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Transcript
The Cytoskeleton • Functions – Structural scaffold creating and supporting cell shape • Framework positioning organelles within cytoplasm – Network of molecular “roads” for intracellular transport of materials – Framework for whole cell movement – Framework for cell division The Cytoskeleton • Three major structural components – Microtubules • Major role: support, intracellular transport – Intermediate filaments • Major role: mechanical strength to resist physical stresses – Microfilaments • Major role: muscle contraction, motility The Cytoskeleton • Microtubules (MTs) – Major role • Intracellular transport – Motor proteins drag cargo along them • Structural support – Resist compression forces – Resist shear (bending) forces – Hollow, rigid – 25nm diameter, 4nm wall thickness – Radiate outward toward plasma membrane from near nucleus (MTOC) The Cytoskeleton • Microtubules (MTs) – Unit = alpha / beta tubulin heterodimer • alpha subunit + beta subunit • Heterodimer is asymmetric • Beta end is called “plus” end • Alpha end is called “minus” end – Not referring to a charge difference plus-end minus-end The Cytoskeleton • Microtubules (MTs) – alpha / beta (a/b)-tubulin heterodimer – Beta subunit is a GTPase • Assembly – Polymer grows by addition of units at the “plus” end – GTP-bound tubulin can add – GTP form hydrolyzes to GDP form – GDP-bound tubulin cannot add – GDP-bound tubulin can release only from “plus” end – GDP-bound tubulin cannot release from “minus” end or from central region The Cytoskeleton • Dynamic instability – MTs can assemble/disassemble at different rates in different locations within a single cell – Various proteins can bind and stabilize MTs The Cytoskeleton • Microtubule-associated proteins (MAPs) – Form bridges crosslinking adjacent MTs for parallel alignment – Increase MT stability – Promote assembly – Regulated by phosphorylation state Anti-tubulin antibody stain The Cytoskeleton • Microtubule organizing centers (MTOCs) – GTP-bound a/b-tubulin spontaneously assembles into MTs very slowly – GTP-bound a/b-tubulin add to an existing MT very rapidly – MTOCs are the nucleation points for MT assembly • Centrosome • Basal body The Cytoskeleton • Microtubule organizing centers (MTOCs) – Centrosome • 2 centrioles at right angles to each other near nucleus – Contain gamma-tubulin subunit – Nucleate “minus” end of a/b-tubulin » Plus-end is oriented outward toward plasma membrane The Cytoskeleton • Microtubule organizing centers (MTOCs) – Basal body • Single centriole at the base of cilia and flagella • Eukaryotic cilia and flagella – Hair-like motile organelle projecting from cell surface – Covered by plasma membrane • Eukaryotic cilia and flagella – Central protein core is called an “axoneme” • Eukaryotic cilia and flagella – Central protein core is called an “axoneme” • Composed of 11 MTs arranged in a “9+2” array – 9 outer MTs – 2 central MTs – Connected by various MAPs – Locomotion caused by sliding outer tubules past each other » Action of motor proteins (dynein) The Cytoskeleton • Motor proteins that “walk” on MTs – Kinesin gene family • Plus-end directed – Outward or “anterograde” transport – Dynein gene family • Minus-end directed – Inward or “retrograde” transport The Cytoskeleton • Kinesins are composed of 2 heavy and 2 light polypeptides – Cargo-interaction domain “tail” • Different kinesins have different specificities – ATPase “head” • Binds to MT • ATP hydrolysis propels heads forward • Highly processive The Cytoskeleton • Kinesins are composed of 2 heavy and 2 light polypeptides – ATPase “head” • Binds to MT • ATP hydrolysis propels heads forward • Highly processive The Cytoskeleton • Motor proteins that “walk” on MTs – Dynein gene family • Minus-end directed – Inward or “retrograde” transport • Very large (1.5MDa) – Involved in cilia/flagella movement The Cytoskeleton • Three major structural components – Intermediate filaments (~65 genes) • Major role: mechanical strength to resist physical stresses – Hemidesmosomes and desmosomes • Intermediate filaments (IFs) – Animal specific Epidermolysis – Strong, rope-like Bullosa Type Simplex Junctional Dystrophic Genes Mutated Keratin-5; Keratin-14; plectin laminin-5; collagen XVII; a6b4 integrin collagen VII • Intermediate filaments (IFs) – Animal specific – Strong, rope-like – Bridged together with other cytoskeletal elements • (e.g. plectin crosslinks MTs and IFs) The Cytoskeleton • Intermediate filaments – Composition and assembly • Monomers form dimers • Dimers form tetramers lacking polarity • Tetramers form larger fibers • Incorporation into existing filaments not limited to end regions The Cytoskeleton • Three major structural components – Microfilaments (MFs) • Major role: muscle contraction, motility • Solid, branched • 8nm diameter • Molecular unit= actin The Cytoskeleton • Microfilaments (MFs) – Actin molecule is asymmetric • “plus”-end versus “minus”-end – Actin is an ATPase – ATP-bound actin can be incorporated into growing MFs – plus-end of MFs grows 10x faster than minus-end – Higher dissociation rate from minus-end leads to treadmilling The Cytoskeleton • Microfilaments (MFs) – Drugs • Cytochalasin D blocks plus-end addition leading to complete MF depolymerization • Phalloidin blocks turn-over locking MFs into polymerized state + cytochalasin D • Actin binding proteins The Cytoskeleton + cytochalasin D The Cytoskeleton • Motors that walk on Microfilaments (MFs) – Myosin gene family • ATPase “head” domain • Cargo-interacting “tail” domain The Cytoskeleton • Motors that walk on Microfilaments (MFs) – Myosin gene family • Type V can walk on actin filaments carrying a bound cargo • Type II forms bipolar filaments via tail - tail interactions The Cytoskeleton • Myosin type II in muscle contraction – Muscle fiber • Large cell, 100mm long, 10100 microns thick • Contain >100 nuclei • Derived from the fusion of many myoblast cells – Myofibrils • thin protein strands composed of repeating units called “sarcomeres” that give muscle its “striated” appearance – Sarcomere • Z, I, A, H and M regions Sliding filament model of muscle contraction Sliding filament model of muscle contraction