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BIO1140 (690)

The Cytoskeleton

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Kathleen Gilmour

Topic 3: The Cytoskeleton -used for structure, support, and shape in terms of the cytoplasm of the cell (especially in animal cells) -used to organize the contents of the cell -organelles in a eukaryotic cell are placed according to cytoskeleton -important for movement within the cell and moving the cell itself (i.e.: cell division and actual movement of the cell) 1) Microtubules -the largest cytoskeletal element (in terms of diameter) -25 nm in diameter -like hollow straws -relatively stiff -function to support the cell and cell contents are draped over the microtubules -found in 2 places of the cell: 1) in the body of the cell -known as cytoplasmic microtubules -microtubule network in the body of the cell is dynamic and changes to meet the requirements of the cell 2) in cilia and flagella -known as axonemal microtubules -are very stable -structure of microtubules -microtubules are supermolecular structures -are made up of proteins which are built into a complex pattern -the protein building block of microtubules is tubulin -the 2 different forms of tubulin involved in microtubule formation include α and β tubulin -the basic building block is a heterodimer between these 2 isoforms -the 2 proteins are linked by non-covalent interactions but the bonding is strong -each of the 2 proteins can bind a molecule of GTP -the GTP attached to the a-tubulin is protected by being between the 2 proteins and remains as GTP -the GTP attached to the B-tubulin can be hydrolyzed to GDP -in order to form the microtubule, the heterodimers are lined up to form a protofilament (a linear arrangement of tubulin dimers) -the dimers always line up in the same orientation with a-tubulin at the bottom and B-tubulin at the top -new dimers are always added to the B-tubulin end and this end is called the + end -the a-tubulin end of the microtubule is called the - end -the 2 ends of the microtubule will always be different -it takes 13 protofilaments in a cylinder arrangement to make up 1 microtubule -the microtubule has polarity involving its +/- ends -microtubules are held together by the interaction of non-covalent bonds between the dimers and different microtubules are linked by non-covalent interactions as well -microtubules are thus susceptible to minor changes in B-tubulin when GTP is hydrolyzed to GDP -the dynamic nature of microtubules -B-tubulin with GTP bound to it is highly likely to polymerize and grow by joining a microtubule -once the GTP is hydrolyzed to GDP, a small conformation change occurs and this increases the likelihood of that dimer to dissociate from the polymer (break down) -GTP bound to tubulin = growth and formation of microtubule -GDP bound to tubulin = break down / dissociate from microtubule -as time goes on and the dimers sit in the microtubule, GTP is likely to be hydrolyzed to GDP -as the hydrolysis occurs, the microtubule is more likely to break down at the - end -if hydrolysis begins to occur near the +end, the microtubule will begin to break down -the presence of GTP-tubulin determines growth or dissociation of the microtubule -if very low levels of GTP-tubulin are present, the rate of addition to the +end won't be high and the +end will tend to become hydrolyzed -the microtubule will collapse at the +end due to hydrolyzation and lack of GTP tubulin -this is known as catastrophe -if there are high levels of GTP-tubulin present, the +end of the microtubule will grow quickly -dissociation may still occur at the -end but the +end will grow -growth at the +end and shrinkage at the -end can be perfectly balanced and this is known as treadmilling -the microtubule doesn't grow in length -because of the roles microtubules play in the cell, the cell needs to have control where microtubules are growing, how fast they are growing, and the patterns that they grow in -microtubule associated proteins (MAPs) help stabilize the microtubule -stabilize microtubules -control microtubule initiation -regulate concentration and availability of tubulin to regulate growth -can bind microtubules together (parallel or crosslink) -organize microtubules -link them into networks or bundle them into clusters -MAPs itself are regulated by other processes in the cell (i.e.: an activation of a receptor on the surface) -microtubules can be controlled using drugs -colchicine -a plant compound -sequesters (seizes) tubulin monomers and prevents them from polymerizing to form microtubules -preventing polymerization will cause a lack of growth or a collapse of the microtubule -taxol -stabilizes microtubules -stops microtubules from being dynamic -in chemotherapy, taxol can be used to stabilize the microtubules involved with cancer in order to inhibit cell division -all cells in the body that are exposed to taxol will experience this effect, but the cancer cells are most affected because of the inhibition of division in the rapidly dividing cells -the Dynamic Instability Model describes the idea of a dynamic balance between growth and shrinkage of the microtubule -in microtubules, GTP hydrolysis also results in the behavior known as dynamic instability, in which individual microtubules alternate between cycles of growth and shrinkage -whether a microtubule grows or shrinks is determined by the rate of tubulin addition relative to the rate of GTP hydrolysis -as long as new GTP-bound tubulin molecules are added more rapidly than GTP is hydrolyzed, the microtubule retains a GTP cap at its + end and microtubule growth continues -if the rate of polymerization slows, the GTP bound to tubulin at the +end of the microtubule will be hydrolyzed to GDP -if this occurs, the GDP-bound tubulin will dissociate, resulting in rapid depolymerization and shrinkage of the microtubule -Microtubule-Organizing Centres (MTOC) -increase the likelihood that microtubules will form by overcoming the difficulty of initiating the process -involve gama-tubulin (g-tubulin) -g-tubulin has a high likelihood of binding a-tubulin -g-tubulin forms a template -g-tubulin is arranged with other proteins into a microtubule template -it then binds a-tubulin to set the -end of the microtubule, thus initiating the process of microtubule