Muscles & Movement: Microtubules & Microfilaments Flashcards
What do all physiological processes depend on?
movement
What are 6 physiological processes that rely on movement?
cell division
cell motility
intracellular transport
cell shape changes
muscle contractions
animal locomotion
T or F: all movement is caused by the same machinery
true
What are the 2 components of cellular machinery that cause movement?
cytoskeleton and motor proteins
What is the cytoskeleton?
an intracellular network of proteins
What is the cytoskeleton made of?
microtubules and microfilaments
In what 3 ways is the cytoskeleton used for movement?
as a road for motor protein carriers
as a way to reorganize the cytoskeletal network
as a rope for motor proteins to pull on
What are microtubules?
1 of 2 components of the cytoskeleton
tubelike polymers made of tubulin which can exist in different isoforms
What protein are microtubules formed from?
tubulin polymers
T or F: there’s only one type (isoform) of microtubules
false, there’s multiple and they’re made from similar proteins to tubulin in different animal groups
How are microtubules organized in the cytoskeleton?
they are anchored at both ends
the negative end is anchored to the MTOC near the nucleus
the positive end is connected to integral proteins in the membrane
What does MTOC stand for? what’s another term for it?
microtubule-organization center
aka centrosome
What anchors the - end of microtubules?
the MTOC near the nucleus
What anchors the + end of microtubules?
integral proteins in the membrane
What is the function of microtubules?
they are ‘roads’ for motor proteins to transport subcellular components
What are the motor proteins that travel along microtubules?
kinesin and dynein
What are some examples of subcellular components that motor proteins carry along microtubules?
melanophores to cause change to skin colour rapidly
vesicles from ER to Golgi
organelles like lysosomes and mitochondria
involved in separating chromosomes during mitosis and meiosis
also involved in cytokinesis (division of plasma membrane to produce 2 cells)
Describe how microtubules and motor proteins are involved in moving pigment granules in African Clawed frogs
African Clawed frogs can rapidly change their skin pigmentation to camouflage with their surroundings
melanophore cells have pigment granules (melanin) which when triggered by release of melanostimulating hormone are moved along microtubules by motor proteins to turn the skin colour darker and reverse by inhibiting hormone to turn lighter
Describe the structure of tubulin
a heterodimer made of alpha and beta tubulin
How do microtubules form?
spontaneously - does not require an enzyme
T or F: the formation of microtubules requires an enzyme to catalyze the reaction
false, it’s spontaneous
explain how microtubules are polar
they have - end and a + end
What is at the + end of a microtubule?
beta tubulin
What is at the - end of a microtubule?
alpha tubulin
Describe the steps for assembling a microtubule
alpha monomer and beta monomer both separately bind to GTP and are activated
alpha and beta dimerize, hydrolyzing GTP to GDP (alpha at - end, beta at + end) = tubulin
multiple tubulin dimers bind in alternating - alpha-beta-alpha-beta + orientation to form a single-stranded protofilament
multiple (13) protofilaments bind in rows (all - to + direction) to form a sheet
the sheet of protofilaments rolls up to form a microtubule (in - to + direction)
microtubule continues to grow by adding monomers to the + end and shrink by removing monomers from - end
What are the monomers that make up the tubulin dimer?
alpha and beta tubulin
how many protofilaments form a sheet?
13
T or F: dimers are only added to the + end and removed from the - end of microtubules
false, it’s more common for them to be added to the + end and removed from the -, but can be added or removed from either
What does it mean for microtubules to have asymmetrical growth?
they usually grow faster at the + end and shrink at the - end
Which end of a microtubule typically grows faster?
+
Which end of a microtubule typically shrinks?
-
How does a cell regulate growth and shrinkage rates of microtubules?
concentrations of tubulin
dynamic instability
MAPs
temperature
chemicals
How does the concentration of tubulin affect microtubule growth/shrinkage?
high tubulin = more growth
How does dynamic instability affect microtubule growth/shrinkage?
GTP-hydrolysis on Beta tubulin = more unstable
more GTP hydrolysis on B-tubulin = more binding to a-tubulin? maybe?
How do MAPs affect microtubule growth/shrinkage?
