PM132 Cytoskeleton and movement DG 2024 PDF

The Cytoskeleton: the movers and shapers in the cell Deya Gonzalez Professor of Molecular Medicine Reproductive Biology and Gynaecological Oncology Group ILS2 building, 2nd floor room 210 d.gonz...

The Cytoskeleton: the movers and shapers in the cell Deya Gonzalez Professor of Molecular Medicine Reproductive Biology and Gynaecological Oncology Group ILS2 building, 2nd floor room 210 [email protected]  Cytoskeleton The cytoskeleton is a network of fibers extending throughout the cytoplasm It organizes the cell’s structures and activities, anchoring many organelles It is composed of three types of molecular structures: -Microtubules -Microfilaments -Intermediate filaments  Roles of the Cytoskeleton: Support, Motility, and Regulation ❑ The cytoskeleton helps to support the cell and maintain its shape ❑ It interacts with motor proteins to produce motility ❑ Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton ❑ Essential component of the cell division machinery ❑ Recent evidence suggests that the cytoskeleton may help regulate biochemical activities Cell biology, Gerald Karp, chapter 13. Functions Functions Functions o Organization and maintenance of animal cell o Structural support o Muscle contraction shape and polarity o Maintenance of animal shape o Cell locomotion o Chromosome movements o Formation of nuclear lamina and scaffolding o Cytoplasmic streaming o Intracellular transport/trafficking and o Strengthening of nerve cell axons o Cytokinesis movement of organelles o Maintenance of animal shape (neurofilament protein) o Axonemal: cell motility o Keeping muscle fibers in register (desmin) o Intracellular transport/trafficking Cytoskeletal filaments are dynamic and adaptable Microtubules  Microtubules ❑ Microtubules participate in a wide variety of cell activities. ❑ Most involve motion that is provided by protein “motors” that use ATP. ❑ They determine the positions of membrane-enclosed organelles and direct intracellular transport. ❑ The migration of chromosomes during mitosis and meiosis takes place on microtubules that make up the spindle fibers.  The structure of a microtubule and its subunit Adapted from Essential Cell Biology (© Garland Science 2010)  GTP hydrolysis controls the dynamic instability of microtubules A. Growing microtubule B. Shrinking microtubule +GTP Cytosol pool of GDP-tubulin dimers GTP-tubulin dimers Adapted from Essential Cell Biology (© Garland Science 2010) ❑ Each microtubule grows and shrinks independently of its neighbours. Microtubule-binding proteins (MAPs) organize microtubules and affect their stability. Some MAPs prevent or promote cytosolic microtubule polymerization; other MAPs organize microtubules into bundles or cross-link them to membranes and intermediate filaments or both.  Microtubules guide the transport of organelles, vesicles and macromolecules MT guides intracellular transport in both directions along the nerve axon. Microtubules and Motor proteins position organelles in the cytoplasma. Kinesins bind the Endoplasmatic reticulum from its point of connection with the nucleus, stretching it along microtubules like a net. Cytoplasmic dyneins attached to Golgi membranes pull the Golgi along microtubules inward toward the nucleus Kinesins and cytoplasmic Dyneins are MT motor proteins that move on opposite direction along the MT Each protein forms dimers with globular heads, which bind and hydrolise ATP, and interact with MT The single tail interacts with the cargo.  Cilia and Flagella contains stable MT moved by Dynein Cilia cilia Scanning electron microscopic photograph of ciliated and secretory cells within the human Fallopian tube epithelium. Flagella flagellum The bending of cilia and flagella is driven by the arms of a motor protein, dynein. Microfilaments  Microfilaments Some functions of actin filaments are: ❑ to provide mechanical strength to the cell by forming a band under the plasma membrane ❑ link transmembrane proteins to cytoplasmic proteins ❑ form contractile ring during cytokinesis in animal cells ❑ cytoplasmic streaming ❑ generate locomotion in cells such as white blood cells and amoeba ❑ Interact with myosin to provide force of muscular contraction  G actin monomers polymerise into F-actin Microfilaments The actin subunit is a single globular polypeptide chain and is thus a monomer Like tubulin, each actin subunit has binding site for a