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The Cytoskeleton

Biochemistry, Medicine, Phase 1

Cytoskeleton a network of protein filaments that extend throughout the cell
The cytoskeleton determines cell shape and polarity; tissue structure; adhesion; cell movement; intracellular movement (of vesicles and of chromosomes)
Three classes of protein polymer (or filament) actin, intermediate filaments, microtubules
Actin microfilaments; composed of actin-binding proteins; comprises 5% total protein
Microtubules composed of micro-tubule-associated proteins (MAPs)
Actin filaments (1) called F-actin (filamentous actin or microfilaments); are polymers of individual actin proteins called G-actin (globular actin)
Actin monomers G-actin (globular actin)
Actin filament (2) polarised double helix; 13 actin subunits for every complete turn
Diameter of actin filament 7nm
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
G-actin adds more rapidly to (+) end of the filament; once incorporated, ATP is hydrolysed to ADP
G-actin is removed more rapidly from (-) end of the filament
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
Cell migration happens a lot during embryogenesis but also adulthood; also pathogensis (metastasis); actin filaments turn over rapidly
Major function mechanical support; cell shape changes and maintenance; cell motility
Actin-binding proteins modulate polymerisation dynamics and function;
Membrane-attachment proteins spectrin; bind to G-actin
Severing proteins geisolin, severin; bind to F-actin
Cross-linkingproteins transgelin; bind to F-actin
Capping proteins prevent filament grown; bind to F-actin
Actin-sequestering proteins bind to G-actin and prevent its polymerising;
Actin-bundling protein alpha-actinin in muscle
Motor proteins myosin in muscle; bind to F-actin
Side-binding proteins interaction with other proteins; bind to F-actin
Intermediate filaments polymer of individual intermediate filament proteins; 10nm in diameter; visible by electro microscope; not dynamic
Intermediate filament network typically more dense around the nucleus; can extend into the periphery
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)
Intermediate filament names by cell type keratin, vimentin, glial fibriallary acidic protein, neurofilamin
Mice lack neurofilaments (NF+/-) reduces axon diameter
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
Tetramer fundamental unit of the intermediary filament; formed from two helical dimers
Growth of an intermediate filament slow, not dynamic; subunit exchange occurs throughout the length of the filament
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
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
Tubulin monomer of microtubules; consists of one molecule or alpha and beta-?
Alba tubulin negative charge
Beta tubulin positive charge
Cylindrical microtubule 25nm in diameter; thirteen columns of tubulin polymer
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
Microtubules are polymerised in centrosomes; minus end remains close the centrosome and the plus end points outward towards the cell periphery
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);
Microvilli Actin filament bundles provide for absorption in the gut byt forming an adhesion belt
Stereocilia contains actin filaments; detect vibration in the cochlea; cells are depolarised or hyperpolarised by deflections caused by sound; actin filaments keep them rigid
Shape of axons intermediate filaments and microfilaments support shape
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
Form meshwork around the cell nucleus hold it into position Intermediary filaments
Hold synaptic vesicles close to the presynaptic membrane actin filaments
Organise the ER of a cell microtubules
Anchoring cells cytoskeleton is essential; to extracellular matrix at cell junctions and to each other
Actin-based movement cell motility; e.g.migration of neutrophils (WBC) to sites of infection for phagocytosis F
Event 1 of actin-based movement cell pushes out protrusions at the front (leading edge); actin filament polymerisation provides to force of membrane protrusion
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
Event 3 of actin-based movement the rest of the cell pulls against the anchorage points to drag itself forward
Event 4 of actin-based movement actin depolymerises at the rear
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
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
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)
Myosin "head region" interacts with actin and binds ATP; energy release from ATP hydrolysis forces the myosin tail to move, generating force;
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
Microtubule based movement cilia and flagella; microtubules slide along one another causing the cilium to bend
Dynein a minus-ended microtubule associated motor protein;
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)
Kinesin moves towards (+) ends (cell periphery); stays attached to the microtubule throughout the ATP hydrolysis cycle (unlike mysosin)
Dynein moves toward (-) ends (near nucleus)
Vesicles move 10cm/day which take more than a week down an entire axon
Processive motor protein e.g. kinesin and dynein; capable of moving great distances along microtubules; stays bound to the microtubule
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
Involves in separation of chromosomes during cell division (interphase, metaphase, telophase) microtubules
Colchicine, vinblastine and taxol anti-cancer therapeutics; inhibit the function of the mitotic spindle and thus cell division
Colchicine and vinblastine destabilise microtubules; inhibits microtubule polymerization by binding to tubulin
Taxol stabilises microtubules; also acts on tubulin
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
Myosin VII mutations causes Usher's Syndrome; hereditary deafness and blindness
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
Epidermolysis bullosa symplex (EBS) and muscular dystrophy disease of the intermediate filaments caused my plectin mutations
Amyotrophic lateral sclerosis (ALS) or Motor Neuron disease some hereditary forms are caused by mutations in neurofilamin genes
Microtubules alongside amyloid plaques, AD brains display neurofibrillary tangles comprising MAP, Tau
Tau hyperphosphorylated in tangles and cannot bind microtubules
Hereditary Spastic Paraplegia most commonly caused by mutations in spastin
Spastin microtubule severing protein
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
Listeriosis causes infections of the central nervous system (meningitis, meningoencephalitis, brain abscess, cerebritis) and bacteremia in those who are immunocompromised; from eating contaminated foods
Created by: emmaallde



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