ER through a Ca2+-antagonized transport into the ER (CaATiER) mechanism.[35] The CaATiER mechanism shows sensitivity to cytosolic Ca2+ ranging from high nM to low μM range, with the Ca2+-sensing element yet to be identified and validated.[citation needed]

Clinical significance

Increased and supraphysiological ER stress in pancreatic β cells disrupts normal insulin secretion, leading to hyperinsulinemia[36] and consequently peripheral insulin resistance associated with obesity in humans.[37] Human clinical trials also suggested a causal link between obesity-induced increase in insulin secretion and peripheral insulin resistance.[38]

Abnormalities in

colon, XBP1 anomalies have been linked to the inflammatory bowel diseases including Crohn's disease.[40]

The

neurodegenerative diseases and the inhibition of the UPR could become a treatment for those diseases.[42]

References

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External links
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