Introduction to the Cell Membrane and Organelles PDF

Introduction to the cell membrane and organelles Konstantinos Voskarides Learning objectives 1. Describe the basic structure of the human cell 2. Outline the structure and function of the cell membrane 3. Outline the basic structure and key functions of cellular organelles 4. D...

Introduction to the cell membrane and organelles Konstantinos Voskarides Learning objectives 1. Describe the basic structure of the human cell 2. Outline the structure and function of the cell membrane 3. Outline the basic structure and key functions of cellular organelles 4. Discuss, with examples, the implications of dysfunction of cellular organelles Light microscope, Electron microscope All Eukaryotic Cells Have the Same Basic Set of Membrane-enclosed Organelles All Eukaryotic Cells Have the Same Basic Set of Membrane-enclosed Organelles All Eukaryotic Cells Have the Same Basic Set of Membrane-enclosed Organelles All Eukaryotic Cells Have the Same Basic Set of Membrane-enclosed Organelles  The cell membrane is a mosaic lipid bilayer Figure 10-1 Molecular Biology of the Cell (© Garland Science 2008) Phosphoglycerides, Sphingolipids, and Sterols Are the Major Lipids in Cell Membranes Two different modes of transport through the cell membrane Evolutionary Origins May Help Explain the Topological Relationships of Organelles A simplified “road map” of protein traffic within a eukaryotic cell Vesicle budding and fusion during vesicular transport Examples of signal sequences that direct proteins to different intracellular locations NUCLEUS Nuclear Pore Complexes (NPC) Perforate the Nuclear Envelope Nuclear pores control export and import of molecules The Ran GTPase Imposes Directionality on Transport Through NPCs MITOCHONDRION THE TRANSPORT OF PROTEINS INTO MITOCHONDRIA PEROXISOMES Peroxisomes Use Molecular Oxygen and Hydrogen Peroxide to Perform Oxidation Reactions Detoxify blood from several toxic substances, alcohol and reactive oxygen species (ROS)  ENDOPLASMIC RETICULUM (ER) n Endoplasmic reticulum has two major regions: smooth endoplasmic reticulum and rough endoplasmic reticulum. Rough ER contains attached ribosomes, smooth not n Via the attached ribosomes, rough endoplasmic reticulum synthesizes proteins. Rough ER also manufactures membranes n Smooth endoplasmic reticulum serves as a transitional area for transport vesicles. It also functions in carbohydrate and lipid synthesis, like cholesterol and phospholipids, and detoxification of metabolic wastes and drugs n Rough and smooth ER are typically connected to one another so that the proteins and membranes made by the rough ER can freely move into the smooth ER for transport to other parts of the cell The ER Is Structurally and Functionally Diverse A Signal-Recognition Particle (SRP) Directs the ER Signal Sequence to a Specific Receptor in the Rough ER Membrane A Signal-Recognition Particle (SRP) Directs the ER Signal Sequence to a Specific Receptor in the Rough ER Membrane In Single-Pass Transmembrane Proteins, a Single Internal ER Signal Sequence Remains in the Lipid Bilayer as a Membrane-spanning a Helix Combinations of Start-Transfer and Stop-Transfer Signals Determine the Topology of Multipass Transmembrane Proteins Rhodopsin Most Proteins Synthesized in the Rough ER Are Glycosylated by the Addition of a Common N-Linked Oligosaccharide Oligosaccharides Are Used as Tags to Mark the State of Protein Folding The unfolded protein response (UPR, or ER stress)  Dufey et al, 2014 Ozawa et al, 2009 Role of the UPR in physiology and diseases. Genetic manipulation of major UPR components has revealed its relevance in the function of diverse organs and cell types, in addition to its contribution to a variety of diseases using preclinical mouse models. Important functions have been reported in brain, bone marrow, heart, liver, pancreas, intestine, and gastric system (blue boxes). Pathologies where abnormal ER stress levels play a relevant role in disease include diabetes, neurodegeneration, ischemia, cancer, and other diseases (red boxes) GOLGI apparatus n The Golgi apparatus modifies proteins and lipids that it receives from the endoplasmic reticulum. n These molecules leave the Golgi to be delivered to different intracellular or extracellular targets. Protein processing – carbohydrate regions of glycoproteins are altered by addition, removal or modification of carbohydrates. Lipid processing – adds phosphate groups and glycoproteins to lipids from the endoplasmic reticulum (such as cholesterol) to create the phospholipids that make up the cell membrane The Golgi Apparatus Consists of an Ordered Series of Compartments The Golgi Apparatus Consists of an Ordered Series of Compartments Oligosaccharide Chains Are Processed in the Golgi Apparatus Proteoglycans Are Assembled in the Golgi Apparatus Significance of glycosylation Correct folding of proteins. More than 50% of proteins are glycosylated Quality control in ER Cell-to-cell adhesion (e.g. in lectins, proteins that recruit immune cells) ABO blood group proteins Envelopes of viruses may have glycans to shield from immune recognition  Examples of congenital disorders of glycosylation Reily et al, 2019 Protein trafficking Konstantinos Voskarides Learning objectives 1. Outline the basic routes of protein trafficking inside the cell 2. Outline the basic structure