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Rough endoplasmic reticulum (RER): is a series of connected flattened sacs that have many ribosomes on their outer surface.
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Chapter: Endoplasmic Reticulum
Introduction Origin of Endoplasmic reticulum Composition and Structure of Endoplasmic Reticulum. Types of Endoplasmic Reticulum Rough Endoplasmic Reticulum Smooth Endoplasmic Reticulum Difference between Rough and Smooth Endoplasmic reticulum Functions of Endoplasmic Reticulum Import of proteins into Endoplasmic reticulum Protein Folding and Processing Protein and Lipid Export from ER Summary Exercise/ Practice Glossary References/ Bibliography/ Weblinks
and lipids for most of the cell's organelles (the Golgi apparatus, lysosomes, endosomes, secretory vesicles, and the plasma membrane). The ER membrane also contributes to mitochondrial and peroxisomal membranes by producing most of their lipids.
Origin of Endoplasmic reticulum The origin of endoplasmic reticulum is not definitely known. According to Dallmer (1966), endoplasmic reticulum originated from the plasma membrane by the process of invagination. According to De Robertis 1970, endoplasmic reticulum originates from the evagination of nuclear envelope (Figure 1). At telophase, the nuclear envelope is reformed with the help of vesicles of endoplasmic reticulum.
Figure 1: Origin of Endoplasmic Reticulum Source: https://commons.wikimedia.org
Value Addition: Screenshot of historical research article by K.R. Poter on sarcoplasmic reticulum.
Structure and Composition of Endoplasmic Reticulum
The endoplasmic reticulum, enclosed by a continuous membrane, is the largest organelle of most eukaryotic cells. Its membrane account for approximately 50% of all cell membranes and the space enclosed by the ER represents about 10% of the total cell volume. The Endoplasmic reticulum membrane is like a unit membrane (typical of three-layered) in some regions while at other regions it may show a micellar (globular) structure. Hence shows a combination of both structures. The endoplasmic reticulum membrane is thinner than the plasma membrane which measures about 50 A.
Types of Endoplasmic Reticulum
Endoplasmic reticulum (ER) is of two types– Smooth endoplasmic reticulum (SER) and Rough endoplasmic reticulum (RER) (Figure 3). RER arises from nuclear membrane. RER consists of tubules studded with ribosomes and is associated with protein modification and trafficking. SER is primarily associated with synthesis of lipids and helps in detoxification. The two contiguous membrane domains-RER and transitional ER function in protein processing. The transitional ER is the site where vesicles exit to the Golgi apparatus. The smooth ER is involved in lipid, rather than protein, metabolism. ER encompasses a membrane system that enfolds a lumen, estranged from the surrounding cytosol. The symphony of the luminal space is different from that of the adjacent cytosolic space. RER and SER share many proteins
and activities like synthesis of lipid and cholesterol. Fluorescent microscopy using labeled proteins and lipids indicates that their membranes are incessant as these could diffuse from one side of lumen to another side. Besides, RER and SER have various different protein components which mark their structural and functional differences.
So, there are basically two morphological types of endoplasmic reticulum: the Rough endoplasmic reticulum (RER) or the granular form (ergastoplasm) and Smooth endoplasmic reticulum (SER) or agranular form (Figure 2 & Table 1). The RER is composed of flattened sacs-cisternae. Outer membrane of the nuclear envelope continues to form RER which bears ribosomes on its cytosolic façade. The difference is that the rough endoplasmic reticulum is covered in ribosomes, giving it a rough appearance in the electron microscope. The main function of RER is protein synthesis; integral membrane proteins in the plasma membrane, and proteins that the cell will export to the extracellular medium (such as the proteins of the extracellular matrix). Hence presence of RER is predominantely in cells which are actively synthesizing proteins, e.g. enzyme secreting cells. Whereas Smooth endoplasmic reticulum is one lacking ribosomes. SER forms highly curved, tubular and interconnecting system. The membranous elements of the SER are highly curved and tubular, forming an interconnecting pipelines system. Smooth endoplasmic reticulum is one lacking ribosomes. The function of the smooth endoplasmic reticulum varies from tissue to tissue. It is characteristic of cells in which synthesis of non- protein substance like phospholipids, glycolipids and steroids takes plce, e.g. adipose tissues cells, adrenocorticals etc. In the ovaries, testes, and the adrenal gland synthesis of steroid hormones takes place; in the liver it is the site of detoxication. RER transfers the synthesized product, protein, to the Golgi bodies and also helps in store of minerals such as calcium. Inversely the function of the smooth endoplasmic reticulum is the storage and sudden
A modified form of smooth endoplasmic reticulum is found which is referred as “ Sarcoplasmic reticulum ”, in the striated muscles. It is a plexus surrounding the myofibrils. The myeloid body , which is present in the pigment cells of the retina of the frog, is probably modified smooth endoplasmic reticulum.
