Cytoskeletal Filaments PDF Lecture Notes

Figure 14.1 Cytoskeletal filaments https://app.jove.com/embed/player?id=11788&t=1&s=1&fpv=1 https://www.hhmi.org/beautifulbiology/scroll-and-tell/cytoskeleton https://learninglink.oup.com/access/cooper9e-instructor-resources#tag_chapter-13  The Cytoskeleton and Cell Motility Functions of...

Figure 14.1 Cytoskeletal filaments https://app.jove.com/embed/player?id=11788&t=1&s=1&fpv=1 https://www.hhmi.org/beautifulbiology/scroll-and-tell/cytoskeleton https://learninglink.oup.com/access/cooper9e-instructor-resources#tag_chapter-13  The Cytoskeleton and Cell Motility Functions of Cytoskeleton: A dynamic scaffold for structural support of the cell Framework for positioning organelles A network of tracks that direct movement of materials: organelles, vesicles, mRNA Force generating apparatus for cell movement a. Microfilaments b. microtubules c. intermediate filaments d. all Essential component of cell division machinery The cytoskeleton is a network of filamentous structures: - microtubules - intermediate filaments - microfilaments (actin filaments), Loading… each is a polymer of proteins subunits held together by non- covalent bonds. allows for rapid assembly and disassembly http://thepsychguru.com/wp-content/uploads/2012/10/image005.jpg Overview of the Major Functions of the Cytoskeleton The cytoskeleton has many roles: Serves as a scaffold in maintaining cell shape. Directs the movement of materials within the cell. Directs cellular locomotion. Contractile ring formation for cell division. Serves as an Structure and functions of the cytoskeleton internal framework to organize organelles within the cell.  Structure and Organization of Actin Filaments Learning Objectives dynamics of actin filaments and the roles of actin-binding proteins. the organization of actin filaments underlying the plasma membrane. Loading… structure and function of microvilli. remodeling of actin filaments and cell motility. https://www.hhmi.org/beautifulbiology/media-detail/lights-camera-actin#content-details Figure 14.2 Actin filaments Figure 14.3 Assembly and structure of actin filaments (Part 1) Actin monomers (G actin) polymerize to form actin filaments (F actin). The first step is the formation of dimers and trimers, which then grow by the addition of monomers the plus ends grow faster by the addition of ATP-actin monomers Structure of an actin monomer with ATP bound. Figure 14.4 Treadmilling and the role of ATP in actin filament polymerization Actin bound to ATP associates with the rapidly growing plus ends ATP is hydrolyzed to ADP ADP-actin dissociates from filaments more readily than ATP-actin. Actin monomers bound to ADP dissociate from the minus end, monomers bound to ATP are added to the plus end. Figure 14.5 Initiation and growth of actin filaments (Part 1) In the cell, polymerization and depolymerization are governed by actin- binding proteins Formins (green) initiate and stimulate the elongation of actin filaments. Profilin (purple) stimulates exchange of ADP for ATP Figure 14.5 Initiation and growth of actin filaments (Part 2) In the cell, polymerization and depolymerization are governed by actin-binding proteins The Arp2/3 complex binds to actin filaments near their plus ends and initiates the formation of branches. Animation 13.1 Figure 14.6 Stabilization of actin filaments Loading… Actin filaments can be stabilized by capping proteins that bind to their plus or minus ends and by filament-stabilizing proteins (e.g., tropomyosin) that bind along their length. FIGURE 13.7 Actin bundles and networks Actin bundles (arrowheads) projecting from the actin network (arrows) underlying the plasma membrane of a macrophage. The bundles support cell surface projections called microvilli (see FIGURE 13.14) and filopodia (see FIGURE 13.15). Actin-binding proteins can stabilize and organize filaments into bundles or networks. Figure 14.8 Filament severing by cofilin Cofilin binds to and severs actin filaments. The newly formed filament ends are available for polymerization or depolymerization Important for cell motility and cell division Figure 14.10 Association of the erythrocyte cortical cytoskeleton with the plasma membrane Actin and associated proteins form a network underlying the PM The spectrin-actin network is linked to the membrane by ankyrin (green), which binds to both spectrin (blue) and band 3. An additional link is provided by the binding of protein 4.1 to spectrin/actin junctions and to glycophorin (another abundant transmembrane protein). Figure 14.9 Structure of spectrin - structural basis for cytoskeleton on erythrocytes Spectrin is a tetramer consisting of two α and two β chains. Each β chain has a single actin-binding domain (ABD) at its amino terminus. Both α and β chains contain multiple repeats of α-helical spacer domains Provides structural basis for cytoskeleton on erythrocytes Figure 14.11 Stress fibers and focal adhesions Many cells have specialized regions of the plasma membrane that form contacts (focal adhesions) with the extracellular matrix. These regions serve as attachment sites for bundles of actin filaments (stress fibers) Fluorescence microscopy of fibroblasts. Actin stress fibers are stained magenta and focal adhesions are stained bright green with antibody against vinculin. Figure 14.12 Attachment of stress fibers to the plasma membrane at a focal adhesion Bundles of actin filaments (stress fibers) anchor cells to the extracellular matrix at sites of attachment Focal adhesions are mediated by the binding of integrins to the extracellular matrix. Stress fibers (bundles of actin filaments cross-linked by α-actinin) are bound to the cytoplasmic domain of integrins by complex associations involving a number of proteins. Figure 14.13 Attachment of actin filaments to adherens junctions Actin bundles are also anchored to regions of cell-cell contact Cell–cell contacts at adherens junctions are mediated by cadherins, which serve as sites for attachment of actin filaments. In sheets of epithelial cells, these junctions form a continuous belt of actin filaments around each cell. Figure 14.14 Microvilli Microvilli: Actin-based cell protrusions