Cytoskeleton Notes PDF

6 Cytoskeleton By: Dr. Manar Mansour The Cytoskeleton The cytoskeleton: A network of fibers extending throughout the cytoplasm The cytoskeletal matrix is a dynamic structure ( assembled when needed and disassembled when not) Function:  Maintains cell shape and support the cel...

6 Cytoskeleton By: Dr. Manar Mansour The Cytoskeleton The cytoskeleton: A network of fibers extending throughout the cytoplasm The cytoskeletal matrix is a dynamic structure ( assembled when needed and disassembled when not) Function:  Maintains cell shape and support the cell structure.  Provides the mechanisms for cell movement.  Acts as tracks for “motor proteins” that help move materials within cells. 6-2  There are three major types of cytoskeletal components:  Actin filaments (= Microfilaments); ≈7 nm  Intermediate filaments ≈ 10 nm and  Microtubules; hollow tubes ≈ 25 nm. 6-3 There are three main types of protein fibers that make up the cytoskeleton 4 The Cytoskeleton Overview of the physical and biochemical functions of the three cytoskeletal systems in animal cells. a) Biophysical and biochemical properties shown for each filament. b) Culture cells stained for actin (green) and sites of actin attachment to the substratum (orange). c) Localization of microtubules (green) and Golgi apparatus (yellow). d) Cytokeratins, a type of intermediate filaments. 6-5  The Cytoskeleton Cell signaling regulates cytoskeleton function. Cells use cell-surface receptors to sense external signals from the extracellular matrix, other cells, or soluble factors. These signals are transmitted across the plasma membrane and activate specific cytosolic signaling pathways. Signals – often integrated from more than one receptor – lead to the organization of the cytoskeleton to provide cells with their shape, as well as to determine organelle distribution and movement. 6-6 1 – Microfilaments (actin filaments) Microfilaments are made of the protein actin, and may exist as single filaments( twisted double chain), in bundles, or in networks. Actin is a globular protein, or spheroproteins are spherical ("globe-like") proteins The structural role of microfilaments is to make cells bear tension (pulling forces) Microfilaments are needed for cell contraction, as in muscle cells, and add structure to the plasma membrane and shape to cells. They are involved in cytoplasmic streaming, and the formation of pseudopodia (amoeboid movement). Actin filaments also involved in animal cell division 6-7  Microfilaments Structures of F-actin filaments. An actin filament appears as two strands of subunits. One repeating unit consists of 28 subunits (14 in each strand, indicated by * for one strand), covering a distance of 72 nm. The ATP- binding is oriented in the same direction in all actin subunits in the Actin can be labeled by filament. The end of the filament with fluorescent phalloidin. an exposed binding cleft is the (-) end; the opposite end is the (+) end which In general, proteins may is more dynamic(elongates and shrinks be labeled by indirect fast). immunofluorescence.  Microfilaments Polymerization of G-actin in vitro occurs in three phases. (a) In the initial nucleation phase, ATP – G-actin monomers (red) slowly form stable complexes of actin (purple). These nuclei are rapidly elongated in the second phase by the addition of subunits on both ends of the filament. In the third phase, the ends of actin filaments are in a steady state with monomeric G- actin. (b) Time course of the in vitro polymerization reaction reveals the initial lag periods associated with nucleation, the elongation phase and steady state. (c) In some short stable actin filaments fragments are added at the start of the reaction to act as nuclei, elongation immediately starts without any lag period. 6-9 Microfilaments for Support Certain epithelial cells have microvilli, which enlarge the surface area. Microvilli are finger-like protrusions that are supported by bundels of actin microfilaments 6-10  Microfilaments Myosin is the motor protein of the actin system The myosin superfamily in human. Computer analysis of the relatedness of S1 head domains. ~ 40 myosin genes by the human genome. Indicated are examples in which loss of a specific myosin causes a disease. 6-11 Figure 16-54a Molecular Biology of the Cell (© Garland Science 2008)  Microfilaments Motor proteins: myosin Neither actin nor tubulin are contractile proteins ! Three common classes of myosin. The three indicated myosins(I, one headed; II, two headed) have different head domains and variable number of light chains, but all move toward the (+) end of actin filaments. 6-12  Microfilaments Sliding filament assay is used to detect myosin- powered movement. (a) In presence of ATP, the myosin heads (S1) walk towards the (+) end of the filament. (b) The photographs show the positions of three actin filaments (numbered 1, 2, and 3) at 30 second intervals recorded by video microscopy. The rate of filament movement can be determined from such recordings. 6-13  Microfilaments The sliding-filament model of contraction in striated muscle. The arrangement of thick myosin and thin actin filaments in the relaxed state is shown in the top diagram. In the presence of ATP and Ca2+, the myosin heads extending from the thick filaments walk toward the (+) ends of the thin filaments. Because the thin filaments are anchored at the Z disks (purple), movement of the myosin pulls the actin filaments towards the centre of the sarcomere, shortening its length in the contracted state, as shown in the bottom diagram. 6-14  Microfilaments Actin filaments are associated with myosin forming a contracting belt ( cleavage furrow) that pinches a dividing animal cell into two daughter cells. 