Cellular Movement:

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Chapter 16

• Cellular Movement
• Motility and Contractility

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Definitions

• Motility involves the movement of cell thru its
  environment, the environment thru or past the
  cell, movement of components in the cell or
  shortening of the cell
• Contractility usually used to describe the
  shortening of the muscle cell

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Motile Systems

• MF and MT act as scaffold for specialized motor
  proteins, also called mechanoenzymes
• movement at the cellular level
• 2 major systems
• microtubule-based movement fast axonal
  transport center to dendrites
• microfilament-based movement muscle
  contraction, actin and myosin

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Microtubule Based Motors
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Kinesins

• Consists of 3 parts
• globular head region on the MT that functions as
  an ATPase
• coiled helical region
• light chain region links to vesicle/organelle
  to the motor protein
• Movement has globular head walking in a
  hand-over-hand fashion each step requires ATP
  hydrolysis on the head region
• Moves towards the positive end of the MT

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Kinesin Motor Protein
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KIFs Kinesin Family Members

• Have similar motor domains but in different
  locations within the cell
• move and localize substances in the cell
• to mitotic and meiotic spindle or kinetochores
• functions in cytokinesis

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Dyneins

• 2 classes
• cytoplasmic made up of 2 heavy chains that
  interact with MT, 3 intermediate chains and 4
  light chains
• move to the minus end of MT
• cant bind organelles on its own, interacts with
  dynactin helps link cargo and moves by binding
  to proteins such as spectrin
• axonemal
• 4 types identified so far

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Dynein Motor Protein
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Movement of Cellular Parts

• Dynein brings vesicles from ER to Golgi moves
  toward centromere
• Kinesin brings vesicles from the Golgi towards
  the plasma membrane

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MT-based Motility

• Cilia and flagella
• share common structure
• differ in length and function

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Cilia

• Large numbers
• Bound by plasma membrane, makes them
  intracellular structures
• In multicellular organisms, most cilia move
  things over cell rather than move cell like an
  amoeba
• Move in a coordinate, wavelike beating generated
  perpendicular to cell surface
• inhibited by smoking

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Flagella

• Moves cells through liquid usually from behind
• 1 to a few on a cell
• Bound by extension of plasma membrane
• Movement is more undulating, force is parallel to
  flagella

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Cilia Structure

• Axoneme is connected to basal body
• axoneme is the main cylinder of tubules
• Between axoneme and basal body is a transition
  zone
• basal body takes on shape of the axoneme
• Basal body 9 sets of tubules, each being a
  triplet (1 complete and 2 incomplete)
• centriole moves to and contacts the plasma
  membrane, forms nucleation site for MT assembly,
  making 9 outer doublets of axoneme, now a basal
  body

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Cilia Structure
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Axoneme

• 92 pattern
• 9 outer an extension of 2 of 3 subfibers of
  basal body
• A tubule complete MT
• B tubule incomplete MT
• 2 inner are the central pair
• complete MT

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Axoneme

• A and B tubules share a common wall made mostly
  of tektin (related to IF)
• Contain 2 side arms project from A towards B
  clockwise
• made of axonemal dynein, aids in sliding of MT
• Interdoublet links, less frequent, limit movement
  of doublets
• Radial spokes at regular intervals, project in
  from doublets to projections off central pair
• translates sliding into bending
• Outer doublets linked by nexin and aids in
  converting sliding into bending
• Movement is dependent on ATP

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Actin-Based Cell Movements - Myosins

• Actin acts as motor protein pathway
• Mechanoenzymes that move are myosins
• 1 heavy chain, globular head and a tail of
  various length
• head binds actin, hydrolyzes ATP to move
• tails various length to interact with various
  proteins, different functions
• 1 light chain, binds to head
• helps regulate myosin ATPase

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Myosins

• Some myosins bind to actin at head and tail
• Myosin I and V bind to membrane
• role in cell movement
• other functions
• Myosin II best understood, many types
• 2 heavy chains each head, hinge region and
  rod-like tail, 4 light chains
• found in skeletal, heart and smooth muscle
• can form thick filaments
• converts ATP energy into mechanical force
  contraction

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Movement in Muscle

• Muscle ? bundle ? fibrils ? myofibril ? sarcomere
  (fundamental contractile unit)

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Arrangement of Filaments

• Thick filament myosin
• Thin filament mainly actin
• Pattern of thin filament around thick filament in
  a hexagonal pattern

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Muscle Striation

• Dark bands A bands thick overlapping thin
• Light region in A band H zone (only thick)
• Middle of H zone M line (myomesin links myosin
  tails together)
• Light bands I bands thin (only actin)
• Middle of I band Z line (thin filaments join)
• Z line to Z line is a sarcomere

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Think Filaments

• Globular heads link to actin filaments

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Thin Filaments

• 3 proteins actin, tropomyosin, troponin
• Tropomyosin like the tail of myosin, fits in
  groove of actin filaments
• Troponin 3 polypeptide chains
• TnT binds tropomyosin
• TnC binds calcium
• TnI inhibits muscle contraction

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Myosin and Actin Interaction

• ? actinin keeps actin in parallel
• Cap Z keeps the plus end attached at Z line
• Tropomodulin binds the minus end and maintains
  length and stability
• Myomesin in H zone bundles myosin
• Titin attaches thick filaments to Z line
• Nebulin stabilizes thin filaments

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Muscle Contraction

• Complex interaction between all proteins
• Sliding filament model
• A bands stay constant
• I bands shorten considerably
• thin filaments slide past thick filaments but
  both fibers stay the same length sarcomeres
  shorten
• force muscle can generate is proportional to
  shortening reach a point where fibers can no
  longer overlap

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Cross-Bridges and ATP

• Transient cross-bridges between F-actin of thin
  filaments and myosin head of thick filaments
• Cross-bridges must form and break repeatedly when
  muscle is contracting
• Myosin head walks towards the Z line

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Cycle of Events

• Step 1 myosin in high energy state (ADP and Pi)
  binds specific actin subunit making a more
  tightly bound shape that removes the Pi
• Step 2 power-stroke, release ADP, thick
  filament pulls against thin filament
• Step 3 cross-bridge dissociation, bind ATP
  which causes a shape change and disengagement of
  myosin from actin
• decrease in ATP leads to stiff, rigid state
  rigor, rigor mortis is when no more ATP after
  death, cross-bridges stay intact
• Step 4 energy of ATP hydrolysis returns the
  myosin head to high energy state to move further
  down the actin filament

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Regulation by Ca2

• Muscle regulates Ca in sarcoplasmic reticulum
• Tropomysosin and troponin regulate availability
  of myosin binding sites on actin dependent on
  Ca
• Tropomyosin blocks myosin binding site on actin
• Troponin C binds Ca when levels increase and
  conformation change moves the tropomyosin away
  from actin to let myosin bind
• Ca levels and troponin C releases Ca and
  tropomyosin moves back to original place