nucleotide , but for actin is ATP or ADP As for tubulin, actin subunits assemble head to tail to generate filaments with structural polarity Microfilaments are designed to resist tension. With other proteins, they form a three-dimensional network just inside the plasma membrane. Actin polymerization is similar to tubulin MT but involves ATP hydrolysis instead of GTP  Actin binding proteins (ABPs) 3 groups: ▪ banding and cross linking proteins ▪ regulatory proteins: Organisation of polymerization/depolymerization, actin filaments severing proteins,capping proteins ▪ Motor proteins - sliding on MF (myosin) Sliding Actin-binding proteins regulate the organization of Actin Actin Filaments are often nucleated at the plasma membrane Consequently, the highest density of actin filaments in most cells is at the cell periphery Actin associates with Thicker filaments, composed of a motor protein, Myosin to form contractile structures  -Myosin-I is the simplest myosin -Its globular head attaches to actin filaments and a tail attaches to vesicles or organelles -the tail may bind to the plasma membrane and pull it to a different shape Figure 17-36 Essential Cell Biology (© Garland Science 2010)  Myosin-II molecules can associate with one another to form myosin-II filaments Actin filaments A small bipolar myosin-II filament can slide two actin filaments of opposite orientation. This sliding movement mediates contraction of interacting actin and Actin filaments myosin filaments in both muscle and non-muscle cells (C) Essential Cell Biology Muscles contract by a sliding-filament mechanism In muscle cells, thousands of actin filaments are arranged parallel to one another. Myosin II, interdigitate with the thinner actin fibers. Intermediate Filaments Intermediate Filaments ❑ Intermediate filaments provide mechanical strength and resistance to shear stress ❑ There are several types of intermediate filaments, each constructed from one or more proteins characteristic of it. o Keratins are found in epithelial cells, hair and nails o Nuclear lamins form a meshwork that stabilizes the inner nuclear membrane o Neurofilaments strengthen the long axons of neurons o Vimentins provide mechanical strength to muscle and other cells CLASSES OF INTERMEDIATE FILAMENTS Model for IF assembly in vitro A) two polypeptides twist around each other to form a two-chain coiled coil B) two dimers align laterally to form a tetrameric protofilament C) Protofilaments assemble into larger filaments by end-to-end and side-to-side alignment D) The fully assembled IF is thought to be eight protofilaments thick at any point  Intermediate filaments, are intermediate in size at 8 - 12 nanometers, are specialized for bearing tension. Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. They reinforce cell shape and fix organelle location.  Disorders associated with Mutations in Genes encoding Cytoskeletal proteins Microtubules: ❑ Ciliary and Flagellar diseases including Kartagener's syndrome and Multiple Morphological Abnormalities of the sperm Flagella ❑ Lissencephaly ❑ Amyotrophic lateral sclerosis 22 ❑ Alzheimer disease Microfilaments ❑ Myopathy and Cardiomyopathy ❑ Malignant tumors including lung, breast, liver, prostate, colorectal and salivary glands cancers Intermediate Filaments ❑ Skin diseases including epidermolysis bullosa ❑ Amyotrophic lateral sclerosis (motor neuron disease) ❑ Cardiomyopathies ❑ Liver diseases ❑ Inflammatory Bowel Disease IBD ❑ Cataract, Crohn’s disease, rheumatoid arthritis, and human immunodeficiency virus  Clinical applications Drugs that inhibit microfilaments are used in biomedical research Actin-specific drugs Phalloidin binds and stabilises filaments Cytochalasin cap filament plus ends Latrunculin prevents polymerisation Swinholide severs filaments Therapeutic drugs that inhibit Microtubules Microtubule-specific drugs Taxol binds and stabilises microtubules Colchicine Prevents tubulin polymerisation Vinblastine, vincristine Prevents tubulin polymerisation Auristatins Prevents tubulin polymerisation Maytansinoid Prevents tubulin polymerisation Main components of the eukaryotic cytoskeleton Microfilaments: ▪ actin 7nm Microtubules: ▪ tubulins ( ) 25 nm Intermediate filaments: ▪ lamin ▪ cell specific prot. (e.g. vimentin) 8-12 nm + Associated proteins (e.g. motor proteins) Bibliography -Essential Cell Biology, 4th edition. -Molecular Biology of the Cell, 5th edition.