Endoplasmic Reticulum Structure & Function 2024-2025 PDF

Structure and Function of the Endoplasmic Reticulum Asst. Prof. Burcu TÜRKGENÇ Dept. of Medical Biology  ENDOPLASMIC RETICULUM “The endoplasmic reticulum (ER) is a network of membrane-enclosed tubules and sacs (cisternae) that extends from the nuclear membrane throughout t...

Structure and Function of the Endoplasmic Reticulum Asst. Prof. Burcu TÜRKGENÇ Dept. of Medical Biology  ENDOPLASMIC RETICULUM “The endoplasmic reticulum (ER) is a network of membrane-enclosed tubules and sacs (cisternae) that extends from the nuclear membrane throughout the cytoplasm The largest organelle of most eukaryotic cells ER can comprise >10% of the total cell volume 2  Endoplasmic Reticulum 1- Rough (granular) ER (GER or RER) 2- Smooth (agranular) ER (SER) The rough ER is covered by ribosomes on its outer surface. GER involved in production, processing, folding, quality control of proteins. The smooth ER is not associated with ribosomes and involved in lipid metabolism. 3  Endoplasmic Reticulum 1- Rough (granular) ER (GER or RER) 2- Smooth (agranular) ER (SER) 4  SMOOTH ER (SER) Tubular or vesicular in form SER is not involved in protein synthesis SER perform different functions of in the different cells types; lipid synthesis, metabolism, muscle contraction, detoxification. 5 6  The SER functions 1. Lipid synthesis (membrane lipids are synthesized; phospholipids, cholesterol and ceramide). 2. Biosynthesis of steroid hormones. 3. Involved in the metabolic reactions in the liver cells (metabolize lipid soluble compounds, harmful drugs, alcohol). 4. Participates in the contraction process in the muscle cells (sarcoplasmic reticulum). 5. Lipid transport. 7 Extra Information Cell membrane lipids Phospholipids are a group of lipids that consist of two hydrophobic fatty acids, a hydrophilic glycerol unit and a phosphate group which is esterified to an organic molecule (X) such as choline, ethanolamine, inositol, etc. Esterified: a chemical compound produced by a reaction between an acid and an alcohol, in which the hydrogen of the acid has been replaced by an alkyl group of atoms 8 Extra Information Cell membrane lipids A ceramide is composed of sphingosine and a fatty acid. Ceramides are a family of waxy lipid molecules. Ceramides are found in high concentrations within the cell membrane of eukaryotic cells, since they are component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer. 9 Extra Information Cell membrane lipids Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. More than 90% of cellular cholesterol is located at the plasma membrane. With high cholesterol, you can develop fatty deposits in your blood vessels. 10  Smooth ER and Lipid Synthesis ER is the major site at which membrane lipids are synthesized in eukaryotic cells. Membrane lipids are synthesized in association with already existing membranes (rather than in the aqueous environment of the cytosol, because they are hydrophobic). Then transported from ER to their final destinations either in vesicles or by carrier proteins. 11  Smooth ER and Lipid Synthesis In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. A carrier protein is a membrane protein that moves solutes across the membrane by creating conformational changes in the protein. 12  Smooth ER and Lipid synthesis Most of the phospholipids of the membrane are derived from glycerol They are synthesized on the cytosolic side of the ER membrane from water soluble cytosolic precursors Resulting phospholipid (phosphatidic acid) is inserted into the membrane 13  Smooth ER and Lipid synthesis Enzymes on the cytosolic side of the ER membrane then modify phosphatidic acid or catalyze the addition of different polar head groups Resulting in formation of; Phosphatidyl choline, Phosphatidyl serine, Phosphatidyl ethanolamine. Phosphatidyl inositol is formed from phosphatidic acid via a modified diacylglycerol. 14  The SER functions in biosynthesis of steroid hormones SER contains the enzymes required for steroid synthesis and they are abundant in steroid producing cells ; Leydig’s cells of testes: testosteron adrenal cortex cells: corticosteroids corpus luteum cells of ovarium: progesteron Steroid hormones are synthesized from cholesterol 15  SR participates in the contraction process in muscle cells Sarcoplasmic reticulum (SR) Sarcoplasmic reticulum is a smooth endoplasmic reticulum that is found within the muscle cells. The main function of the SER is to store calcium ions (Ca2+). The sarcoplasmic reticulum specifically regulates Ca +2 ion flow. Ca+2 ions regulate muscular contraction. The Ca2+ ions bind to the Troponin C component of the actin filament, which exposes the binding site for the myosin head to bind to in order to stimulate a muscle contraction. SER membrane contains Ca+2 - ATPase pumps The sarcoplasmic reticulum system consists of a branching network of SER 16 cisternae surrounding each myofibril  Extra Information Types of Muscle Muscle is a type of contractile tissue found in animals, and its main function is to facilitate movement. There are three types of muscle tissue found in the human body, and these are: Smooth muscle – Smooth muscle contracts involuntarily and is found in the internal organs (except the heart) and blood