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Cell Bio > L15. Cytoskeleton > Flashcards

L15. Cytoskeleton Flashcards

1
Q

what are microtubules

A
  • hollow cylinders
  • made of the protein tubulin
  • one end is attached to the centrosome
  • is the most rigid and biggest
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2
Q

what are intermediate filaments

A
  • very flexible and rope like
  • forms the nuclear lamina
  • used for mechanical and tensile support
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3
Q

what are actin filaments

A
  • they are flexible fibers
  • they are most highly concentrated in the cortex
  • the most smallest
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4
Q

microtubules - what is the centrosome

A
  • microtubule organizing center
  • microtubules grow out of the centrosomes
  • located near the cell center
  • it consists of a pair of centrioles that is surrounded by a matrix of proteins
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5
Q

microtubules - what are its purposes

A
  • create tracks for transport and organelle positioning
  • during mitosis, they disassemble and reassemble as the mitotic spindle
  • they also make up cilia and flagella (bacterial flagella is different)
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6
Q

microtubules - explain how the tubes are made

A
  • each tubulin subunit is a dimer composed of 2 globular proteins (alpha and beta tubulin)
  • the tubulin dimers are stacked into 13 parallel protofilaments
  • these protofilaments have structural polarity
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7
Q

microtubules - explain its polarity

A
  • alpha tubulin makes up the - end
  • beta tubulin makes up the + end
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8
Q

microtubules - how does tubulin polymerize

A
  • they polymerize from nucleation sites on the centrosome
  • the centrosome organizes an array of microtubules that radiate outwards through the cytoplasm
  • the centrosome matrix has gamma tubulin and it serves as a starting point for microtubules (- end is embedded in the centrosome)
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9
Q

microtubules - how do they grow

A
  • dynamic instability permits rapid remodeling
  • alpha/beta tubulin dimers are added to the + end as it grows
  • alpha/beta tubulin dimers are lost from the - end as it shrinks
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10
Q

microtubules: dynamic instability - how can + and - ends be stabilized

A
  • minus ends: being linked to the centrosome
  • plus ends: stabilized by binding to specific proteins (capping proteins)
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11
Q

microtubules: dynamic instability - why do microtubules grow this way

A

it is easier to add gamma tubulin than to nucleate from scratch

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12
Q

microtubules: dynamic instability - what is it driven by

A
  • GTP hydrolysis
  • GTP-tubulin binds more tightly than GDP-tubulin
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13
Q

microtubules: GTP hydrolysis - growing microtubules

A
  • polymerization
  • each dimer binds to GTP
  • GTP is hydrolyzed shortly after the addition of the dimer and GDP will remain tightly bound to beta-tubulin
  • polymerization being faster than GTP hydrolysis creates a GTP cap
  • this then packs the microtubule more efficiently
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14
Q

microtubules: growing microtubule - what is the GTP cap

A

it is where all of the tubulins are GTP-bound and can bind more strongly

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15
Q

microtubules: GTP hydrolysis - shrinking microtubules

A
  • depolymerization
  • GDP-tubulin associates less tightly and fall off
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16
Q

microtubules - create tracks for transport

A
  • all the microtubules in the axon points in the same direction
  • plus end is towards the termini and serves as a track for transport
  • backward and outward transport is driven by different motor proteins
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17
Q

microtubules: create tracks for transport - motor proteins for forward and backward transport

A
  • kinesins and dyneins use ATP hydrolysis to move cargo along microtubules via globular heads
  • both are homodimers
  • kinesins move towards the + end
  • dyneins move towards the - end
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18
Q

microtubules: create tracks for transport - explain the motor proteins specificity

A
  • kinesins and dyneins transport different cargo
  • their tail determines cargo specificity
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19
Q

microtubules - organelle positioning

A
  • kinesin pulls the ER outward and stretches it
  • dyneins pull the Golgi inward towards the nucleus
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20
Q

microtubules: cilia - explain the cilia of an epithelial cell

A
  • hair-like structures
  • grows from a cytoplasmic basal body that serves as an organizing center
  • has a core of bundled microtubules that grow from the basal body
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21
Q

microtubules: cilia - explain their movements

A
  • whip-like
  • power stroke: cilium is fully extended and fluid is driven over the surface of the cell
  • recovery stroke: cilium curls back into position with minimal disturbances
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22
Q

microtubules: cilia and flagella - what are their uses

A
  • respiratory tract (cilia): mucus swallowing
  • oviduct (cilia): helps carry egg
  • flagella: present in sperm and protozoa
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23
Q

microtubules: flagella - explain their movements

A
  • dyneins causes flagella to beat
  • they are present at regular positions and serve as cross-links to hold microtubule bundles
  • other protein generate the force that causes the bending
  • without dynein, the force is sliding instead of bending
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24
Q

microtubules: cilia and flagella - how are the microtubules arranged

A

in a 9 + 2 array: 9 double microtubules in a ring around a pair (2) of single microtubules

