BMS1025 - Cell Biology 2023 Endoplasmic Reticulum PDF

BMS1025 – Cell Biology • Dr Penny Lympany 28AY04 April Chloe Terrazas ; Cellular Biology: organelles, structure, function Content In these sessions we will be looking at Ribosomes Protein synthesis This Photo by Unknown Author is licensed under CC BY Endoplasmic reticulum Ribosomes and Endo...

BMS1025 – Cell Biology • Dr Penny Lympany 28AY04 April Chloe Terrazas ; Cellular Biology: organelles, structure, function Content In these sessions we will be looking at Ribosomes Protein synthesis This Photo by Unknown Author is licensed under CC BY Endoplasmic reticulum Ribosomes and Endoplasmic Reticulum Ribosomes – Assemble proteins Large and small subunits Endoplasmic reticulum – Rough – making cell membrane and secretory proteins, connected to nuclear envelope and cell membrane with ribosomes on outside Smooth – make and store proteins, chy, lipids First – the nucleolus Discrete area within the nucleus of eukaryotic cell Site for synthesis and processing of rRNA and assembly of ribosomes Not bound by a membrane – condensate of macromolecules Endoplasmic Reticulum Source: Fig Alberts et al. Molecular Biology of the Cell 7th Ed Endoplasmic reticulum Membrane of the endoplasmic reticulum (ER) >50% total membrane of an average animal cell. Labyrinth of branching tubules and flattened sacs extending throughout cytosol Tubules and sacs interconnect, and membrane is continuous with the outer nuclear membrane. Membrane system encloses a single internal space - ER lumen, which is continuous with the space between the inner and outer nuclear membranes. Source: Fig 12-14 Alberts et al. Molecular Biology of the Cell 7th Ed The ER often occupies more than 10% of the total cell volume. What is the function of the ER? Central role - biosynthesis of both lipids and proteins Lumen stores intracellular Ca2+ that is mobilized in many cell signalling responses Membrane - site of production of many of the transmembrane proteins and lipids of the cell’s organelles; ER, Golgi apparatus, lysosomes, endosomes, secretory vesicles, peroxisomes, and the plasma membrane ER membrane also site where most of the lipids for mitochondrial membrane are made Most of the proteins that will be secreted to the cell exterior (and those destined for the lumen of the ER, Golgi apparatus, or lysosomes) are initially delivered to the ER lumen The ER is Structurally and Functionally Diverse Distinct regions of the ER become highly specialized. Entails changes in the proportional abundance of different parts of the ER Rough ER - ribosomes engaged in protein synthesis bound to the surface of this part of the ER Smooth ER – lack of ribosomes, other functions Source: Fig 12-15 Alberts et al. Molecular Biology of the Cell 7th Ed EM of rough and smooth ER Source: Fig 12-15 Alberts et al. Molecular Biology of the Cell 7th Ed (A, courtesy of Lelio Orci. B, courtesy of Daniel S. Friend, by permission of E.L. Bearer. C, after R.V. Kristić, Ultrastructure of the Mammalian Cell. New York: Springer-Verlag, 1979. With permission from Springer Nature.) Rough vs Smooth Rough ER • Most secreted proteins are synthesized by the ribosomes • Cells specialised to secrete large amounts of protein are packed with rough ER. e.g exocrine cells of the pancreas (rough ER makes up 60% of these cells’ membranes) Smooth ER • Transitional ER - transport vesicles carrying newly synthesized Functions for the smooth ER are more diverse and can become highly specialized. • proteins and lipids bud off for transport to the Golgi apparatus. • Cells that synthesize steroid hormones contain prominent smooth ER to accommodate the enzymes that make cholesterol and modify it to form a variety of steroid hormones • Membranes contain enzymes that catalyse detoxification of drugs and various harmful compounds produced by metabolism e.g. cyt P450 • Sequesters Ca2+ from the cytosol. Muscle cells have an abundant, modified smooth ER called the sarcoplasmic reticulum. The release and reuptake of Ca2+ by the sarcoplasmic reticulum trigger myofibril contraction and relaxation, respectively, during each round of muscle contraction. Golgi Apparatus Newly synthesised proteins cross ER membrane from cytosol to enter biosynthetic-secretory pathway Proteins modified as pass from ER to Golgi apparatus to cell surface Balance of pathways – forward and backward This Photo by Unknown Author is licensed under CC BY Pathway from ER to cell surface comprises number of sorting steps The Golgi Apparatus – compartments Collection of flattened, membrane-enclosed compartments (cisternae) Each stack typically consists of four to six cisternae In animal cells, tubular