Microtubule-associated proteins stabilize the growth/shrinkageW
What does MAP stand for?
microtubule-associated proteins
How does temperature affect microtubule growth/shrinkage?
lower temperature (under 25 celsius) causes microtubules to disassemble
How do chemicals affect microtubule growth/shrinkage?
can have varying effects
Even though they live in very cold waters, Antarctic fish have stable microtubules - how? what is the minimum temperature microtubule assembly can occur?
their microtubules have amino acid variations which allow stable formation of tubulin polymers at colder temperatures (min -1.8 C)
When the concentration of tubulin is low, is the + end of a microtubule growing or shrinking? the - end?
both ends are shrinking
When the concentration of tubulin is increasing/mid, is the + end of a microtubule growing or shrinking? the - end?
the + end is growing
the - end is shrinking
this is called the treadmilling range
What is the treadmilling range?
the range of tubulin concentration at which the + end of the microtubule is growing and the - end is shrinking
When the concentration of tubulin is high, is the + end of a microtubule growing or shrinking? the - end?
both ends are growing
What regulates the length of microtubules?
Microtubule Associated Proteins (MAPs)
What are MAPs?
Microtubule-associated proteins that bind to the surface of microtubules to either stabilize or destabilize the structure
How do the MAPs that bind to + microtubule end affect the microtubule structure?
they prevent transition from growth to shrinkage = they maintain the microtubule length
What are some examples of MAPs?
STOPs
Tau
MAP2
+TIPs
Katanin
Which MAPs provide rigidity and prevent shrinkage/growth?
STOPs
Tau (neuron)
MAP2 (neuron)
Which MAPs increase growth of microtubules? which end do they bind to?
+TIPs bind to the + ends of microtubules
Which MAPs cause microtubule shrinkage?
Katanin = binds to GDP-bound tubulin and severs microtubules
What regulates the activities of MAPs?
protein kinases and protein phosphatases
What determines the direction of motor protein movement along microtubules?
polarity and type of motor protein
What are the 2 types of motor proteins that move along microtubules?
kinesin and dynein
Which direction does kinesin move along microtubules?
in the + direction
Which direction does dynein move along microtubules?
in the - direction
T or F: movement of motor proteins along microtubules requires an energy input
yes, ATP hydrolysis
What provides energy for the movement of motor proteins along microtubules?
ATP hydrolysis
What determines the rate of motor protein movement along microtubules?
ATPase domain of the motor proteins and regulatory proteins
How might axons and dendrites differ in their microtubule polarity?
axons have fixed polarity, the - end of microtubules is always toward the cell body and the + end is always toward axonal terminals
vs.
dendrites can have mixed polarity
- end may be toward axonal terminals and + end may be toward cell bodies or vice versa
What did scientists use to measure step size of kinesin and dynein?
Peroxisomes were labelled and used as cargo by dynein and kinesin to see how far it moved
How did the average movement of peroxisome of kinesin and dynein compare? what were they?
roughly equivalent
~8.6-8.9 nm
How do the velocities of kinesin and dynein compare? what are they?
they are roughly the same
~1.5-1.7 um/s
What imaging technology was used to determine the way kinesin walks?
FIONA
fluorescence imaging with one nanometer accuracy
Does kinesin walk hand-over-hand or inchworm along microtubules? how was this concluded?
hand over hand
predicted that if it were hand over hand, the movement of one labelled hand should be double the perixosome movement
OR if inchworm, the movement should be equal to the peroxisome movement
they found it was double = hand over hand
What was the average step size of kinesin? what does this conclude about how it walks?
17.3 nm which is ~ double the peroxisome movement = hand over hand
What has been concluded about how dynein walks?
neither hand over hand or inchworm
an active head moves ahead and drags the inactive head forward
What do unicellular organisms or different organs in the body use for movement?
cilia and/or flagella
What are cilia? give examples
numerous filaments that line tissue cells and produce wave-like motions
ex. in fallopian tubes, respiratory epithelium
What are flagella? give examples
microscopic hair-like structures that can be in pairs or single and produce whip-like motions
ex. sperm
What are cilia and flagella composed of? how many are there?
a bundle of parallel microtubules arranged into axoneme
9 pairs of microtubules around a central pair
What causes the movement in cilia and flagella?
asymmetric activation of dynein
What motor proteins move along the microtubules in cilia and flagella?
only dynein
What 5 physiological processes do microtubules have a role in?
cytokinesis
axon structural support
vesicle transport
pigment dispersion
movement in flagella
movement in cilia
How are microtubules involved in cytokinesis?
microtubules make sure chromosomes are equally divided after mitosis
How are microtubules involved in axon structure?
microtubules provide structural support to long axons
How are microtubules involved in vesicle transport?
they can carry hormones
How are microtubules involved in pigment dispersion?
they control pigment granule movement throughout a cell
How are microtubules involved in reproduction?
sperm flagella are composed of microtubule bundles which allow for the movement of sperm
How are microtubules involved in respiration and digestion?
cilia are composed of microtubule bundles which help cilia move fluids over epithelial cells
What are microfilaments?
polymers of actin that are the second type of cytoskeletal fiber for movement
What cell types are microfilaments in?