and key functions of cellular organelles 3. Discuss, with examples, the implications of dysfunction of cellular organelles VESICLES TRANSPORT Intracellular transport is the movement of vesicles and substances within a cell. Intracellular transport is required for maintaining homeostasis within the cell by responding to physiological signals. Proteins synthesized in the cytosol are distributed to their respective organelles, according to a special signal peptide, in the N-terminus of the protein Intracellular vesicles main routes  Vesicle are formed by budding, from the membrane Vesicles are coated with special proteins Clathrin-coated vesicles, are responsible for the uptake of extracellular molecules from the plasma membrane by endocytosis as well as the transport of molecules from the trans Golgi network to lysosomes COPII-coated vesicles bud from the ER and carry their cargo forward along the secretory pathway, to the Golgi apparatus COPI-coated vesicles transport resident ER proteins marked by the KDEL or KKXX retrieval signals back to the ER Electron micrographs of clathrin-coated, COPI-coated, and COPII-coated vesicles There Are Various Types of Coated Vesicles The Assembly of a Clathrin Coat Drives Vesicle Formation The assembly and disassembly of a clathrin coat  Vesicle fusion is mediated by interactions between specific pairs of proteins, called SNAREs, on the vesicle and target membranes (v-SNAREs and t- SNAREs, respectively) Specific SNAREs are required for different vesicle fusions in different locations The formation of complexes between v-SNAREs on the vesicle and t-SNAREs on the target membranes that leads to membrane fusion, is not fully understood Tethering of a transport vesicle to a target membrane Endocytosis of viruses has similarities with vesicle fusion TRANSPORT FROM THE ENDOPLASMIC RETICULUM THROUGH THE GOLGI APPARATUS n Proteins Leave the ER in COPII-coated Transport Vesicles n Only Proteins That Are Properly Folded and Assembled Can Leave the ER n Vesicular Tubular Clusters Mediate Transport from the ER to the Golgi Apparatus n The Retrieval Pathway to the ER Uses Sorting Signals n Many Proteins Are Selectively Retained in the Compartments in Which They Function Proteins Leave the ER in COPII-Coated Transport Vesicles Vesicular Tubular Clusters Mediate Transport from the ER to the Golgi Apparatus The Retrieval Pathway to the ER Uses Sorting Signals EXOCYTOSIS The three best-understood pathways of protein sorting in the trans Golgi network The formation of secretory vesicles The process is illustrated schematically (top) and in an electron micrograph that shows the release of insulin from a secretory vesicle of a pancreatic β cell Some proteins are produced as pro-peptides and mature by proteolysis inside the vesicles Four examples of regulated exocytosis leading to plasma membrane enlargement Two ways of sorting plasma membrane proteins in a polarized epithelial cell ENDOCYTOSIS n Phagocytosis n Pinocytosis n Lysosomes are very important for the endocytotic pathways Endosome maturation: the endocytic pathway from the plasma membrane to lysosomes A low-density lipoprotein (LDL) particle The receptor-mediated endocytosis of LDL Storage of plasma membrane proteins in recycling endosomes: The example of glucose transporters  Impaired recycling of glucose transporters can couse insulin resistance. This is the first step for the development of diabetes mellitus. Fazakerley et al, 2019 Pathways synopsis n Endocytosis -> early endosome -> Recycling endosome -> Plasma membrane n Endocytosis -> early endosome -> late endosome -> Lysosome Many neurotoxins directly affect SNARE complexes. Such toxins as the botulinum and tetanus toxins work by targeting the SNARE components. These toxins prevent proper vesicle recycling and result in poor muscle control, spasms, paralysis, and even death. 33 THE DEGRADATION AND RECYCLING OF MACROMOLECULES IN LYSOSOMES n Lysosomes Are the Principal Sites of Intracellular Digestion n Lysosomes Are Heterogeneous n Plant and Fungal Vacuoles Are Remarkably Versatile Lysosomes n Multiple Pathways Deliver Materials to Lysosomes n Cells Can Acquire Nutrients from the Extracellular Fluid by Macropinocytosis n Specialized Phagocytic Cells Can Ingest Large Particles n Cargo Recognition by Cell-surface Receptors Initiates Phagocytosis n Autophagy Degrades Unwanted Proteins and Organelles Lysosome acid hydrolases are hydrolytic enzymes that are active under acidic conditions Four pathways to degradation in lysosomes (A) Scanning electron micrograph of a mouse macrophage phagocytosing two chemically altered red blood cells. The red arrows point to edges of thin processes (pseudopods) of the macrophage that are extending as collars to engulf the red cells. (B) An electron micrograph of a neutrophil phagocytosing a bacterium, which is in the process of dividing Autophagy is mediated by receptors that recruit cargo to the autophagosome membrane Lysosome storage diseases Developmental delay, movement disorders, seizures, dementia, deafness, and/or blindness Some people with lysosomal storage diseases have enlarged livers or spleens, pulmonary and cardiac problems, and bones that grow abnormally Bellettato and Scarpa, 2010 https://www.ucl.ac.uk/immunity-transplantation/clinical-services/diseases- treatments/inherited-diseases/lysosomal-storage-disease