formation -where MTOCs are found, -ends of microtubules are also found and this provides a means for the cell to organize microtubules within the cell -the centrosome is an example of a MTOC in animal cells -consist of centrioles (a pair at right angles) surrounded by pericentriolar material (includes g-tubulin) and tends to attract tubulin dimers, thus initiating the process of microtubule formation -where centrosomes / MTOCs are found in the cell, microtubules will be arranged around it with -ends arranged around the centrosome and the +ends radiating out from that -often, centrosomes are in the center of the cell and the microtubules move toward the periphery -e.g.: in a neuron, centrosomes can be found close to the cell nucleus and microtubules extend along the axon, with the +ends at the end of the axon -e.g.: the basal bodies that anchor cilia and flagella are an example of an MTOC - -ends at the basal end and +ends at the apical end of the flagella Functions of Microtubules -a key function of microtubules is the movement of cells or within a cell -microtubules within a cell form a highway for movement, but require something to transport things along the highway -motor proteins are the 'things' that move along the 'highways' -motor proteins convert chemical energy in ATP to movement / mechanical energy -convert ~60% of the energy from ATP into movement -consist of a motor domain (comparable to the engine in a car) and a tail (comparable to the body of the car) -the motor domain is responsible for breaking down ATP and for generating the movement (the stepping motion) -the tail hauls the 'cargo' that the motor protein is pulling -kinesin is the motor protein that works specifically with microtubules -involves 2 molecules with 2 domains altogether -the kinesin binds to the molecule and hydrolyzes ATP and a conformational change results which causes it to move along the microtubule -for each step it takes, it requires 1 molecule of ATP -the 2 motor proteins that walk along microtubules are: 1) Kinesins -involves a family of around 45 proteins with similar structure -always step toward the +end of the microtubule and are considered +end directed motor proteins -therefore, kinesins tend to move things to the periphery of the cell (outbound cargo) -i.e.: moving cargo from the golgi complex to the membrane 2) Dyneins -always step to the -end of the microtubule and are -end directed motor proteins -therefore, tend to move things toward the centre of the cell, where the MTOC and the -end exists (inbound cargo) -the use of motor proteins is evident in neurons: -cell body contains the nucleus and nutrients must be delivered across the axon -microtubule network provides a mechanism for this to occur (axonal transport) -vesicles, compounds, etc. move along the microtubules via motor proteins -kinesins move things away from the cell body -dynein move things toward the cell body -motor protein-driven movement in chromatophores results in the color change of fish, amphibians, and reptiles -chromatophores are pigment containing cells -pigment is bound in small, membrane-bound granules that move along microtubules -the pigment can move toward the periphery of the cell and so the pigment spreads throughout the cell -in toadfish, urea transporters are located in the vesicles -when the toadfish excrete urea, the urea vesicles are transported to the membrane, fused to the membrane, in order to insert the urea transporter in the membrane -this can be tested experimentally by using colchicine -if microtubules are involved, urea excretion should fall Microtubules and Cellular Motility -microtubules are found in cilia and flagella -cilia are short -flagella are long -there are typically many cilia -typically 1 or 2 flagellum -flagella have a wave-like / whip-like motion -cilia have a rowing, back and forth movement -flagella are always used to move the cell around -cilia can be used to move the cell around, but can also be attached to stationary cells and used to move things over the cell -cell membrane and a 9+2 arrangement of microtubules are found in both the flagella and cilia -the 9+2 arrangement involves 2 central microtubules surrounded by 9 doublets (a pair of microtubules) -1 microtubule in the doublet is complete (the A-microtubule) and the 2nd is partial (the B-microtubule) -everything is held together by protein spokes that go out from the centre to the doublets, as well as protein connections between the doublets (nexin) - -ends are at the basal body / origin of the cilia / flagellum and the +ends are at the tip of the cilia / flagellum -axonemal dynein is used to move the cilia / flagellum -it is a -end directed motor protein -different from cytoplasmic dynein -is attached to the A-tubules and reaches over to walk along the B-tubules of the adjacent doublet in the - direction and pulls the tubules against each other -because doublets are tied together by nexin links, the walking movement can't slide the microtubules very far and causes only bending -this allows the cilia / flagellum to bend -when this movement is coordinated so that it occurs from one side to the other, this creates the back and forth movement Microtubules and Mitosis -2 roles of microtubules in mitosis -in mitosis, centrosomes duplicate and the 2 centrosomes move to the poles of the cell (in animal cells) and this causes a rearrangement of microtubules in the cell - -ends of microtubules are at the centrosomes at each pole of the cell - microtubules extend out toward the centre of the cell from the centrosomes -if the microtubule (as they extend toward the centre of the cell) bumps into chromosomes, it will attach to the kinetochore (a cluster of proteins at the point where the 2 chromosomes are joined) -these are known as chinetochore microtubules and they attach to the chromosomes -if the microtubules (as they extend toward the centre of the cell) miss the chromosomes, then it will continue to extend until it meets with a microtubule coming from the opposite direction -these are known as polar microtubules -they extend from the poles towards the centre without hitting the chromosomes -these 2 types of microtubules have different roles in cell division -kinetochore microtubules are important in separating
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