ALL eukaryotic cells
What motor protein is used by microfilaments?
myosin
T or F: microfilaments are only in some eukaryotic cells
false, they’re in all eukaryotic celsl
What activates movement of microfilaments?
actin polymerization
sliding filaments using myosin
What are the monomers of microfilaments? what structure do these have?
G-actin
globular
What roles do microfilaments have?
important in vesicle transport
movement of microfilament allows cells to change shape and move around the cytoskeleton
What are polymers of G-actin called (ie., when they assemble into filaments)?
F-actin
Does the polarity of microfilaments differ from microtubules?
no, they both have fixed polarity linked to the arrangement of the monomers
How does the assembly and disassembly of microfilaments compare to microtubules?
while microtubule assembly or disassembly requires an input of energy from GTP/GDP, microfilament dynamics are spontaneous
What does it mean that microfilament dynamics are spontaneous?
they can grow and shrink without energy input
When does actin polymerize?
when its concentration is high enough, it will polymerize
T or F: actin filaments can grow from both + and - ends
true but growth is faster at +
usually shrinks at - end
Do microfilaments also undergo treadmilling?
yes, when growth = shrinkage
What regulates the growth of actin?
Capping proteins
What type of capping proteins are associated with microfilaments?
Tropomodulins
Cap Z
Cofilin
Profilin
ARP
How does tropomodulin function?
it caps the - end of microfilaments to prevent disassembly of actin/shrinkage
how does Cap Z function?
it caps the + end of microfilaments to prevent growth/polymerization
How does cofilin function?
it binds to ADP actin of microfilaments to accelerate shrinkage
How does profilin function?
it binds to G-actin of microfilaments to accelerate growth
How does ARP function?
it nucleates F-actin = initiates polymerization of monomers of actin to produce the microfilament
Describe the steps of microfilament growth
G-actin monomers nucleate to form F-actin polymers
F-actin polymers continue to associate G-actin monomers to the + end to elongate the structure while G-actin is dissociated at the - end
capping proteins can bind to either end of the microfilament to control the dynamics
What are the 4 arrangements of actin within microfilaments?
actin network: cross-linking between networks (connecting protein looks like a 90 degree angle)
actin bundles: cross linking between parallel bundles (connecting protein looks like a straight line)
actin assembly: the actual actin assembly where actin monomers are polymerized into filaments which wrap around each other
membrane attachment: cross-linker protein connects actin in microfilament to integral membrane protein
What type of microfilament capping proteins increase shrinkage?
cofilin
What type of microfilament capping proteins decrease shrinkage?
tropomodulins
What type of microfilament capping proteins increase growth?
profilin
What type of microfilament capping proteins decrease growth?
Cap Z
What is an example of microfilament associated protein(s) cross-links actin to form parallel bundles?
fascin
fimbrin
alpha-actinin
What is an example of microfilament associated protein(s) cross-links actin to form networks?
filamin
What is an example of microfilament associated protein(s) cross-links actin to the membrane?
dystrophin
T or F: different arrangements of actin within microfilaments = different functions
true
What attaches networks and bundles of microfilaments to the cell membrane? what is the purpose?
dystrophin
to maintain cell shape and movement
How can actin polymerization cause movement? Ex. Amoeboid movement
Filipodia and lamellapodia are two types of amoeboid movement
Describe filapodia and lamellapodia in amoeboid movement
filapodia: rod-like extensions of the cell membrane that have neural connectons
lamellapodia: sheetlike extension of cell membrane
both are projections that have movement caused by the dynamics of microfilaments
these spines extend and retract from dendrites
What cell types have lamellapodia?
leukocytes
macrophages
Why would it be important for some cell types to be able to change shape as allowed by the dynamics of microfilaments?
because they need to be able to move around the body
ex. immune cells need to be able to reach injured parts of the body
How is actin polymerization involved in fertilization?
What is the model that describes how myosin moves along microfilaments?
the sliding filament model
Does myosin require energy to move along microfilaments?
yes, it is an ATPase
How many classes of myosin are there?
> 18 with multiple isoforms in each one
T or F: while there’s multiple isoforms of myosin, they all have similar structures
yes
What is the basic structure of myosin?
head with ATPase activity
tail for binding subcellular components (cargo)
neck to regulate ATPase
All but what type of myosin move toward the + end of microfilaments?
all but myosin 6
What is the head of myosin responsible for?
