Cytoskeleton-RSK-Lecture2-4.pdf

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(1)M. Sc. Botany / Zoology / Microbiology SEMESTER - I COURSE – 101. CELL BIOLOGY UNIT - 3. LECTURES : 3 – 5. CYTOSKELETON DR. RAHUL KUNDU Associate Professor DEPARTMENT OF BIOSCIENCES SAURASHTRA UNIVERSITY RAJKOT 1.

(2) CYTOSKELETON is providing structural support to the cell, the cytoskeleton also functions in. cell motility and regulation. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 2.

(3) Structural Support • Mechanical support – Maintains shape. • Fibers act like a geodesic dome to stabilize and balance opposing forces • Provides anchorage for organelles • Dynamic – Dismantles in one spot and reassembles in another to change cell shape.

(4) • The cytoskeleton is a network of fibers extending throughout the cytoplasm. • The cytoskeleton organizes the structures and activities of the cell..

(5) • The cytoskeleton also plays a major role in cell motility. – This involves both changes in cell location and limited movements of parts of the cell.. • The cytoskeleton interacts with motor proteins. – In cilia and flagella motor proteins pull components of the cytoskeleton past each other. – This is also true in muscle cells..

(6) • Motor molecules also carry vesicles or organelles to various destinations along “monorails’ provided by the cytoskeleton. • Interactions of motor proteins and the cytoskeleton circulates materials within a cell via streaming. • Recently, evidence is accumulating that the cytoskeleton may transmit mechanical signals that rearrange the nucleoli and other structures.. There are three main types of fibers in the cytoskeleton:, Microfilaments (Actin Filaments), Intermediate Filaments (IF) and Microtubules (MT)..

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(8) Actin Filaments • Microfilaments, the thinnest class of the cytoskeletal fibers, are solid rods of the globular protein actin. – An actin microfilament consists of a twisted double chain of actin subunits. • Microfilaments are designed to resist tension. • With other proteins, they form a three-dimensional network just inside the plasma membrane.. The shape of the microvilli in this intestinal cell are supported by microfilaments, anchored to a network of intermediate filaments..

(9) Actin Filaments Monomers of the protein actin polymerize to form long, thin fibers. These are about 8 nm in diameter and, being the thinnest of the cytoskeletal filaments, are also called microfilaments. (In skeletal muscle fibers they are called "thin" filaments.) Some functions of actin filaments: form a band just beneath the plasma membrane that • provides mechanical strength to the cell • links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins • anchors the centrosomes at opposite poles of the cell during mitosis • pinches dividing animal cells apart during cytokinesis • generate cytoplasmic streaming in some cells. • generate locomotion in cells such as white blood cells and the amoeba. • interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of muscular contraction 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 9.

(10) Actin Microfilament Formation • Globular actin monomer ( g actin) polymerise to Filamentous actin ( f actin) – Cells approx 50:50 – Monomer can add to either (+ or - ) end • Faster at + end • Actin-ATP hydrolysed (ADP) following addition – Destabilises (like MT). 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 10.

(11) Actin Binding Protein Interactions. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 11.

(12) 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 12.

(13) • In muscle cells, thousands of actin filaments are arranged parallel to one another. • Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers. – Myosin molecules walk along the actin filament, pulling stacks of actin fibers together and shortening the cell..

(14) • In other cells, these actin-myosin aggregates are less organized but still cause localized contraction. – A contracting belt of microfilaments divides the cytoplasm of animals cells during cell division. – Localized contraction also drives amoeboid movement. • Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments..

(15) • In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. – This creates a circular flow of cytoplasm in the cell. – This speeds the distribution of materials within the cell..

(16) Cell Movement. • Whole or part of cell : -Amoeba, neutrophil, macrophages, Neuron processes • axon, dendrites : Common structures, Contraction • Intracellular transport. Motile Structures • Leading/Trailing Edge : extension/retraction, Actin nucleation • Lamellipodia : Sheet-like extensions • Filopodia : Thin protrusions • Integrins anchor to ECM. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 16.

(17) Cell Migration. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 17.

(18) Adhesive Functions Cell signalling • Modify cell cytoskeleton, • Activate intracellular Signalling pathways • Cell motility. Adherens Junctions • microfilaments anchor plaque that occurs under membrane of each cell • plaques not as dense also occur as hemiform. 1/22/2011. Adhesion Junctions. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 18.

(19) Adhesion Junctions • Locations – heart muscle, layers covering body organs, digestive tract. • Transmembrane proteins – Cadherin. • Adherens (cell-cell) – cadherin (E-cadherin) – Links to cadherin in. neighboring cell • Adherens (cell-matrix) – Integrin – Links to extracellular matrix 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 19.

(20) Adhesive Signalling. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 20.

(21) Actin Signalling • Rho – Family of small GTPases organize the actin cytoskeleton – Rho, RAC, CDC42 – Form different actin structures • Cell 1995 Apr 7;81(1):5362 • Wasp – Wiskott-Aldrich syndrome protein – a downstream effector – transfers signal from tyrosine kinase receptors and small GTPases to actin cytoskeleton. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 21.

(22) MF Associated Proteins • Tropomyosin – Reinforces MF – Different tropomyosins • Muscle/non-muscle • Regions • Structures – Phosphorylation • Changes location on MF Thin filaments showing positions of TM strands on actin (gold) in (a) the absence and (b) presence of Ca2+. In (c) TM strands associated with both positions are superimposed on actin for comparison. (b) Absence of Ca2+ TM (red) occupies a position on inner edge of outer domain of actin. (c) In Ca2+ TM (green) lies along outer edge of inner domain as found for negatively stained filaments. (From Xu et al., 1999, Biophysical J. 77, 985992) 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 22.

