Biochemistry, Medicine, Phase 1
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| Cytoskeleton | a network of protein filaments that extend throughout the cell
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| The cytoskeleton determines | cell shape and polarity; tissue structure; adhesion; cell movement; intracellular movement (of vesicles and of chromosomes)
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| Three classes of protein polymer (or filament) | actin, intermediate filaments, microtubules
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| Actin | microfilaments; composed of actin-binding proteins; comprises 5% total protein
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| Microtubules | composed of micro-tubule-associated proteins (MAPs)
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| Actin filaments (1) | called F-actin (filamentous actin or microfilaments); are polymers of individual actin proteins called G-actin (globular actin)
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| Actin monomers | G-actin (globular actin)
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| Actin filament (2) | polarised double helix; 13 actin subunits for every complete turn
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| Diameter of actin filament | 7nm
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| Growth of actin filament | requires ATP to be bound to the actin monomer (G-actin); filaments are very dynamic (intermediate filaments are not); monomers can be added and removed from both ends of the polymer
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| G-actin adds more rapidly to | (+) end of the filament; once incorporated, ATP is hydrolysed to ADP
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| G-actin is removed more rapidly from | (-) end of the filament
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| Polymerisation of actin filament | when g-actin is ATP bound it can bind to the growing actin filament; not very stable over time; ATP eventually hydrolyses to ADP and will depolarise and come off at the negative end of the filament
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| Cell migration | happens a lot during embryogenesis but also adulthood; also pathogensis (metastasis); actin filaments turn over rapidly
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| Major function | mechanical support; cell shape changes and maintenance; cell motility
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| Actin-binding proteins | modulate polymerisation dynamics and function;
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| Membrane-attachment proteins | spectrin; bind to G-actin
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| Severing proteins | geisolin, severin; bind to F-actin
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| Cross-linkingproteins | transgelin; bind to F-actin
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| Capping proteins | prevent filament grown; bind to F-actin
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| Actin-sequestering proteins | bind to G-actin and prevent its polymerising;
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| Actin-bundling protein | alpha-actinin in muscle
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| Motor proteins | myosin in muscle; bind to F-actin
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| Side-binding proteins | interaction with other proteins; bind to F-actin
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| Intermediate filaments | polymer of individual intermediate filament proteins; 10nm in diameter; visible by electro microscope; not dynamic
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| Intermediate filament network | typically more dense around the nucleus; can extend into the periphery
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| Functions of intermediate filaments | used to anchor cells at some cell junctions; support nuclear structure; can act as diagnostic tools to identify foreign cells as they are expressed differently based on their different locations (e.g. cancer)
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| Intermediate filament names by cell type | keratin, vimentin, glial fibriallary acidic protein, neurofilamin
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| Mice lack neurofilaments (NF+/-) | reduces axon diameter
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| Formation of the intermediate filament polymer | intermediate filament protein (monomer) forms a helical dimer; two timers combine to form a tetrameter (the fundamental units of the IF); tetramers link in a staggered formation and end-to-end to form the filament
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| Tetramer | fundamental unit of the intermediary filament; formed from two helical dimers
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| Growth of an intermediate filament | slow, not dynamic; subunit exchange occurs throughout the length of the filament
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| Plectin | a protein that binds to intermediary filaments; these molecules link to IFs and to actin filaments and microtubules to form the net-like structure
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| Microtubules | long, relatively stiff hollow tubes; approximately 25nm; can be rapidly disassembled and reassembled; visible using EM or light microscopy; polar and highly dynamic i
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| Tubulin | monomer of microtubules; consists of one molecule or alpha and beta-?