ATPase activity (generating energy)
What is the neck of myosin responsible for?
regulating the ATPase activity
What is the tail of myosin responsible for?
carrying cargo
What are the 3 main types of myosin we’re looking at?
1, 5, and 2
I, V, II
Which is the most prevalent type of myosin?
Myosin V
Which is the muscle myosin?
myosin II
Describe the structure of myosin I? (monomer, dimer, etc, how many light or heavy chains)
Monomeric if considering that it’s one head, one neck, one tail with 1 heavy and 1 light chain
dimeric if considering that it’s 1 heavy chain and 1 light chain
What are myosin/calmodulin light chains?
myosin protein binds necks to heads and tails of one myosin protein or to bind 2 myosins together
binds calmodulin to modulate ATPase activity of myosin head
Describe the structure of myosin V
2 myosin proteins, each with 1 calmodulin light chain and 1 heavy chain, wrapped together after the binding of the light chains at neck
dimeric if considering 2 individual myosin proteins
tetrameric if considering total of 2 light chains and 2 heavy chains
Describe structure of myosin II
2 types of light chains: regulatory and essential in each myosin protein
2 myosin proteins, each with 2 light chains and 1 heavy chain
dimeric if considering 2 individual myosin proteins
hexameric if considering that both of the 2 myosin proteins have 2 light chains and 1 heavy chain
What are the 2 types of light chains in myosin II?
regulatory and essential
How is calmodulin related to myosin activity?
light chains have calmodulin binding sites that control ATPase activity - when there’s higher binding of calmodulin = there’s increased ATPase activity
What do myosin regulatory light chains do?
sites of phosphorylation for myosin light chain kinase (MLCK) that when activated increases speed of myosin movement
What are myosin essential light chains for?
to provide structural support for the myosin head
What analogue can be used to describe the sliding filament model?
pulling yourself along a rope where the actin is the rope and the myosin is your arm and hand
What are the basic steps of the sliding filament model?
alternating cycles of grasp, pull, and release
in the rope analogy:
1. hands grasp the rope
2. muscles contract to pull the rope
3. hand releases, extends and grasps the rope further along
describe the steps involved in the sliding filament model
microfilament has myosin head attached, no ATP bound
- ATP binds = myosin releases microfilament
- ATP dephosphorylated by myosin head which then extends and binds to actin further along microfilament
- release of inorganic phosphate triggers a power stroke which pulls the microfilament in the opposite direction (moving the ADP-bound myosin head further along microfilament)
- ADP is released from myosin head
- continues
What are the 2 processes in the sliding filament model?
chemical reaction in which myosin binds to actin - aka cross-bridge
structural change in which myosin bends to create a power stroke when inorganic phosphate is released
What is the cross-bridge cycle?
myosin binds to actin to form the cross-bridge
myosin releases actin because ATP binds
ATP is dephosphorylated by ATPase activity of myosin head which causes myosin to extend
inorganic phosphate is released causing the ADP-bound myosin head to bend and power stroke (pull on the microfilament) to move further along
What happens when there’s no ATP in the sliding filament model?
rigor mortis and myosin cannot release actin
What 2 factors affect myosin movement?
unitary displacement
duty cycle
What is unitary displacement? How does it affect myosin movement along microfilaments?
the step-size/distance myosin moves during each cross-bridge cycle
the step-size depends on the length of myosin neck and the location of myosin binding sites on actin (the helical structure of actin)
What is the duty cycle?
basically it’s the amount of time the myosin head is bound to the microfilament
cross-bridge time divided by the cross-bridge cycle time
What is the average duty cycle for myosin V? what does this mean?
~0.5
this means that 50% of the time, myosin V is bound to actin
How does the duty cycle affect myosin movement along microfilaments?
using more than one myosin dimer to maintain contact with actin can increase the speed of movement
Approximately how often are there myosin binding sites along actin for myosin V? How does this relate to the helical shape of actin filaments in microfilaments? how does this effect myosin movement?
there are myosin binding sites every 36-37 nm which is equal to the length of one turn of the helix of actin
this means that myosin steps are = to the helical turn of actin
What fashion does myosin V move along microfilaments? is it hand over hand or inchworm?
if it were to be hand over hand, the results should show step sizes double that of the cargo movement (~74 nm)
if it were to be inchworm, the results should show step sizes equal to the cargo movement (37 nm = 1 helical turn)
HAND OVER HAND movement was discovered
what physiological processes do microfilaments function in?
vesicle transport
microvilli (digestion)
amoeboid movement
skeletal, cardiac, smooth muscle contraction (only myosin II)
