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Lecture 6

Lecture 6.docx

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Biological Sciences
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Stephen Reid

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What is the cytoskeleton? •Extensive protein network –Made on free ribosomesin the cytoplasm, no targeting sequence •Three main components: 1.Microtubules 2.Actin 3.Intermediate Filaments Cytoskeleton -Functions •Scaffold –structural support and cell shape •Internal framework –organize organelles within a cell •Movement –directs cellular locomotion and movement of materials within the cell •Machinery for cell division Key to Cystoskeletal Functions: 1) Structure and Support 2) Intracellular transport 3) Contractility 4) Spatial organization Memorize table 9.1 Actin: Flexible, helical is used more for contractility and mobility 3) Intermediate Filaments •Heterogenous group of proteins, divided into five classes •~ 10 nm in diameter –intermediate in size between microtubules and actin(micro filaments) •50 gene products •Mainly structural support •Types I –IV form filaments •Type V (lamins) are present in the inner lining of the nucleus •Monomers have a long a-helical stretch –Fibrous proteins –strong, resistant to stretch –Gives cells/cell structures strength •Monomers form homodimers parallel to one another •Dimers come together antiparallel to form tetramers •Tetramers then polymerize into filaments •Both tetramer and IF(intermediate filament) lack polarity/lack direction •Assembly and disassembly are controlled by: phosphorylation and dephosphorylation •Generally for cell strength –not used for intracellular transport etc. Neurofilaments-important for integrity of axons Lamins–inner nuclear membrane cytoskeleton: determines shape of nuclear envelope Keratin: •Keratin: main protein of skin cells (keratinocytes) •Skin is made of many layers –constantly dividing –Makes huge amounts of keratin •As skin cells are pushed towards surface they undergo apoptosis ; programmed cell death •Cell dies –leaves epidermis covered with a dead, waterproof layer of keratin protein: Cornified cells •Plasma membrane forms boundary between a cell and its non-living environment •But cells must respond to external stimuli –Interact in specific ways with other cells and extracellular materials •In multicellular animals –Cells interact with extracellular material to form defined tissues. –These interactions are crucial to the formation of epithelial tissue and connective tissue, which are crucial for various cellular activities Extracellular Matrix •Everything that surrounds the cells –Made by cells (secreted) •Organized network of extracellular materials •Often has key regulatory role in determining the shape and activities of cells Extracellular Matrix -Functions •For support of cells/tissues •Site of cell attachment •Signals to cells –Gives cells “identity” –Sends survival signals, signals to migrate etc. •Substrate for cells to migrate over Extracellular Matrix •Different ECM’s have different amounts of proteins depending on function •Amount of water varies –Ex.; blood versus bone •Amount of bound mineral varies –Ex.; bone has Type I Collagen that binds calcium phosphate Extracellular Matrix –Basement Membrane •Basement Membrane = basal lamina, one of the best defined ECMs •Continuous sheet –Surrounds nerve fibers, muscles, and fat cells –Underlies the basal surface of epithelial tissues –Underlies inner endothelial lining of blood vessels Basement Membrane -Function •Provide mechanical support for attached cells •Generate signals that support cell survival •Serve as substratum for cell migration •Separate adjacent tissues in an organ •Act as a barrier to macromolecules -Basement membrane oif capillaries prevent passage of proteins out of blood into tissues. ECM proteins: 1) Collagen 2) fibronectin 3) laminin 4) Proteoglycan: protein + polosaccharide. 1) Collagen •Most common ECM protein •Most abundant animal protein •Secreted, fibrous glycoproteins–only in ECM •High tensile strength •Trimer of polypeptide chains wound around each other •Each type of collagen is restricted to particular locations in the body •Provide the insoluble framework that determines mechanical properties of the matrix –Tendons –collagen fibrils aligned parallel to the long axis (direction of pulling forces) 2) Laminin •Extracellular glycoproteins •Three polypeptide chains linked by disulfide bonds •Help cell migration during development •Components of basement membranes 3) Fibronectin •Linear array of distinct polypeptides –Modular structure •Each polypeptide is ~ 30 fibronectin molecules •Fibronectin modules are found in other proteins too •Has binding sites for other ECM components: brings proteins in ECM together •Guides migrating cells during embryogenesis 4) Proteoglycans •Protein-polysaccaride complex •Core protein attached to glycosaminoglycans(GAGs) -Each GAG: repeating disaccharide structure •GAGs are negatively charged –attract cations -Attracts water, forming porous, hydrated gel: VERY FLUID Structure: Core protein that has GAG chains. (don’t need to know monomer names) Extracellular Matrix •Continuously
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