6-15 2 – Intermediate filaments Intermediate filaments are found only in multicellular organisms, forming ropelike assemblages in cells. Intermediate filaments are the only components of the cytoskeleton made up of polymerized, true fibrous subunit proteins, as contrasted to the chains of globular subunits that make up microfilaments and microtubules They have two major structural functions: to stabilize the cell structure, and resist tension. In some cells, intermediate filaments maintain the positions of the nucleus and other organelles in the cell. 6-16 6-17 2- Intermediate filaments 6-18 3 – Microtubules Microtubules are hollow cylinders made from tubulin (Globular) protein subunits. Like actin filaments, microtubules are dynamic and grow faster at their plus ends compared with their minus ends. Microtubules provide a rigid intracellular skeleton (compression resisting) for some cells, and they function as tracks that motor proteins can move along in the cell. They regularly form and disassemble as the needs of the cell change. a-Tubulin b-Tubulin 6-19  Microtubules consist of tubulin heterodimers Certain drugs interfere with the polymerization or stability of microtubules Bind tubulin dimers and Bind microtubules prevents their and prevent polymerization depolymerization 6-20 From Alberts et al.: Molecular Biology of the Cell 3 – Microtubules Microtubules originate from a In animals, microtubules originate from structure called the microtubule centrosomes. organizing center (MTOC). Their plus ends grow outward, radiating throughout the cell. Although plant cells typically have hundreds of sites where microtubules start growing, most animal and fungal cells have just one site that is near the nucleus. The two centrioles inside a centrosome consist of microtubules As nine sets of triplets arranged in a ring. 6-21 Cilia are Made up of Microtubules Flagella and Cilia: Moving the Entire Cell Flagella are generally much longer than cilia, and the two structures differ in their abundance and pattern of movement; however, their construction is identical. 6-22 Motor Proteins Use Energy from ATP to Move Things 6-23 Motor Proteins Use Energy from ATP to Move Things (Part 1) 6-24  In function, microtubules are similar to actin filaments: They provide stability and are involved in movement. Motor Proteins Pull Vesicles Along the Tracks Microtubules Serve as Tracks for Vesicle Transport; when ATP is hydrolyzed by kinesin, the protein moves along microtubules in a directional manner: toward the plus end. 6-25 3 – Microtubules Motor proteins move along microtubules. In both cilia and flagella, the microtubules are cross- linked by spokes and the motor protein called dynein. Dynein changes its shape when energy is released from ATP. Many dynein molecules associate along the length of the microtubule pair. Dynein moves vesicles toward the (-) end of the microtubule. Kinesin, another motor protein, moves them toward the (+) end. Thus, the three motor proteins myosin, kinesin, and dynein use ATP to undergo conformational (shape) changes; the shape changes result in movement. 6-26 Extracellular Structures The plant cell wall is composed of cellulose fibers embedded in a matrix of other complex polysaccharides and proteins. The cell wall provides a rigid structure for the plasma membrane under turgor pressure, giving important support. It is a barrier to many fungi, bacteria, and other organisms that may cause plant diseases. 6-27 Extracellular Structures Multicellular animals have an extracellular matrix (ECM) composed of fibrous proteins, such as collagen, and other glycoproteins. Functions of the extracellular matrix:  Holds cells together in tissues.  Contributes to physical properties of tissue.  Helps filter material passing between tissues.  Helps orient cell movements.  Plays a role in chemical signaling. Epithelial cells, which line the human body cavities, have a basement membrane of extracellular material called the basal lamina. 6-28 The ECM The ECM is made up of glycoproteins and other macromolecules EXTRACELLULAR FLUID Polysaccharide Collagen A proteoglycan molecule complex Carbo- hydrates Core protein Fibronectin Proteoglycan Plasma molecule membrane Integrins Integrin Micro- CYTOPLASM filaments Figure 6.29 6-29 Intercellular Junctions Plasmodesmata  Are channels that perforate plant cell walls Cell walls Interior of cell Interior of cell Figure 6.30 0.5 µm Plasmodesmata Plasma membranes 6-30 Cell connections mediate cell-to-cell adhesion 6-31 Animals: Tight Junctions, Desmosomes, and Gap Junctions Types of intercellular junctions in animals TIGHT JUNCTIONS Tight junctions prevent fluid from moving Tight junction At tight junctions, the membranes of across a layer of cells neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continu- ous seals around the cells, tight junctions prevent leakage of extracellular fluid across Tight junctions 0.5 µm A layer of epithelial cells. DESMOSOMES Desmosome Intermediate Desmosomes (also called anchoring filaments junctions) function like rivets, fastening cells Together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. Gap junctions 1 µm GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions Extracellular consist of special membrane proteins that matrix surround a pore through which ions, sugars, Space Gap junction amino acids, and other small molecules may between Plasma membranes of adjacent cells pass. Gap junctions are necessary for commu- cells nication between cells in many types of tissues, Figure 6.31 0.1 µm including heart muscle and animal embryos. 6-32