Lect 20-24: Cytoskeleton and Cell Motility PDF

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...

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 Loading… GTP Figure 14.28 Structure of microtubules Dimers of α- and β-tubulin polymerize to form microtubules, which are composed of 13 ____________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 Loading… 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 ______ (purple sphere). GDP-bound tubulin is less stable in the microtubule, the dimers at the minus end rapidly dissociate. Figure 14.30 Dynamic instability of microtubules Dynamic instability results from hydrolysis of GTP bound to β-tubulin after polymerization. GDP-β-tubulin dimer has less binding affinity for adjacent than GTP-β-tubulin Growth: new GTP-bound tubulin molecules are added more rapidly than GTP is hydrolyzed, so a GTP cap is retained. Shrinkage: GTP is hydrolyzed more rapidly than new subunits are added Figure 14.31 Roles of microtubule-associated proteins in dynamic instability (Part 1) Polymerases accelerate growth by increasing incorporation of GTP-bound tubulin. Polymerase is a MAP Figure 14.31 Roles of microtubule-associated proteins in dynamic instability (Part 2) Depolymerases: GTP- tubulin from the plus end, leading to microtubule shrinkage 13. 3 Figure 14.31 Roles of microtubule-associated proteins in dynamic instability (Part 3) CLASP proteins rescue microtubules from catastrophe by stopping disassembly and restarting growth. Microtubule instability JOVE Figure 14.32 Intracellular organization of microtubules The minus ends of microtubules are anchored in the centrosome (an MTOC). In interphase cells, the centrosome is located near the nucleus and microtubules extend outward to the cell periphery. During mitosis, duplicated centrosomes separate and microtubules reorganize to form the mitotic spindle. Figure 14.34 Structure of centrosomes Loading… microtubules radiating from the pericentriolar X- section of a centriole illustrating material that surrounds a pair of centrioles. its nine triplets of microtubules Figure 14.33 Growth of microtubules from the centrosome A. Microtubules an interphase mouse fibroblasts B. Cell treated with colcemid for 1 hour to disassemble microtubules. The drug was then removed and the cell allowed to recover for 30 minutes, allowing the visualization of new microtubules growing out of the centrosome. Figure 14.35 Organization of microtubules in nerve cells Stable microtubules in both axons and dendrites terminate in the cytoplasm rather than being anchored in the centrosome. Dendrites: microtubules oriented in both directions Axons: microtubules oriented with their plus ends pointing toward the tip of the axon. MAPs: cap both the plus and minus ends, as well as stabilize microtubules by binding along their length (MAP2 in dendrites and tau in axons). Figure 14.36 Microtubule motor proteins Dynein and kinesin I move in opposite directions along microtubules. The globular head domains of the heavy chains bind microtubules and are the motor domains of the molecule. Light chains bind to cargo Figure 14.37 Transport of vesicles along microtubules Kinesin I and other plus-end–directed kinesins transport vesicles and organelles in the direction of microtubule plus ends, which extend toward the cell periphery. Dynein and minus-end–directed members of the kinesin family carry their cargo in the direction of microtubule minus ends, microtubules are anchored in the center of the cell. Figure 14.38 Association of the endoplasmic reticulum with microtubules ER (A) and microtubules (B) in an epithelial cell. Note the close correlation 17.3 17.5 Figure 14.39 Examples of cilia and flagella – microtubule based (A) SEM of cilia covering the surface of Paramecium. (B) SEM of ciliated epithelial cells lining the surface of a trachea. (C) Multiple-flash photograph (500 flashes per second) showing the wavelike movement of a sea urchin sperm flagellum. Video 13.6 Figure 14.40 Structures of primary and motile cilia Primary and motile cilia are anchored in basal bodies, which contain nine triplets of microtubules. Two of the microtubules in each triplet extend to form the axoneme. The axoneme of motile cilia contains an additional central pair of microtubules. The outer doublets are joined to each other by nexin links and to the central pair of microtubules by radial spokes. Each outer microtubule doublet is associated with inner and outer dynein arms. Do not memorize Figure 14.42 Electron micrograph of the mitotic spindle Figure 14.43 Formation of the mitotic spindle After nuclear breakdown, microtubules reorganize to form the mitotic spindle Kinetochore microtubules are attached to the kinetochores of condensed chromosomes. Interpolar microtubules overlap with each other in the center of the cell, and astral microtubules extend outward to the cell periphery. Figure 14.44 Anaphase A- chromosome movement Chromosomes move toward the spindle poles along the kinetochore microtubules. Movement driven by kinesins which act to depolymerize and shorten the microtubules. Figure 14.45 Spindle pole separation in anaphase B The separation of spindle poles : two types of movement. interpolar microtubules slide past each, push the spindle poles apart. Driven by plus- end–directed motor proteins. The spindle poles separate via astral microtubules - driven by minus-end– directed motors anchored to cell cortex  Note Check: 1. Colchicine is a drug that promotes microtubule disassembly. Suggest an experiment to see if microtubules are important for golgi localization. 