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| Alba tubulin | negative charge
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| Beta tubulin | positive charge
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| Cylindrical microtubule | 25nm in diameter; thirteen columns of tubulin polymer
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| Assembly and disassembly of the microtubule | similar to the assembly of actin filaments; there is positive and negative end; GTP bound monomers (alpha-tubulin) assemble onto microtubule; GDP bound beta-tubulin monomers dissociate rapidly
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| Microtubules are polymerised in | centrosomes; minus end remains close the centrosome and the plus end points outward towards the cell periphery
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| Cell shape and orientation | actin filament bundles provide support; dense sheets of actin found in the cortex of cells; maintains the shape of cells e.g. erythrocytes (RBC);
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| Microvilli | Actin filament bundles provide for absorption in the gut byt forming an adhesion belt
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| Stereocilia | contains actin filaments; detect vibration in the cochlea; cells are depolarised or hyperpolarised by deflections caused by sound; actin filaments keep them rigid
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| Shape of axons | intermediate filaments and microfilaments support shape
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| Stabilise the shape of plates | microfilaments provide support; protrusions are activated by cuts and formed by microfilaments allowing them to adhere to one another and form a clot
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| Form meshwork around the cell nucleus hold it into position | Intermediary filaments
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| Hold synaptic vesicles close to the presynaptic membrane | actin filaments
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| Organise the ER of a cell | microtubules
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| Anchoring cells | cytoskeleton is essential; to extracellular matrix at cell junctions and to each other
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| Actin-based movement | cell motility; e.g.migration of neutrophils (WBC) to sites of infection for phagocytosis F
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| Event 1 of actin-based movement | cell pushes out protrusions at the front (leading edge); actin filament polymerisation provides to force of membrane protrusion
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| Event 2 of actin-based movement | protrusions adhere to the surface on which the cell is moving through contact junctions; F-actin connects to the focal adhesions to provide a contractile force for the cell
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| Event 3 of actin-based movement | the rest of the cell pulls against the anchorage points to drag itself forward
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| Event 4 of actin-based movement | actin depolymerises at the rear
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| Lamellipodia | is a cytoskeletal protein actin projection on the leading edge of the cell; sample the environment; extend and withdraw; generated by rapid growth of actin filaments at the cell membrane; the (+) end of actin filaments are oriented towards the periphery
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| Lamellipodia or filopdia touch down | they attach to the extracellular matrix through the formation of focal adhesions (focal contacts); actin filaments connect the focal adhesion to the rest of the cytoskeleton
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| Myosin | motor protein; pull on actin filaments to drag the cell forward; specially myosin II filaments; doesn't stay bound to actin all the time (unusual)
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| Myosin "head region" | interacts with actin and binds ATP; energy release from ATP hydrolysis forces the myosin tail to move, generating force;
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| Myosin head movement | ADP is released from the myosin head and replaced by ATP at which stage the head can detach from the actin filament; the head binds further down the filament
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| Microtubule based movement | cilia and flagella; microtubules slide along one another causing the cilium to bend
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| Dynein | a minus-ended microtubule associated motor protein;
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| Kinesin and dynein | involved in the movement of organelles, e.g. synaptic vesicles along axons to synapses; composed of heavy chain (binds to microtubule) and light chain (binds to what needs to be moved)
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| Kinesin | moves towards (+) ends (cell periphery); stays attached to the microtubule throughout the ATP hydrolysis cycle (unlike mysosin)
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| Dynein | moves toward (-) ends (near nucleus)
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| Vesicles move | 10cm/day which take more than a week down an entire axon
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| Processive motor protein | e.g. kinesin and dynein; capable of moving great distances along microtubules; stays bound to the microtubule
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| Non-processive motor protein | e.g. myosin II; detaches completely from actin filaments at the end of the cycle; will travel only short distances; constantly bound and un-bound depending on hydrolysis of ATP
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| Involves in separation of chromosomes during cell division (interphase, metaphase, telophase) | microtubules
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| Colchicine, vinblastine and taxol | anti-cancer therapeutics; inhibit the function of the mitotic spindle and thus cell division
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| Colchicine and vinblastine | destabilise microtubules; inhibits microtubule polymerization by binding to tubulin
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| Taxol | stabilises microtubules; also acts on tubulin
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| Dystrophin | rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane; mutations causes Duchenne and Becker Muscular Dystrophy
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| Myosin VII mutations | causes Usher's Syndrome; hereditary deafness and blindness
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| Epidermolysis bullosa symplex | disease of the intermediate filaments; mutations in keratin genes results in failure to form proper keratin filaments in epidermis; skin highly sensitive to mechanical energy; blistering in adults and sloughing of epidermis in newborns
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| Epidermolysis bullosa symplex (EBS) and muscular dystrophy | disease of the intermediate filaments caused my plectin mutations
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| Amyotrophic lateral sclerosis (ALS) or Motor Neuron disease | some hereditary forms are caused by mutations in neurofilamin genes
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| Microtubules | alongside amyloid plaques, AD brains display neurofibrillary tangles comprising MAP, Tau
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| Tau | hyperphosphorylated in tangles and cannot bind microtubules
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| Hereditary Spastic Paraplegia | most commonly caused by mutations in spastin
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| Spastin | microtubule severing protein
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| Listeria bacteria | hijack actin of cytoskeleton; engulfed by host cell; escapes from phagocytic vesicle; F-actin is polymerised at the back of it, providing motility; actin "comet" drives it into the neighbouring cell
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| Listeriosis | causes infections of the central nervous system (meningitis, meningoencephalitis, brain abscess, cerebritis) and bacteremia in those who are immunocompromised; from eating contaminated foods
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