Cytoskeleton Handout PDF

8-Cytoskeleton OVERVIEW OF LECTURES FOR TOPIC 8: CYTOSKELETON Reading Ch. 17- Relevant Parts Prior Learning: Campbell pp. 112-118 A. Overview B. Intermediate Filaments a. Structure b. Examples C. Mic...

8-Cytoskeleton OVERVIEW OF LECTURES FOR TOPIC 8: CYTOSKELETON Reading Ch. 17- Relevant Parts Prior Learning: Campbell pp. 112-118 A. Overview B. Intermediate Filaments a. Structure b. Examples C. Microtubules a. Structure b. Self Assembly c. Drugs that affect Microtubules d. Microtubule Functions i. stabilization of cell shape ii. subcellular motility iii. motility at the cellular level flagella and cilia D. Actin Microfilaments a. Structure b. Self Assembly c. Functions d. Myosin & Muscle Cells 8-1 Cell Biology  8-Cytoskeleton Topic 8 Cytoskeleton A. Overview The cytoskeleton consists of a network of ________________________________ protein filaments that provide Structure support + movement tw in the whole all __________________________________________________________________for , , the cell. These filaments are also involved in the systems that enable cells to move 3 types of cytoskeletal filaments: · microtubules - MT actin filaments/micofilaments MF - · intermediate filaments - IF Filaments interact with many other proteins, including ___________________ motor proteins that use energy from ATP to move various cargoes (including vesicles) along tracks made of MT or MF. MT and MF are ___________________________ dynamic undergoing constant polymerization and depolymerization 1. Intermediate Filaments fibrous that - system of proteins Strengthen cells - transmit mechanical stress across alls in a tissue. NOT DyNAMIC /don't change in size) * purely structural 2. Microtubules - rigid , hollow tubes of protein in division · form spindle apparatus all involved in generating all shape + organizing movement. 3. Actin Microfilaments capable of forming the most complex · configurations · Interact in a large raiety of proteins to do so. · involved in muscle contractions. 8-2 Cell Biology  8-Cytoskeleton B. Intermediate Filaments veratin , lamin g e.. 1. Structure of IFs Made of different but closely related proteins Protein type depends on function and cell type, but all IFs have similar structure subunit I = a filaments wrapped - around each other tetramers y - dimers aligh in a staggered configuration end up in a large comm protein Fibre > - very strong 2. Examples of different IF proteins and their functions ↳ When intermediate filaments misfunction: Epidermolysis Bullosa a mutation in one of the feratin genes causes disease. * have weakened IF. has been treated * recently successfully in transgenic skin grafts ! 8-3 Cell Biology  called desmosomes connections 8-Cytoskeleton # · weakened If do not transmit mechanical stress among stress among skin epithelial alls desmosomes usually points of between · contact strong cells + act live Connected to IF +:: IF are snaps. continuous through cells. * causes blistering + tearing of skin cells if IF are weakened. C. Microtubules 2 Types of microtubules are recognized in cells. dynamic MT-loosely organized + Cytoplasmic · MT-highly organized + static · axonemal * found in ciliatflagella (in eukayotes) * found in centrioles 1. Microtubule Structure Microtubules are organized complexes of tubulin protein dimers. tubulin-X B tubulin > - together make dimer. 2 types of + , up a - Dimers assemble end to end into a protofilament. form 13 protofilaments align in a ring to MT Microtubules are rigid, hollow structures. have different ends MT Btubulin minus end (t > - end wd tubulin exposed = (H) end > - end i exposed = NOTE In : RT , (t) and It do not mean charge ! for movement of motor * (t) and () ends are important proteins ↑ L tubulin 8-4 Cell Biology  no enzymes needed ! 8-Cytoskeleton - 2. Self-Assembly of Microtubules a) in vitro MTs arise from self-assembly of subunits a well studied process in vitro - If a high concentration of purified tubulin is placed in a test tube with GTP and Mg +2, microtubules will spontaneously form. can measure rate of MT formation using spectrophotometry scattered ↓ absorbance &. light of + as [MTJ & the amount Platear - Texponential % of tubulin incoporated MT - into lag phaal time b) in vivo the concentration of tubulin is lower, and microtubules require help to get started. from structures where most MT originate MT organizing centers = MTOC -> A microtubules growing in animal cells the MTOC = Centrosome e anchors end. g rings , one of the MT Contains of -.. Olgamma)- tubulin which gives the N/B divers a place to start assembly from The (-) end of MT is embedded end in the ring , and (t) grows from there. always Dynamic Instability of Microtubules and the Role of GTP: growing + Cytoplasmic MT exhibit dynamic instability continuous growth and Shrinking shrinkage of MT through addition and loss of subunits from both ends, although tubulin is added more rapidly to the + end of MT. GTP is required for microtubule assembly each tubulin dimer binds to GTP. · when GTP is bound to tubulin that dimer is efficient in , binding to the end of a MT -+ growth is quick once GTP tubulin is incorporated into GTP is hydrolyzed · a MT , to GDP. e * GDP tubulin has8-5 binding Stable , as long as tubulin less affinity for other tubulin , and Cell Biology at is not as strong. end has GTP bound.  - eventually GtP on end is hydrolyzed to GDP. 8-Cytoskeleton Here, GTP tubulin is less abundant. The end of the microtubutle has had time for its GTP to hydrolyze to GDP no GTP cap is left. GDP tubulin starts to fall of end of the MT = MT Shrinkage MT catastrophe when MT starts shrinking · occurs rapidly MT is. rescued if GTP-tobulin is increased 3. Drugs that Affect Microtubules Colchicine (isolated from Autumn crocus, Colchicum autumnale), vinblastine and vincristine (from periwinkle - - Vinca minor) stop microtibole growth by binding to tubulin Taxol (from Pacific yew, Taxus brevifolis) & MT growth Vinblastine, vincristine and taxol are used as anti-cancer drugs because of their effect on the mitotic spindle of rapidly dividing cells. These drugs are effective because constant assembly and disassembly is required for normal MT function in ne cells. 4. Microtubule Function Microtubules perform multiple functions in the cell 2 general categories of MT function: Different accessory proteins (MAPs) that bind MTs allow this diversity of function a) stabilization of cell shape Microtubules are stabilized by capping proteins Capping proteins bind to the + end of the MT and block disassembly Stabilization of selected MT can polarize the cell Capping proteins are used to form polarized cell structures (i.e. structures with direction) containing microtubules oriented in a certain direction e.g. in nerve axons 8-6 Cell Biology  8-Cytoskeleton m MT also organize how cellulose is deposited in plant cell walls b) motility at the subcellular level rganelles to move along. 1 Motor Proteins head interacts in MT > - Globular head has ATPase activity o cleavage of ATP results in change of protein conformation and movement along MT; motor protein tail associates with the cargo o 2 families of motor proteins: kinesins (move away from centrosome) and dyneins move from plus to minus ↓ * (t) to(-) from 1) to (t) reside fail of Motor protein interacts in or moved. organelle being MT golgi Movement of Organelles 1 F MF 8-7 Cell Biology  8-Cytoskeleton c) Motility at the cellular level flagella and cilia Hairlike structures that project out from the cell surface on some eukaryotic cells Shorter structures are cilia o Cilia function Longer structures are flagella o Flagella function Structure of Cilia and Flagella involves axonemal microtubules in a 9+2 arrangement. X-section - axonemal MT and not > - organized - dynamic 9 doublets in ring a + 2 central MT Proteins dynein and nexin are also part of the axoneme. movement is in a motor proteins Nexin binds the microtubule doublets together it is a flexible protein dynein Dyneins (motor protein) move along the MT and cause the whole structure to bend dynein ↓ in · an axonne , MT are held in linked to each other place + if MT - · When motor proteins move , MT not linked doublets bend rather than slide MT get sliding 8-8 Cell Biology  8-Cytoskeleton D. Actin Microfilaments Major components of the peripheral cytoskeleton cell cortex, and muscle Similarities to Microtubules · exhibit dynamic instability · actin subunits interact Aip in to stabilize ends also has a different ends > - It end and It end Differences from Microtubules · smaller + not a tube 2 activ filaments wound around each other. abundant 130 x More MF than MT most component of cytoskeleton - 1. Microfilament polymerization note similarities to Microtubule assembly 6-actin (globular o Actin monomers are called ______________________________________ U G-actin is __________________ ATP and has ______________bound to it when in solution. o shape o double helix Monomers assemble into a ___________________ (= 7nm filament) with polarity similar to MTs o F-actin (filamentous) Once assembled into a MF, the actin is referred to as _____________________________ o ATP is hydrolyzed to ADP. After G-actin is incorporated into a MF, _______________________________________ ADP bound () bound end. (+)- ATP o This reduces the affinity between monomers and reduces stability of the polymer. o The end with ___________________________ ATP (t) end (= ATP cap) has more rapid assembly ________________ o ADP is bound The end of the MF where ____________________________ 1-) end is less stable ___________________ o Rate of growth is faster at the plus end - * more efficient o Assembly /dissasembly is essential to the function of actin MF o Evidence from action of drugs: o Cytochalasin ___________________________________ prevents MF polymerization o Phalloidin ____________________________________________ stabilizes MF stops ,depolymerization 8-9 Cell Biology  8-Cytoskeleton of MF in different all structures 2. Microfilament function arrangement mesh f cell 7 bundles division MF off (gives sell * ring of pinches Shape to divide cells Actin Binding Proteins allow MF to be organized in many ways G- bind G-actin + it from stop binds to ends of adding on to MF ↑ MF + stabilize then stop growth ↳ create parallel arrangement ↳ e g.. myosin create mesh e. g. Hopomyosin in muscles ore 8-10 ! Cell Biology