connections between corresponding cisternae link many stacks - single complex, usually located near the cell nucleus and close to the centrosome Source: Alberts et al. 7th Edn Fig 13-28 What does the Golgi apparatus do? Major site of carbohydrate synthesis Sorting and dispatching station for products delivered to it from the ER Builds and attaches oligosaccharide chains to the many proteins and lipids that the ER sends to it. Oligosaccharides can act as tags for transport to endosomes. But most proteins and lipids are recognized in other ways for targeting into the transport vesicles going to the cell surface and other destinations. This Photo by Unknown Author is licensed under CC BY How? This will be covered in more detail in other lectures in this module This Photo by Unknown Author is licensed under CC BY-SA-NC The Endoplasmic reticulum is where some proteins are made ➢Ribosomes are still required for translation. ➢The ribosome is recruited to the ER membrane through a lipophilic sequence in the polypeptide chain being produced. ➢The poly peptide chain moves into the ER without the use of ATP, so there is no difference between translation in the cytoplasm and in the ER in terms of energy use. ➢The protein may be released into the ER lumen or is integrated into the membrane. Lecture 5 - The roles of ER Lecture 6 - Fates of proteins in the ER Alberts 5thEd: Fig 12-33b Steroid hormone synthesis in the smooth ER OH O OH Cholesterol Testosterone • Steroid hormones are synthesised on the SER • SER is expanded to accommodate the enzymes which make and modify cholesterol • One Example: Testosterone is synthesised from cholesterol in the abundant SER of the Leydig cells of the testis Detoxification of drugs and the smooth ER Cytosol ER Lumen Water Soluble Metabolite Cytochrome P450 Lipophilic drug Oxidation • Occurs in the hepatocytes of the liver. • Lipophilic drugs are modified to increase water solubility. • Further modification can occur in the cytosol, before excretion from the body in the bile or urine. • The hepatocytes are also the major producers of lipoproteins. Cellular adaptation is to expand smooth ER membrane to enable cells to carry out their expected functions The Sarcoplasmic Reticulum Acts as a calcium store in the muscles + + + + - - - - ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Action potential from nerves depolarises plasma membrane ● ● ● ● ●● ● ● ●●● ● ● ● ● ● ● ● ●● ● ● ● ●● ● ● ● ● ● Rise in cytosolic Ca2+ stimulates muscle contraction Voltage sensitive plasma membrane protein changes conformation Extracellular ● SER Lumen ● ● ● ● ● ● ● ● ● ● ●● ● ● ● ● ● ● ●● Resting: High calcium ion (Ca2+) ion concentration in SER due to Ca2+ pumps + + + + After nerve impulse: - - - Ca2+ channel closes Cytosol 2+ Abundant Ca pumps restore levels ● ● Ca2+ channel opens and Ca2+ passes into the cytosol Adapted from Alberts 5th Ed: Fig 16-77 The ER – a site of membrane synthesis The Phospholipid Bilayer Phospholipids are amphipathic and consequently spontaneously form bilayers choline phosphate Fatty acids glycerol CH2 – N+ (CH2)3 CH2 O O=P–O– O CH2 – CH – CH2 O O C=O C=O CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH2 CH3 Hydrophilic (water loving) head Hydrophobic (water hating) tail Phosphatidyl choline Membrane synthesis The ER is a major site of phospholipid synthesis • New phospholipids are assembled in the outer leaflet of the ER membrane… choline fatty acids • Scramblase is a phospholipid translocator which causes the two leaflets to equilibrate OUTER • New phospholipids are carried to the Golgi, plasma membrane, lysosomes and endosomes by vesicular transport (later next week) INNER • … by membrane bound enzymes which combine their components ER Lumen glycerol In most cells the SER:• is transitional between the RER and the Golgi • is sparse (~1% of cell membrane) • is actually a mixture of rough and smooth ER BUT in some cells the SER is abundant and has specialised roles Alberts 5th Ed: Fig 13-25 Has no ribosomes! Alberts 6th Ed: Fig 13-33c The Smooth ER Alberts 5thEd: Fig 12-33b Steroid hormone synthesis in the smooth ER OH O OH Cholesterol Testosterone • Steroid hormones are synthesised on the SER • SER is expanded to accommodate the enzymes which make and modify cholesterol • One Example: Testosterone is synthesised from cholesterol in the abundant SER of the Leydig cells of the testis What we have covered That’s all folks This Photo by Unknown Author is licensed under CC BY-NC Remember – for more information Alberts B et al. Molecular Biology of the Cell or your favourite Molecular Cell Biology text book The module discussion board Email – [email protected]