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25
Q

intermediate filaments - nuclear lamina

A
  • it supports and strengthens the nuclear envelope
  • the nuclear lamina is a network of filaments called lamins
  • assembly and disassembly is done via phosphorylation and dephosphorylation
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26
Q

intermediate filaments - what are its uses

A
  • has durable networks in the cytoplasm
  • great tensile strength and helps cells withstand mechanical stress
27
Q

intermediate filaments - what are they anchored to

A
  • desmosomes
  • enables connections between cells
28
Q

intermediate filaments - what are they composed of

A
  • the monomer is an alpha-helical rod domain
  • pairs of monomers associate to form a parallel dimer
  • 2 dimers form an antiparallel tetramer
  • tetramers pack together to make the intermediate filament
29
Q

intermediate filaments: what are the 4 classes - cytoplasmic

A
  • keratin filaments (in epithelial cells)
  • vimentin and vimentin-related filaments (in connective-tissue cells, muscle cells, and glial cells)
  • neurofilaments (in nerve cells)
30
Q

intermediate filaments: what are the 4 classes - nuclear

A

nuclear lamins (in all animal cells)

31
Q

intermediate filaments: what are the 4 classes - keratin

A
  • most diverse class
  • in hair, feathers, and claws
  • ends of filaments are anchored to desmosomes
  • it creates a cabling tensile strength that absorbs stress from a stretching force
32
Q

intermediate filaments: keratin - explain an abormaility

A
  • mutant keratin gene: Epidermolysis bullosa simplex
  • makes skin prone to blistering and rupture
33
Q

intermediate filaments - what is plectin

A
  • it is an accessory protein that reinforces intermediate filaments
  • it cross-links filaments into bundles and links them to microtubules, actin, and desmosomes
34
Q

actin filaments - explain the filaments

A
  • they are polymers of actin
  • they also need accessory proteins for stability
  • they have a cleft in the monomer that provides a binding site for ATP/ADP
  • each filament is a 2-stranded helix
35
Q

actin filaments - what cell shapes can they create

A
  • microvilli
  • dynamic protrusions
  • lamellipodium
  • contractile ring
36
Q

actin filaments - how do they grow and shrink

A
  • treadmilling
  • actin monomers carry ATP and it is hydrolyzed after assembly and reduces the strength of the binding
  • plus end receives addition much faster than the ATP is hydrolyzed, making it more stable and it grows
  • at the same time, the - end contracts
37
Q

treadmilling vs dynamic instability

A
  • treadmilling: when rates are equal, the strand stays the same size
  • dynamic stability: rapid switch from growth to shrinkage, thus microtubules undergo more drastic changes than actin filaments
38
Q

actin filaments - what are actin binding proteins

A
  • they bind and sequester actin
  • they hold together bundles in microvilli and cross-link actin in the cell cortex
  • they can also associate with myosin motors to form contractable bundles and form tracks for support
39
Q

actin filament - examples of actin binding proteins

A
  • formins and actin-related proteins (ARPs)
  • they both promote polymerization
40
Q

actin filaments - explain the concentration of actin in the cell

A
  • 5% of all protein is actin
  • 1/2 is assembled (other half are in reserve)
41
Q

actin filament - explain how a cell moves (chemotaxis)

