The cytoskeleton
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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.

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The cytoskeleton
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 cytoskeleton1
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 cytoskeleton2
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 cytoskeleton3
The Cytoskeleton

plus-end

  • 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

minus-end


The cytoskeleton4
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 cytoskeleton5
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 cytoskeleton6
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 cytoskeleton7
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 cytoskeleton8
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 cytoskeleton9
The Cytoskeleton

  • Microtubule organizing centers (MTOCs)

    • Basal body

      • Single centriole at the base of cilia and flagella




  • 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 cytoskeleton10
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 cytoskeleton11
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 cytoskeleton12
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 cytoskeleton13
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 cytoskeleton14
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

    • Strong, rope-like

    • Bridged together with other cytoskeletal elements

      • (e.g. plectin crosslinks MTs and IFs)


The cytoskeleton15
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 cytoskeleton16
The Cytoskeleton

  • Three major structural components

    • Microfilaments (MFs)

      • Major role: muscle contraction, motility

      • Solid, branched

      • 8nm diameter

      • Molecular unit= actin


The cytoskeleton17
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 cytoskeleton18
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


The cytoskeleton19
The Cytoskeleton

  • Actin binding proteins

+ cytochalasin D


The cytoskeleton20
The Cytoskeleton

  • Motors that walk on Microfilaments (MFs)

    • Myosin gene family

      • ATPase “head” domain

      • Cargo-interacting “tail” domain


The cytoskeleton21
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 cytoskeleton22
The Cytoskeleton

  • Myosin type II in muscle contraction

    • Muscle fiber

      • Large cell, 100mm long, 10-100 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




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