vessels. Cardiac muscle – Cardiac muscle is found only in the heart and also contracts involuntarily. Skeletal muscle – Skeletal muscle is attached to the bones. It is the most abundant type of muscle tissue in the human body, and the only type of muscle that can be moved voluntarily. Skeletal muscle contractions pull on the bones of the skeleton and allow those bones (and the body structures they support) to move. 17 Extra Information The Structure of Muscles Muscle cells, commonly known as myocytes, are the cells that make up muscle tissue. Muscle tissue is made up of bundles of muscle fibers. Muscle fibers are also composed of tubular myofibrils. Myofibrils are long filaments that run parallel to each other to form muscle (myo) fibers. Myofibrils are composed of repeating sections of sarcomeres Sarcomere is the functional unit (contractile unit) of a muscle fiber. Actin and myosin are both proteins that are found in all types of muscle tissue. Myosin forms thick filaments (15 nm in diameter) and actin forms thinner filaments (7nm in diameter). Actin and myosin filaments work together to generate force. 18 Extra Information SR participates in the contraction process in muscle cells A sarcomere is the functional unit (contractile unit) of a muscle fiber. The sarcomeric Z-disc defines the lateral borders of the sarcomere. Actin and myosin are both proteins that are found in every type of muscle tissue. They both work together to generate muscle contractions and movement. 19  Extra Information Muscle contractions Actin and myosin work together to produce muscle contractions and, therefore, movement. First, a motor neuron delivers an electrical signal to the muscle cell from the brain. This triggers the release of a chemical called acetylcholine. Acetylcholine causes calcium ions to be released from the sarcoplasmic reticulum. Next, the calcium ions bind to a protein called troponin. Troponin is attached to another protein, called tropomyosin, and is found between the actin filaments in muscle tissue. When calcium ions bind to troponin, the shape of troponin changes. This moves tropomyosin from the myosin-binding sites on the actin filament and ‘unblocks’ them, making it possible for the myosin heads to bind to the actin filament. Once tropomyosin has moved out of the way, the myosin heads can bind to the exposed binding sites on the actin filaments. This forms actin-myosin cross-bridges and allows muscle contraction to begin. The myosin uses ATP energy to move its head groups towards the middle of the sarcomere. This movement pulls the actin filaments towards the center of the sarcomere, causing the sarcomere to shorten and contract. The contraction of the sarcomere causes the muscle fiber to contract and generates muscle movement. 20 Extra Information Muscle contractions 21  The SER is involved in the metabolic reactions in the liver cells SER is abundant in liver It contains specific enzymes that metabolize lipid soluble compounds and inactivates harmfull drugs (i.e. phenobarbital, by converting them water soluble compounds that are eliminated from the body in the urine) Detoxify alcohol, certain hormones and toxic compounds. Cytochrome P450 Enzyme System (The liver hepatocytes contain all the necessary enzymes for the metabolism of drugs. The main enzymes involved in metabolism belong to the cytochrome P450 group. These are a large family of related enzymes housed in the smooth endoplasmic reticulum of the cell). 22  Breakdown of glycogen SER membranes contains a specific enzyme called Glucose-6-phosphatase in liver it is involved in the breakdown of glycogen into glucose 23 Extra Information (Glucose is the main source of fuel for our cells. When the body doesn't need to use the glucose for energy, it is stored as glycogen (many connected glucose) in the liver and muscles). 24  The SER functions in lipid transport Lipids can leave the SER by non-vesicular lipid transport at membrane contact sites by Lipid transport proteins (LTPs). Lipids can also leave the SER by vesicular transport in the form of complete budded membranes. 25  GRANULAR ER (GER) Rough ER (RER) GER is involved in protein synthesis and prominent in cells specialized for protein synthesis and secretion. In light microscope, GER can be detected by staining with basic dyes (Because of attacched ribosomes) 26  The rough ER is covered by ribosomes on its outer surface. In electron microscope, GER appears as parallel membrane limited flattened sacs or cisternae 27 28  GER is particularly well developed in protein secreting cells in the digestive enzyme producing cells of the exocrine pancreas; aciner cells in the collagen and elastin producing cells of the connective tissue; fibroblast in the antibody producing cells; plasma cell in the neurotransmitter producing cells; motor neurons in the phagocytic cells containing lysosomal enzymes; macrophages 29 Three different types of proteins are synthesized in GER: 1. Proteins that are secreted from the cell 2. Lysosomal enzymes 3. Membrane proteins (Nuclear mebrane, plasma membrane, peroxisome membrane proteins) 30  In contrast, proteins destined to remain in the cytosol or to be sent into the nucleus, mitochondria, chloroplasts, or peroxisomes are synthesized on free ribosomes and released into the cytosol when their translation is complete. 