(23) Actin Motors - Myosin. • Myosins – Myosin I • All cells • One head domain – Binds actin – Myosin II • Muscle myosin – Also other cells • Dimer, 2 heads • Bind to each other to form myosin filament – Thick filament 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 23.

(24) INTERMEDIATE FILAMENTS (IF) • Intermediate filaments, intermediate in size at 8 - 12 nanometers, are specialized for bearing tension. – Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins. • Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. • They reinforce cell shape and fix organelle location.. Fig. 7.26.

(25) INTERMEDIATE FILAMENTS (IF) • These cytoplasmic fibers average 10 nm in diameter (and thus are "intermediate" in size between actin filaments (8 nm) and microtubules (25 nm)(as well as of the thick filaments of skeletal muscle fibers). • There are five major types of intermediate filament, each constructed from one or more proteins characteristic of it. • Despite their chemical diversity, intermediate filaments play similar roles in the cell: providing a supporting framework within the cell. For example, the nucleus is held within the cell by a basketlike network of intermediate filaments made of proteins called keratins. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 25.

(26) INTERMEDIATE FILAMENTS (IF) • Intermediate filaments of keratin is present in epithelial cells. • Different kinds of epithelia use different keratins to build their intermediate filaments. Over 20 different kinds of keratins have been found, although each kind of epithelial cell may use no more than 2 of them. • Up to 85% of the dry weight of squamous epithelial cells can consist of keratins.. Some other functions of intermediate filaments: • anchor the thick and thin filaments of muscle cells in a fixed position • provide mechanical strength to the long axons found in some neurons 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 26.

(27) MICROTUBULES (MT) •. • •. •. Microtubules, the thickest fibers, are hollow rods about 25 microns in diameter. – Microtubule fibers are constructed of the globular protein, tubulin, and they grow or shrink as more tubulin molecules are added or removed. They move chromosomes during cell division. Another function is as tracks that guide motor proteins carrying organelles to their destination. In many cells, microtubules grow out from a centrosome near the nucleus. These microtubules resist compression to the cell..



(30) Microtubules • are straight, hollow cylinders • have a diameter of about 25 nm • are variable in length but can grow 1000 times as long as they are thick • are built by the assembly of dimers of alpha tubulin and beta tubulin. • are found in both animal and plant cells • Microtubules grow at each end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP), and • shrink at each end by the release of tubulin dimers (depolymerization) • However, both processes always occur more rapidly at one end, called the plus end. The other, less active, end is the minus end. • Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along the microtubule. • Microtubule motors 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 30.








(38) There are two major groups of microtubule motors: •kinesins (most of these move toward the plus end of the microtubules) and •dyneins (which move toward the minus end). Some examples: •The rapid transport of organelles, like vesicles and mitochondia, along the axons of neurons takes place along microtubules with their plus ends pointed toward the end of the axon. The motors are kinesins. •The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle. Both kinesins and dyneins are used as motors as we shall see. Spindle fibers arise from a microtubule organizing center (MTOC). The MTOC in animal cells is the centrosome.. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 38.

(39) The Centrosome The centrosome is • located in the cytoplasm just outside the nucleus. • Just before mitosis, the centrosome duplicates. • The two centrosomes move apart until they are on opposite sides of the nucleus. • As mitosis proceeds, microtubules grow out from each centrosome with their plus ends growing toward the equatorial plate forming spindle fibers.. The photo shows microtubules growing in vitro from an isolated centrosome. The centrosome was supplied with a mixture of alpha and beta tubulin monomers. These spontaneously assembled into microtubules only in the presence of centrosomes.. 1/22/2011. Dr. Rahul Kundu | Cell Biology-Unit-3 | Lectures 3-5. 39.

(40) • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring.. • During cell division the centrioles replicate.. Fig. 7.22.

(41) • Microtubules are the central structural supports in cilia and flagella. – Both can move unicellular and small multicellular organisms by propelling water past the organism. – If these structures are anchored in a large structure, they move fluid over a surface. • For example, cilia sweep mucus carrying trapped debris from the lungs..

(42) • •. Cilia usually occur in large numbers on the cell surface. – They are about 0.25 microns in diameter and 2-20 microns long. There are usually just one or a few flagella per cell. – Flagella are the same width as cilia, but 10-200 microns long.. •. A flagellum has an undulatory movement. – Force is generated parallel to the flagellum’s axis.. •. Cilia move more like oars with alternating power and recovery strokes. – They generate force perpendicular to the cilia’s axis..

(43) •. In spite of their differences, both cilia and flagella have the same ultrastructure. – Both have a core of microtubules sheathed by the plasma membrane. – Nine doublets of microtubules arranged around a pair at the center, the “9 + 2” pattern. – Flexible “wheels” of proteins connect outer doublets to each other and to the core. – The outer doublets are also connected by motor proteins. – The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole..

(44) • The bending of cilia and flagella is driven by the arms of a motor protein, dynein. – Addition to dynein of a phosphate group from ATP and its removal causes conformation changes in the protein. – Dynein arms alternately grab, move, and release the outer microtubules. – Protein cross-links limit sliding and the force is expressed as bending.. Fig. 7.25.





(49) Next Lecture. JUNCTIONS BETWEEN CELLS. THANKS 22 January 2011. Dr. Rahul Kundu: Cell Biology: Lectures - 1-2. 49.

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