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Cell Biology > Lecture 3 - The cytoskeleton > Flashcards

Lecture 3 - The cytoskeleton Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

What are the functions of the cytoskeleton?

A
  • cell morphology
  • migration
  • elongation
  • chromosome separation
  • vesicle transport
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2
Q

What are the three componants of the cytoskelteon?

A
  • microfilaments (7-9nm)
  • intermediate filaments (10nm)
  • microtubules (24nm)
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3
Q

What are eight features of a migrating cell?

A
  • actin bundle
  • lamellepodia
  • filopodia
  • focal adhesions
  • tail
  • microtubule organising centre
  • stress fibres
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4
Q

What are lamellopida?

A

broad membrane extensions that move forward and are typical of fastly migrating cell

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5
Q

What are filopodia?

A

fine cytoplasmic extensions

typical of slower moving cells

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6
Q

What are membrane ruffles?

A

assemblies of cytoplasm not tighly adheared to a substrate

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7
Q

What does an actin polymer consist of?

A

-consist of globular monomers of G-actin which nucleate and elongate in the same direction to form a filamentous helical polymer

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8
Q

What is the orientation and structure of the monomeric G-actin?

A
  • Denatures in the absence of ADP or ATP
  • Has an Mg2+ molecule bound
  • Has orientation = ATP binding cleft orientatated towards top (-) end
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9
Q

How many subunits of G actyin make up one repeating helical turn?

A

28 G-actin subunits

72nm long

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10
Q

What is actin polymerisation dependent upon?

A

the equilibrium between concentration of monomers in cell and active elongation/severing

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11
Q

How does the critical actin concentration (Cc) affect actin formation?

A

Cc determines whether the filament can extend

  • Under steady state conditions the dissociation rate = association rate (ATP hydrolysis to ADP is slower than the opposite reaction)
  • During elongation the G monomer concetration is higher than critical concentration, and ATP is hydrolysed to ADP
  • During dissasociation the G monomer concentration is lower than the Cc
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12
Q

What is involved in the active process of actin length regulation?

A

Severing and capping proteins

-interact with filamentous actin and cap and/or severe

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13
Q

Give an example of a severing/capping protein

A 
Gelsolin - caps and severes (+) end
CapZ - caps (+) end
Tropomodulin - caps (-) end
Cofilin - severes
gCAP39 - caps (+) end
Severin - caps and severes
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14
Q

How can capping and severing proteins alter the actin cytoskeleton?

A
  • protect it by capping and consequently stabilising
  • shortern by severing
  • can cap at both ends so that it is permenantly stabilised
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15
Q

What are the features of Gelsolin?

A
  • capping and severing protein that can help to regulate actin filament length alongside Cc
  • activated by high Ca2+ concentration leads to a conformational change e.g. when growth faction binds to it’s ligand
  • conformational change allows it to bind to the F-actin polymer and disrupt the subunit organisation causing severing
  • then binds to the (+) end of the actin fragment
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16
Q

What is the structure of contractile bundle (stress fibres and cytoplasmic contractile ring) and what protein are they crosslinked by?

A
  • contractile bundle with a loosely packed, antiparallel organisation (allow contraction by myosin II)
  • crosslinked by alpha actinin
17
Q

What is the structure of filopodia and what protein are they crosslinked by?

A
  • tightly packed bundles of parallel F-actin (prevents myosin II binding)
  • crosslinked by fimbrin
18
Q

What is the structure of the gel-like network of actin in the cell cortex and what is it crosslinked by?

A
  • gel-like network of randomly orientated fibres, provides structure
  • fibres crosslinked by filamin at right angles
  • required for lamellipodia formation and cell migration
19
Q

What are four crosslinking proteins and what are their structures?

A

Fimbrin - monomer (filopodia)
alpha-actinin - dimer (stress fibres)
Filamin - dimer with a ‘hook like’ structure (network of fibres)
Spectrim - tetramer (red blood cells)

20
Q

What does the branching of actin filaments require and what are these proteins?

A

Actin-related proteins (ARPs)

  • complexes that can interact with actin and different actin binding proteins
  • bind and act as a nucleation point for the formation of other fibres/filaments
21
Q

Give an example of an ARP (actin relatied protein)

A

Arp2/3 complex
Binds at 70* to the side of an actin microfilament to nucleate daughter filaments
-dynamic and changes often in response to other things in the cell

22
Q

What are mysosins, what are the three?

A
  • multi gene family
  • composed of 1 or 2 heavy chains and several light chains (may have a refulatory role in the action of the proteins)
  • Myosin I = monomer
  • Mysoin II and V = dimers
  • bind to actin via head section which has ATPase activity and can generate force to walk along actin filament
23
Q

What is the structure of Myosin I?

A
  • monomer
  • globular head region with ATPase activity (ATP binding site)
  • 2Xcalmodulin light chains at neck region (regulate)
  • tail region interact with plasma membrane
24
Q

What is the structure of Myosin II?

A
  • dimer
  • 2 globular head regions with ATPase activity (ATP binding site)
  • 2X regulatory light chains at neck region, and 2Xessential light chains
  • tail region (130nm) interact with plasma membrane
25
Q

What is the structure of Myosin V?

A
  • dimer
  • 2 globular head regions with ATPase activity (ATP binding site)
  • light chains at neck region
  • tail region interact with vesicle membrane
26
Q

What are the functions of Myosin I, II, V?

A

Myosin I: cytoskeletal - plasma membrane interactions (formation and movement of filopodia and microvilli)
Myosin V: interaction between cytoskeleton and vesical membrane (transport)
Myosin II: muscle contraction and cytokinesis
-head region bind actin, light chains at neck region regulate head domain, coiled domains pack side by side to form a thick filament

27
Q

How is formation of Myosin II bipolar filament regulated?

A

By myosin light chain kinase (MLCK)

  • following activation mysin light chains in the neck region are phosphorylated by MLCK (elavated Ca2+)
  • activates actin binding domain and causes a conformational change
  • results in release of the tail and spontaenous self assembly to form a bipolar filament structure
28
Q

What experimental assay can be done to detect Myosin II activation?

A

The sliding microfilament assay
1- myosin molecules are attached to a coverslip
2- washed with bovine serum album (protein wash) to get rid of non-adhearent protein molecules
3-Actin filaments fluoresently labelled (Rhodamin phalloidin) are added to the coverslip and bind physically with the actin molecules
4- when ATP is provided myosin II goes through contracile interaction movement, moving the labelled actin filaments which can be viewed by a fluoresence microscope

29
Q

What is the process of muscle contraction?

A

1- Myosin heads are bound to the actin filament in the resting state
2- Influx of CA2+ binds to troponin on actin filament causes a conformational change in tropomyocin, exposing myosin binding sites
3- ATP binding to myosin II causes a conformational change in myosin, disrupting the actin binding site and releasing the myosin heads
4- ATP is hydrolysed to ADP and Pi restoring the actin binding site and causing a conformational change so that the myosin head pivots and binds to a new actin subunit
5- Pi is then released causing the ‘power stroke’ conformational change which pulls the actin filament
overlap to shorten

30
Q

What is the Z disk?

A

Actin microfilaments are anchored to Z disk to allow myosin II to ‘pull’ and generate suffient force to move

Cell Biology (15 decks)

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