A
  • they use actin mobilization
    1. they push out protrusions at the leading edge
    2. protrusions adhere to a surface
    3. cell drags itself forward
42
Q

actin filament: chemotaxis - how do they push out protrusions using lamellipodia

A
  • it extends a thin sheet of lamellipodia which has a dense network of actin
  • it is driven by actin polymerization
  • plus end is close to the PM
43
Q

actin filament - explain animal cell migration

A
  • along with lamellipodia, the cell can form filopodia (thin stiff protrusions)
  • these help probe the environment
  • lamellipodia and filopodia both grow via rapid local actin growth
  • the filaments will push out of the membrane without breaking it
44
Q

actin filaments - explain lamellipodia formation

A
  • actin related proteins (APRs) promotes actin web formation
  • it promotes a continual assembly at the leading edge and disassembly at the back
45
Q

actin filaments - explain filopodia formation

A
  • they use formins
  • these proteins attach to the + end and promote unbranched actin assembly
46
Q

actin filaments - explain the myosin family

A
  • they are all actin-dependent motor proteins
  • they bind to and hydrolyze ATP
  • they provide energy to move towards the plus end of actin
47
Q

actin filaments: myosin family - myosin I

A
  • in all cell types
  • has a head and a tail domain
  • the head binds to actin and hydrolyses ATP to generate motor activity to move the myosin
  • the tail binds to the cargo
48
Q

actin filaments: myosin family - myosin II

A
  • in muscle cells
  • this subfamily are all composed of dimers
  • it has 2 globular ATPase heads and a single coiled-coil tail
  • clusters of myosin molecules bind to each other through tails to form a myosin filament
  • the 2 sets of heads face and pull in opposite directions
49
Q

actin filaments: myosin II - explain it in muscle cells

A
  • if organized in a bundle with actin filaments, myosin II can create a strong contractile force
  • thus creating muscle contractions
  • also seen in contractile bincles (non-miscle cells) during cell division
50
Q

actin filaments - explain skeletal muscle cells

A
  • skeletal muscle fibers are huge multicellular cells formed through fusion
  • the nuclei is just beneath the PM
  • most cells are made of myofibrils
51
Q

actin filaments: skeletal muscle cells - what are myofibrils

A
  • contractile elements
  • they are formed from chains of sarcomeres
52
Q

actin filaments: skeletal muscle cells - what are sarcomeres

A
  • they are contractile units of muscle
  • it gives the muscle its striped appearance
  • they are a high organized assembly of 2 types of filaments
53
Q

actin filaments: sarcomeres - what are the 2 filaments

A
  • myosin II: thick filament
  • actin: thin filament
  • the actin filaments are anchored at the + ends at z discs and overlaps with myosin
54
Q

actin filaments: myosin II - how does myosin walk along the actin filament

A
  1. attached
  2. released
  3. cocked
  4. force-generating
  5. attatched (then it repeats)
55
Q

actin filaments: myosin II movement - attached (first one)

A
  • rigor conformation
  • myosin head that lacks a bound ATP or ADP is attached tightly to the actin filament
56
Q

actin filaments: myosin II movement - released

A
  • ATP binds and causes the dissociation of myosin
  • actin steps forward
57
Q

actin filaments: myosin II movement - cocked

A
  • ATP binding causes the displacement of the myosin head
  • movement of the myosin is driven by ATP hydrolysis and ADP and Pi binds
58
Q

actin filaments: myosin II movement - force-generating

A
  • the myosin head is bound weakly and Pi is released
  • this triggers the power stroke: the force generating change in the shape of the myosin head
  • this causes the head to regain its original rigor confirmation
59
Q

actin filaments: myosin II movement - attached (second one)

A
  • head is bound again but it is in a new position with no ADP
  • cycle starts again
60
Q

actin filaments: skeletal muscle cells - explain skeletal muscle contraction

A

it is triggered by the release of Ca2+ from the sarcoplasmic reticulum

61
Q

actin filaments: skeletal muscle contraction - how is Ca2+ released

A
  • motor neurons provides the signal (neurotransmitter) to the skeletal muscle and this triggers the action potential
  • the action potential then goes into transverse (T) tubules that extend inward from the PM around each myfibril
  • the electrical signal is then relayed to the sarcoplasmic reticulum
  • this causes voltage gated channels to release Ca2+
62
Q

actin filaments: skeletal muscle contraction - what happens after Ca2+ is released

A

Ca2+ activates accessory proteins tropomyosin and troponin complex

63
Q

actin filaments: skeletal muscle contraction - what is tropomyosin

A
  • it is rigid and rod-like
  • it binds in the groove of the actin
  • this prevents myosin from associating with actin
64
Q

actin filaments: skeletal muscle contraction - what the troponin complex

A
  • is also binds to Ca2+
  • it is associated with tropomyosin
  • in high levels of Ca2+, it triggers a change in the troponin complex
  • this then causes a shift in tropomyosin and allows it to be accessed by actin
  • once it is accessed by actin, contraction occurs

Cell Bio (21 decks)

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