2. Name a similarity and a difference between dynein and kinesin I - end tend 3. Describe the structure of microtubules to your classmate, starting with the tubulin dimer  Intermediate Filaments Intermediate filaments (IFs)– flexible rope-like fibers. Provide mechanical strength to cells subjected to physical stress, including muscle cells, neurons, epithelial cells E.g.: Keratins, Lamins Consist of a heterogeneous group of proteins, divided into five major classes. IFs classes I–IV are used in the construction of filaments; type V (lamins) are present in the inner lining of the nucleus. IFs are often connected to other cytoskeletal filaments by protein cross-bridges. Table 14.1 Intermediate Filament Proteins Keratin in an epithelial cell Figure 14.46 Structure and assembly of intermediate filaments The central rod domains of two polypeptides wind around each other to form dimers. Dimers then associate in a staggered antiparallel fashion to form tetramers. Tetramers associate end to end to form protofilaments and laterally to form filaments. Each filament contains approximately eight protofilaments wound around each other in a ropelike structure. Figure 14.48 Attachment of intermediate filaments to desmosomes and hemidesmosomes Desmosomes- Regions of contact between epithelial cells -involving IFs The desmosomal cadherins (desmoglein and desmocollin) link adjoining cells to intermediate filaments Hemidesmosomes - regions of contact between cells and the ECM - involving IFs integrins link the extracellular matrix to intermediate filaments through plectin. IF function Skin from a normal mouse Skin from mouse with mutant keratin. Note severe disruption of layers by mechanical trauma. Resembles a human genetic skin disease Extracellular Matrix (ECM)- learning goals 3 major ECM components: Section 15.2 Matrix structural proteins Collagens Matrix polysaccharides GAGs Cell–Matrix Interactions Learning Loading… Proteoglycans Objectives Integrins Adhesion proteins Focal adhesions Fibronectin hemidesmosomes laminins Figure 16.8 Examples of extracellular matrix ECM fill spaces between cells and binds cells and tissues together Several types with various secreted proteins and polysaccharides Basal Lamina (ex) supports epithelial cells, surrounds muscle cells, fat cells, others ECM most abundant in connective tissue Sheets of epithelial cells rest on a thin layer of ECM called a basal lamina (BL). Beneath the BL is connective tissue, which consists largely of ECM secreted by fibroblasts. The ECM contains fibrous structural proteins in a gel-like polysaccharide ground substance. Figure 16.9 Structure of collagen The major structural proteins of the extracellular matrix are members of the collagen family. The amino acid sequence of a collagen triple helix domain consists of Gly-X- Y repeats, in which X is frequently proline and Y is frequently hydroxyproline (Hyp). Why did sailors in the 1800s (and earlier) get scurvy? Figure 16.10 Collagen fibrils in connective tissue Collagen molecules assemble to form fibrils. The molecules overlap to leave a short gap between the N-terminus of one molecule and the C-terminus of the next. covalent cross-links between side chains of lysine or hydroxylysine residues strengthen the assembly Type IV collagen of basal laminae forms networks rather than fibrils The Gly-X-Y repeat structure of type IV collagen (yellow) is interrupted by multiple nonhelical sequences. More flexible and forms networks instead of fibrils Major types of glycosaminoglycans (GAGs) Fibrous structural proteins (like collagen) are embedded in polysaccharide gels GAGs negatively charged Bind positive ions and trap water to form hydrated gels Provide mechanical support and cushioning to the matrix Glycosaminoglycans (GAGs) rerepeating disaccharide units. Figure 16.13 A proteoglycan. (GAGs plus core protein) Contain long chains of GAGs joined to a core protein. Each GAG can contain up to 100 sugar residues and as many as 100 GAGs may be linked to a core protein. Figure 16.14 Structure of fibronectin- major adhesion protein Adhesion proteins link components of matrix to each other and to surface of cells Figure 16.15 Laminins – major adhesion proteins of basal laminae (A) Laminins consist of three polypeptide chains (B) Laminins assemble into a network. (C) A laminin network in the basal lamina surrounding a nerve Figure 16.16 Structure of integrins Major receptor responsible for Attachment of cells to ECM Bind to short amino acid sequences present on components of the ECM, including collagen, fibronectin, laminin, and proteoglycans. Figure 16.17 Cell–matrix junctions mediated by integrins Integrins also mediate two types of stable junctions in which the cytoskeleton is linked to the extracellular matrix via accessory proteins. Actin-Integrin-ECM IF-Integrin-ECM