common to epithelial cells lining the intestine. Increase surface area for nutrient absorption https://www.hhmi.org/beautifulbiology/media-detail/microvilli-management - content- details https://www.hhmi.org/beautifulbiology/media-detail/actin-out-intestine#content-details FIGURE 13.15 Examples of cell surface projections involved in phagocytosis and movement (A) pseudopodia of a macrophage engulfing a bacteria during phagocytosis. (B) An amoeba with extended pseudopodia. (C) A tissue culture cell with lamellipodia (L) at the leading edge of a fibroblast and filopodia (arrow). Transient protrusions of PM driven by growth of actin filaments at the leading edge are responsible for phagocytosis and cell locomotion Figure 14.16 Cell migration mediated by actin movements The movement of cells across a surface can be viewed as three stages of coordinated movements: (1) extension of the leading edge (2) attachment of the leading edge to the substratum (3) retraction of the rear of the cell into the cell body. FIGURE 13.17 Actin filament remodeling at the leading edge New branches formed, existing filaments cleaved at leading edge Note check 1. 2. Focal adhesions vs Adherens junctions 3. What kinds of things can actin binding proteins do? 14.2 Myosin Motors Explain the molecular basis of muscle contraction. Figure 14.18 Structure of muscle cells Muscles composed of bundles of single large cells (muscle fibers). Form by cell fusion and contain multiple nuclei. Muscle fiber contains many myofibrils, which are bundles of actin (thin filaments) and myosin (thick filaments). Myofibrils organized into a chain of repeating sarcomeres, bounded by Z discs. Figure 14.19 Sliding filament model of muscle contraction- sarcomere Organization of actin (thin) and myosin (thick) filaments in a sarcomere. The actin filaments slide past the myosin filaments toward the middle of the sarcomere during contraction. Within each sarcomere, dark bands alternate with light bands and correspond to the presence or absence of myosin filaments. The I bands contain only thin filaments, the A bands contain thick filaments. The actin and myosin filaments overlap in at ends of the A band Middle region (called the H zone) contains only myosin. The actin filaments are attached at their plus ends to the Z disc. The myosin filaments are anchored at the M line in the middle of the sarcomere. Figure 14.19 Sliding filament model of muscle contraction- sarcomere During contraction, sarcomere shortens, bringing the Z discs closer. No change in the width of the A band, but I bands, and the H zone almost completely disappear. Changes due to actin and myosin filaments sliding past one another so that the actin filaments move into the A band and H zone. The molecular basis for contraction is the binding of myosin to actin filaments, allowing myosin to function as a motor that drives filament sliding. Figure 14.20 Myosin II The myosin II molecule. Two heavy chains and two pairs of light chains (called the essential and regulatory light chains). The heavy chains have globular head regions and long α-helical tails, which coil around each other to form dimers. FIGURE 13.21 Organization of actin and myosin filaments Loading… Thick filaments are formed by the association of several hundred myosin II molecules in a staggered array. The globular heads of myosin bind actin. The orientation of both actin and myosin filaments reverses at the M line, so their relative polarity is the same on both sides of the sarcomere. Molecules of protein titin extend from the Z disc to the M line and act as springs to keep the myosin filaments centered in the sarcomere. Figure 14.22 Model for myosin action The binding of ATP dissociates myosin from actin. Myosin is a motor ATP hydrolysis induces a conformational protein powered by change that alters the position of the ATP hydrolysis myosin head group. Myosin head binds to a new position on the actin filament and release of Pi. The return of the myosin head to its original conformation, coupled to the release of ADP, drives actin filament sliding. Figure 14.23 Association of tropomyosin and troponins with actin filaments The contraction of skeletal muscle is triggered by nerve impulses, which stimulate the release of Ca2+ from the sarcoplasmic reticulum. Tropomyosin binds lengthwise along actin filaments. Is associated with a complex of three troponins In the absence of Ca2+, the tropomyosin–troponin complex blocks the binding of myosin to actin. Binding of Ca2+ to TnC shifts the complex, relieving this inhibition and allowing myosin to bind and contraction to proceed. Video 13.2 FIGURE 13.24 Contractile assemblies in nonmuscle cells Examples of contractile assemblies in non-muscle cells: Stress fibers Adhesion belts Contractile ring of cytokinesis Both calcium ions and ATP are needed for muscle contraction. Why?  Microtubules Learning Objectives structure and dynamic instability of microtubules. how growth of microtubules is initiated within cells. microtubule-associated proteins regulate the organization of microtubules  The Cytoskeleton and Cell Motility Functions of Cytoskeleton: A dynamic scaffold for structural support of the cell Framework for positioning organelles A network of tracks that direct movement of materials: organelles, vesicles, mRNA Force generating apparatus for cell movement a. Microfilaments b. microtubules c. intermediate filaments d. all Essential component of cell division machinery  Structure of a Microtubule and its Subunits GTP/GDP GTP Figure 14.28 Structure of microtubules Dimers of α- and β-tubulin polymerize to form microtubules, which are composed of 13 protofilaments arranged around a hollow core. Figure 14.29 The role of GTP in microtubule polymerization Alpha-tubulin (green) always has bound GTP- not shown in this figure Tubulin dimers with GTP bound to β-tubulin (blue sphere) associate with the growing plus ends while they are in a flat sheet Sheet zips up into the mature microtubule behind the region of growth. After polymerization, GTP hydrolyzed to GDP (purple sphere). GDP-bound tubulin is less stable in the microtubule, the dimers at the minus end rapidly dissociate.