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BIOL 2070 > Week 8: Cytoskeleton and Muscle Function > Flashcards

Week 8: Cytoskeleton and Muscle Function Flashcards

1
Q

What are the 3 cytoskeletal elements?

A

Intermediate filaments, microtubules and actin filaments

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2
Q

Describe intermediate filaments.

A
  • Help cells withstand mechanical stress when stretched
  • Diameter between microtubules and actin filaments
  • Most durable
  • Form the nuclear lamina - strengthens nuclear envelope
  • Link cells via desmosomes
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3
Q

Describe assembly of intermediate filaments

A

Alpha-helical region of monomer joins with another to form a coiled-coil dimer. Two of these join to form staggered tetramer of two coiled-coil dimers. Then there is a lateral association of 8 tetramers which add together to form a filament.

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4
Q

What are the 4 classes of intermediate filaments?

A

From cytoplasm - keratin filaments (in epithelial cells - most diverse class), vimentin and vimentin-related filaments (in connective-tissue cells, muscle cells and glial cells) and neurofilaments (in nerve cells). In nucleus - nuclear lamins (in all animal cells).

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5
Q

What is plectin?

A

aids in bundling of intermediate filaments and links them to other cytoskeletal protein networks

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6
Q

Is nuclear envelope supported by intermediate filaments?

A

Yes

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7
Q

How does nuclear lamina change during cell division?

A

Nuclear lamina disassembles and reforms at each cell division. Regulated by phosphorylation of lamins.

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8
Q

Describe microtubules

A
  • Extend toward cell periphery - creating system of tracks within cell.
  • Facilitate intracellular transport
  • Form mitotic spindle in mitosis
  • Form cilia and flagella
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9
Q

Describe structure of microtubules

A

Hollow tubes with structurally distinct ends: + and -. Made of tubulin heterodimers with alpha and beta subunits

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10
Q

Describe the centrosome

A

The centrosome is the major microtubule-organizing center in animal cells. Consists of a pair of centrioles, surrounded by a matrix of proteins including gamma-tubulin.

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11
Q

Describe how microtubules grow and shrink.

A

Each microtubule grows and shrinks independent of its neighbour. They display dynamic instability - go back ad forth between polymerization and depolymerization. This is driven by GTP hydrolysis. Tubulin dimers bound to GTP bind more strongly to each other than tubulin dimers bound to GDP.

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12
Q

How can microtubule dynamics be modified?

A

Drugs. Taxol - binds and stabilizes them. Colchicine (colcemid) - binds tubulin dimers and prevents polymerization and vinblastine (vincristine) does the same.

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13
Q

How do microtubules organize cell interior?

A

Microtubules organize the cell interior - guide transport of organelles, vesicles and macromolecules

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14
Q

Describe motor proteins

A

Drive intracellular transport. Kinesin - towards plus end (membrane), dynein - towards negative end - nucleus.

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15
Q

Describe actin filaments (microfilaments)

A
  • Polymers of actin
  • Present in all eukaryotic cells
  • Enable cells of move, divide and engulf material
  • Associate with actin-binding proteins
    Microvilli

B) Contractile bundles in cytoplasm

C) Finger-like filopodia protruding from leading edge of migrating cell

D) Contractile ring during cell division

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16
Q

How do actin and tubulin polymerize similarly?

A

new actin monomers are added to plus end. It uses ATP to bind and then hydrolyzes it to detach.

17
Q

How can actin dynamics be modified?

A

Drugs. Phalloidin - binds and stabilizes filaments. Cytochalasin - caps filament plus ends preventing polymerization there. Latrunculin - binds actin monomers and prevents their polymerization

18
Q

What different proteins associate with actin filaments?

A

Nucleating protein, monomer-sequestering protein, severing protein, cross-linking protein, capping protein, side binding protein, myosin motor protein, bundling protein.

19
Q

How do actin filaments allow cells to migrate?

A

Allow cells to migrate by forming lamellipodium (extend from mobile edge) and filopodium (extends from leading edge)
A web of polymerizing actin filaments pushes leading edge of lamellipodium forward.
A cortex rich in actin filaments underlies the plasma membrane of most eukaryotic cells. Cell crawling depends on cortical actin. Myosin motor proteins plays important role in allowing the cell to detach at back

20
Q

How do extracellular signals affect actin filament arrangement?

A

Can alter, ie. Rho activation, Rac activation and Cdc42 activation

21
Q

What do actin and myosin form?

A

Contractile structures.

22
Q

Describe myosin I

A

simplest myosin, walks in + direction along actin filament.

23
Q

Describe myosin II

A

Can associate with each other to form myosin filament.

24
Q

How do actin and myosin interact in muscle contraction

A

Actin filaments slide against myosin. Bipolar myosin-II filament can slide two actin filaments of opposite orientation past each other. This occurs in both muscle and non-muscle cells (ie. dichtyostelium)

25
Q

Describe composition of skeletal muscle cell

A

A skeletal muscle cell is composed of myofibrils - contractile element of muscle cell - chain of identical tiny contractile units called sarcomeres (composed of actin and myosin-II). Myosin filaments are thick filaments and actin filaments are thin filaments.
Sarcomeres - contractile units of muscle

26
Q

How do muscles contract?

A

Muscles contract via. sliding-filament mechanism. During contraction, actin and myosin slide past without shortening.

27
Q

Describe power stroke

A

ATP binds to actin filament - providing energy to allow myosin to migrate in plus direction, ATP hydrolysis, ADP is released causing power stroke - actin moves.

Rigor Mortis → ATP binding (reduces affinity of myosin head for actin) → ATP hydrolyzed (myosin head moves along actin) → phosphate released (myosin head again binds tightly to actin) → rigor mortis

28
Q

Describe the association of T tubules and SR around each myofibril

A
  • Sliding of myosin along actin occurs only when the skeletal muscle receives a signal from motor neuron
  • Neurotransmitter released from nerve terminal triggers action potential in muscle cell membrane
  • Electrical excitation spreads into series of membranous tubes - T (transverse) tubules - extend in from plasma membrane around each myofibril.
  • Electrical signal relayed to SR - sheath of interconnected flattened vesicles that surround each myofibril.
29
Q

What triggers skeletal muscle contraction?

A

sudden rise in cytosolic Ca2+

30
Q

What controls skeletal muscle contraction?

A

Tropomyosin and troponin.

When Ca2+ is low, tropomyosin blocks myosin-binding sites on actin filament, preventing interaction with myosin. When Ca2+ is high, it binds to troponin complex causing tropomyosin to shift and allows myosin to bind

Ca2+ falls again and tropomyosin moves back to blocking position.

BIOL 2070 (10 decks)

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