In epithelial cells of the frog retina and interstitial cells of the testis the endoplasmic reticulum is completely Smooth (SER).
Figure 3: Rough and Smooth Endoplasmic Reticulum Source: https://en.wikipedia.org
Table 1: Difference between Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum
Smooth Endoplasmic Reticulum Rough Endoplasmic Reticulum
Not associated with ribosomes. Studded with ribosomes.
Mainly composed of tubules. Mainly composed of cisternae.
Generally associated with plasma membrane.
Generally associated with nuclear membrane.
Less stable More stable
Found in Epithelial cells, Intestinal cells, Sarcoplasmic Reticulum
Found in Pancreatic Exocrine cells
Main function is lipid synthesis and hence mostly seen in cells associated with synthesis of steroid hormones.
Main function is protein synthesis and hence mostly seen in cells associated with secretion of proteins.
1. Mechanical Support: The endoplasmic reticulum gives additional mechanical support to the cytoplasm as it divides the fluid content of the cell into compartments. 2. Synthesis of secretory proteins:
The proteins are synthesized on RER with a hydrophobic signal by which they can be embedded in the lipid bilayer. Ribosomes present on these ER are the actual site of protein synthesis. In addition ER is also the site of synthesis of a group of proteins which are for storage or for export outside of the cell.
the drugs water soluble, enabling it to leave the cell and are excreted in the urine. SER membranes contain this enzyme system with detoxification properties. Phenobarbital injections produce increased synthesis of detoxification enzymes in the endoplasmic reticulum. Abundant SER with P450 carry out biotransformation of xenobiotics which enter the body through the digestive system, which are inactivated by P450 oxidation.
7. Circulation and exchange:
Endoplasmic reticulum acts as an intracellular transport system for various substances. The exchange between the nucleus and the cytoplasm occurs through nuclear openings which communicate with the Endoplasmic reticulum. This was suggested by Watson. It also provides an extensive internal surface for exchange of material between the maxtrix of cytoplasm and the lumen of the cisternae. It was postulated that presence of permeases and carriers present on ER membrane facilitates the active transportation.
8. Synthesis of cholesterol and steroid hormones:
Cholesterol is an important precursor of steroid hormones. Endoplasmic reticulum is the major site of cholesterol synthesis. The SER of liver cells is concerned with both the synthesis and storage of cholesterol. Other sites of synthesis of steroid hormones are the SER of Testis, ovary and the adrenal. the capacity to synthesize steroid hormones is directly realted to the amount of SER present in cells.
9. Other functions:
Sarcoplasmic reticulum, which is SER of muscles is involved the the concentration of Ca2+^ ions utilizing ATP. Calcium ions which are stored in the sarcoplasmic reticulum are released when the muscles are stimulated by
impulse or hormones resulting in contraction of muscles. Oxyntic cells present in stomach ER are implicated in chloride ion secretion.
Endoplasmic reticulum is connected with the formation of nuclear envelope. Both are connected to each another with the endoplasmic reticulum space opens into the perinuclear space between the two membranes. Both endoplasmic reticulum and nuclear envelope resembles physically and chemically to some extent. Hence it is supposed to that endoplasmic reticulum is involved in nuclear membrane formation.
Transport of ions, molecules and particles into and out of the cells may also take place through membrane flow (Figure 4). Thus substances like RNA and nuclear proteins may pass out from the nucleus outside the cell by the following route:
Figure 4: Rough Endoplasmic Reticulum at work Source: http://welkescience.wikispaces.com
Electron microscopic studies advocate that the endoplasmic reticulum in plants also acts in the formation of the interconnection of cells called plasmodesmata, through the cytoplasmic strands.