31 The mechanism of the protein synthesis in the GER is explained with the SIGNAL HYPOTHESIS This hypothesis was first suggested by Blobel and Sabatini in early 1970’s 32  All protein synthesis initiates on ribosomes that are free in the cytosol. Ribosomes that are engaged in the synthesis of secretory proteins then targeted to the endoplasmic reticulum by a signal sequence at the amino terminus of the growing polypeptide chain. 33  If mRNA lacks a signal sequence The protein will be synthesized entirely in the cytoplasm on free ribosomes If an mRNA contains signal sequence; initially mRNA binds to the free ribosomes in the cytoplasm. During that time the signal peptide is being synthesized, the ribosome remains in the cytosol. 34  The signal peptide is about 16- 20 aminoacids and it appears at the beginning of the polypeptide chain. As the signal peptide emerges from the ribosome it is recognized by a special molecule in the cytosol (signal recognation particles; SRP) (protein-RNA complex) SRP binds to the signal peptide and ribosome (At this moment protein synthesis stops) 35  Then SRP and ribosome complex attaches to the ER at specific sites called SRP receptor (or docking protein) Upon binding to the docking protein SRP is released from ribosome and SRP returns to cytosol. 36  This process is coordinated by GTP binding to both the SRP and SRP receptor With Hydrolysis of GTP to GDP results in dissociation of SRP from receptor and ribosome 37  Ribosome then binds to a protein translocon complex Ribophorin I and II proteins (pore proteins) (Translocon) are found on the ER membrane, ribosomal large subunit attaches to this pore proteins Ribophorins form hydrophilic channels through the hydrophobic core of ER membrane. 38  Transfer of ribosome - mRNA complex to the translocon opens the gate Protein synthesis restarts, the signal peptide and the growing polypeptide chain is inserted into the the channel of translocon The signal peptide is removed from the polypeptide by the SIGNAL PEPTIDASE 39  These signal sequences are short hydrophobic amino acids that are cleaved from the polypeptide chain during its transfer into the ER lumen. And polypeptide is released into the cisterna (lumen) of GER. 40 41  The last step; When the protein synthesis is completed, the ribosomes detache from the RER membrane and mRNA , and they may participate in other round of protein synthesis The proteins are then packaged into transport vesicles and transfer to Golgi complex for the other modifications 42 43  Post-Translational Modifications in the Rough ER Newly synthesized polypeptides in the membrane and lumen of the ER undergo five principal modifications before they reach their final destinations: 1. Addition and processing of carbohydrates 2. Formation of disulfide bonds 3. Specific proteolytic cleavages 4. Proper folding 5. Assembly into multimeric proteins 44  Protein processing within the ER Modification and synthesis of carbohydrate portions of glycoproteins Addition and processing of carbohydrates in ER, GLYCOSYLATION 45  Proteins are modified by the addition of N-linked oligosaccharides consisting of 14 sugar residues. (9 mannose, 3 glucose, 2 N- acetylglucosamine) The oligosaccharide is synthesized on a lipid carrier anchored in the ER membrane. It is then transfered to asparagine residues in the polypeptide. Then three glucose residues and one mannose residue are removed while the proteins still in the ER. 46  Protein folding and processing in the ER. ER is also the site of protein folding, assembly of multisubunit proteins, disulfide bond formation, initial stages of glycosylation. Primary role of luminal ER proteins is to assist the folding and assembly of newly translocated polypeptides. Proteins are translocated across the ER membrane as an unfolded polypeptide chain. 47  Protein folding and processing in the ER Proteins are translocated across the ER membrane as unfolded polypeptide chains. These polypeptides fold into their three dimentional conformation within the ER assisted by molecular chaperons that facilitate the folding. The Hsp70 chaperon; Bip binds to unfolded polypeptide as it crosses the membrane and mediate folding BIP; Binding protein (Proper folding) 48  Protein folding and processing in the ER The formation of disulfide bonds (S-S) between the side chains of cystein residues is an important aspect of protein folding and assembly within in the ER. Disulfide bonds increase the stability of proteins 49  Correctly folded and assembled proteins are released from ER and transport to the Golgi Apparatus Abnormally folded or improperly assembled proteins are targets for degeneration 50 Export of Proteins from the ER Proteins travel along the secretory pathway in transport vesicles, which bud from the membrane of (ER) and then fuse with the membrane of (Golgi). 51  Export of Proteins and Lipids from the ER Molecules are exported from the ER in vesicles that bud from the ER and carry their cargo to the Golgi apparatus. 52 Videos: Endoplasmic Reticulum https://www.jove.com/science- education/12123/the-endoplasmic- reticulum?playlist=74b3c7fd Smooth Endoplasmic Reticulum https://www.jove.com/science- education/12124/smooth-endoplasmic- reticulum?playlist=74b3c7fd 53