major functions • cytoskeleton • network of filamentous structures cytoskeletal • microtubules actin filaments • intermediate filaments • microfilaments elements major roles: chapter 17 • intermediate filaments • scaffolding • organization of organelles • locomotion • movement of materials within microtubules the cell motor proteins microtubules microtubules • microtubules • function of • structure and composition microtubules • hollow, cylindrical • determine shape of cell • globular proteins arranged in • function in axons rows • involved in intracellular • protofilaments transport • α- and ß-tubulin dimers • structural support • 13 protofilaments • shape cell • microtubule associated • maintain internal proteins (MAPs) organization • increase stability • influence cell wall 15 µm • promote assembly microtubules microtubules • axonal transport • microtubules in plants • facilitate movement of vesicles • 4 distinct arrays in dividing plant cells • neurotransmitters 1. widely distributed throughout cortex • movement of vesicles down length of an axon along tracks of 2. single transverse band microtubules 3. mitotic spindle • anterograde 4. phragmoblast • retrograde microtubules microtubules • motor proteins • kinesin heavy chains light chains • convert ATP into mechanical • structure energy • motor domain • move unidirectionally • globular head • three categories • stalk tail neck stalk • kinesin • tail heads • dynein • movement processive • myosin • move cargo toward plasma membrane • anterograde transport (axons) microtubules microtubules • cytoplasmic dynein • MTOCs (microtubule-organizing centers) • structure light chains • huge protein intermediate and • specialized structures for stalk stem nucleation of microtubules • globular force-generating head heads • movement of vesicles • centrosomes retrograde transport (axons) • PCM - pericentriolar material • cell body • axonemal dynein • centrioles • responsible for movement of cilia / flagelladynactin complex • microtubule nucleation • requires dynactin complex dynein • plant cells lack MTOCs kinesin axon terminus microtubules microtubules • construction of microtubules • branching of microtubules • pericentriolar material • new microtubules branch from γ-tubulin on existing molecule • γ-tubulin • changes in spatial organization: • organized into ring • rearrangement of existing microtubules • γTuRC • disassembly of existing microtubules and reassembly of new • GTP-bound α- and ß-tubulin dimers ones in different locations • dimers join to γTuRC • from “-” end to “+” end • “+” = ß-tubulin exposed new microtubule “-” = α-tubulin exposed • γ-tubulin • critical concentration - spontaneous complex • dynamic instability microtubules microtubules • microtubule dynamics • cilia and flagella • GTP required for microtubule assembly • structurally similar • hydrolysis of GTP -> replaced by new GTP • different mechanism of locomotion • catastrophe • cilia beat together - gliding motion • microtubule capping proteins • flagella - variety of patterns • treadmilling • simultaneously growing and shrinking power stroke recovery stroke microtubules microtubules • axoneme • axoneme • central core of cilia and • central core of cilia and flagella flagella • 9 + 2 microtubule • 9 + 2 microtubule arrangement arrangement • central sheath • central sheath • connected to A tubules by • connected to A tubules by radial spokes radial spokes • doublets connected by • doublets connected by radial spoke interdoublet bridge interdoublet bridge interdoublet bridge dynein arm outer dynein arm A tubules B tubules inner dynein arm radial spoke microtubules microtubules • basal bodies and axonemes • ciliary dynein • cilia / flagella emerge from • huge protein basal bodies (MTOCs) • three heavy chains, several intermediate and light chains • growth of axoneme • provides mechanism for ciliary movement • + end of MT • swinging cross bridges • IFT - intraflagellar transport • materials transported for assembling flagella microtubules intermediate filaments • cilia and flagella • intermediate filaments (IFs) • functional examples • structure • movement - protozoans, gametes • rod-tetramer • multicellular organisms • two antiparallel dimers • movement • coiled coils (helices) • flatworms • movement of fluids • sponges • rotifers • ancestral molluscan gills • flame cells • lining of mammalian bronchi intermediate filaments intermediate filaments • types and functions • intermediate filaments (IFs) • keratin-containing intermediate filaments (kIFs) • assembly • epithelial tissue • 8 tetramers associate (17nm) • skin, liver, pancreas • immature filament (20nm) • neurofilaments • mature filament (10nm) • lamins • spontaneous assembly • less sensitive to chemical agents than other cytoskeletal elements © 2010 Nature Education All rights reserved. microfilaments minus end microfilaments • actin filaments • actin • thin polymers of actin proteins • polymerization • forms filaments (F-actin) • three steps • polarity • nucleation • shaped like an arrow • elongation • barbed (feathered) and pointed end • steady state • addition occurs more quickly at + end • actin binding proteins • microfilaments organized by controlling shuttle monomers to site equilibrium between assembly (+) and of actin assembly disassembly (-) • actin filaments found wherever large scale movement / tensile strength required plus end microfilaments microfilaments • actin-binding proteins • cellular migration • monomer-binding proteins • 3 characteristic forms of actin filaments • assist in controlling availability of monomers • filopodia • nucleating proteins • thin, parallel bundles of filaments • stabilize small clusters of monomers (nuclei) • push plasma membrane (+ ends) • ARP2/3 complex • lamellopodia • capping proteins • branched networks of filaments • inhibit elongation • broad, shallow distortions of • severing proteins plasma membrane • depolymerizing proteins • contractile filaments • cross-linking proteins • actin-binding motor proteins microfilaments microfilaments • myosin motors • unconventional myosins • conventional myosins (type II) • myosin I • motor head - binding actin / hydrolyzing ATP • only one head • tail - forms filaments • does not form filaments • filaments can be bipolar • myosin V head • involved in transport of materials ATP • hand-over-hand movement essential light chain • bypasses twists in helix neck tail microfilaments microfilaments • myosin Va • hair cells • transports cargo • stereocilia • organelles • displacement leads to nerve • travel between impulse microtubules • parallel bundles of actin fibers • cooperates with kinesin • multiple types of • myosin VI unconventional myosin • moves in reverse arranged throughout hair cell • towards “-” end of actin filaments microfilaments muscle fibers • myosin motors • levels of organization in • contractile cycle skeletal muscle • tails bind to integrin complexes • muscle fiber • heads bind to actin fibers • myofibrils • myosin II crawls towards plus end • sarcomeres of fiber • actin and myosin • myosin head binds to ATP arranged in bundles • actin released • thin filaments reverse Power • actin fibers stroke • ATP hydrolyzed • reverse power stroke - actin • thick filaments binding • myosin bundles • power stroke - phosphate • titin filaments released • prevent over stretching • ADP released cs muscle fibers H zone I band muscle fibers • sarcomere • actin fibers • contraction • double helix T tubules mitochondrion • actin-myosin contractile cycle • associated with troponin and • myosin heads bend, myosin tropomyosin necks flex like lever • troponin • actin filaments moved inward • in contact with actin and • contraction is inactive state relaxed tropomyosin sarcoplasmic reticulum troponin tropomyosin contracted summary summary • three types of fibers • myosin motors • microtubules, intermediate filaments, microfilaments • contractile cycle • microtubules • reverse power stroke • assembled from tubulin, form part of mitotic spindle, centrioles, • power stroke core of cilia and flagella • striated muscle fibers • motor proteins - kinesins, dyneins, myosins • levels of organization • assembly occurs in association with MTOCs • myofibrils • MTs are dynamic polymers • sarcomeres • intermediate filaments • actin-myosin contractile cycle • composed of many different proteins • actin filaments • double helical polymer of actin - key in contractility and movement.