Figure 5: Structure of Signal Recognition Particle (SRP) with six protein subunits and one molecule of SRP-RNA Source: Author
Thus the import of proteins into endoplasmic reticulum is co-translational as compared to other organelles viz. Mitochondria, chloroplast, peroxisomes etc. where it is post-translational. The proteins are never released in to the cytosol, which were destined for endoplasmic reticulum, and there is no requirement of any chaperones to keep these proteins unfolded. These proteins which are destined to endoplasmic reticulum can be of two types:
i. Soluble proteins: can pass across the endoplasmic reticulum and get to its lumen. ii. Transmembrane proteins: intended for the endoplasmic reticulum membrane or other cell membrane. These proteins are not released in the lumen and stay anchored in the lipid bilayer. This anchoring is facilitated by one or more hydrophobic α-helical regions , which functions as start-transfer or stop-transfer signals during translocation.
The ribosome binding to protein translocation complex in ER membrane is followed by insertion of signal sequence into the membrane channel or translocon. GTP binds to SRP and SRP receptor during the process.
Hydrolysis of GTP to GDP leads to dissociation of SRP from both the receptor and ribosome-mRNA complex.
Proteins from yeast and few proteins from mammals are known to get targeted to ER after complete translation rather than co-translationally. Such protein does not require SRP for the transfer to the lemen of ER. In this case, signal sequences are recognized by separate receptor proteins, known as Sec62/63 complex, associated with translocon. Also, cytosolic chaperons (HSP70) are required to maintain the unfolded conformation of polypeptide in cytosol. For entering in the lumen, there is a separate chaperon (HSP70), called as BiP, is required to pull the polypeptide inside.
Protein Folding and Processing
The ER is site for the protein folding and modification where assembly of different subunits of a protein, formation of disulfide bond, glycosylation at initial stages and addition of glycoprotein anchor take place. Lumen proteins help in folding of translocated proteins (Figure 6). Protein translocation through the membrane takes place cotranslationally in an unfolded state. Three dimensional folding of proteins takes place in the lumen of ER. This folding is assisted by molecular chaperons. As the unfolded polypeptide crosses the membrane, a HSP70 chaperon, BiP, binds to it to assist the protein folding and assembly of multi subunits of the protein (Figure 6). After correct folding, the protein, released from the BiP, is ready to be transported to the golgibody. In case of any misfolding, the protein gets degraded. During the process of folding, the disulfide bond is formed by the enzyme disulfide isomerase present in the lumen.
Summary
Endoplasmic reticulum was reported by Porter, Claude and Fullman (1945). Morphologically Endoplasmic reticulum consists of three types: cisternae, vesicles, and tubules. ER is of two types – Smooth endoplasmic reticulum (SER) and Rough endoplasmic reticulum (RER). RER is one which possesses ribosomes, made up of cisternae. These arise from nuclear membrane. RER primarily functions in protein synthesis. SER lack ribosomes, consisting of tubules mainly. SER helps to synthesize lipids and helps in detoxification. In muscles, endoplasmic reticulum is called sarcoplasmic reticulum, in eyes called as myeloid bodies and in nerves as Nissle granules.
Microsomal membrane is lipoprotein. Lipids are mostly phospholipids and ribophorins are the membrane proteins of RER. ER forms intracellular transport system and provides mechanical support to cytoplasm. ER helps in synthesis of secretory proteins. ER also helps in protein glycosylation and carbohydrate metabolism. Detoxification and electron transport with the help of cytochrome P is also one of the important function of ER. The proteins which are imported into the ER lumen always carry a special hydrophobic signal peptide which is recognized by a signal recognition particle (SRP) that binds to the growing polypeptide chain and also to the ribosomes taking part in the synthesis. Protein folding is assisted by molecular chaperons. A HSP70 chaperon, BiP, binds to it to assist the protein folding and assembly of multi subunits of the protein inside the lumen of ER. A sequence Lys-Asp-Glu-Leu (KDEL) at carboxy terminus of native proteins of ER directs their retrieval to ER otherwise those proteins would be lost to cytoplasm.
Exercise/ Practice
