Cytoskeleton 2 Flashcards
Lamellipodium:
- a protrusion at the leading edge of a migrating cell.
- caused by actin polymerization
Two parts of cell migration via actin polymerization:
- formation of lamellipodium at the leading edge due to actin polymerization.
- collapse of the cell tail by actin depolymerization due to myosin II.
Contractile Ring:
- the ring formed during cytokinesis whe two cells are dividing.
- formed by actin and myosin II.
Adhesion belts:
- contractile bundles of actin and myosin filaments near the apical surface of epithelial cells.
- important for morphogenesis:
- myosin II contraction drives tissue invagination and formation of the neural tube.
The three classes of cytoskeletal motors:
- dynein
- kinesin
- myosin
all are ATPases and have multiple isoforms, encoded by multiple genes
Kinesins:
- carry cargo from the centrosome (minus end) to the periphery (plus end) on microtubules
Dyneins:
- carry cargo from the periphery (plus end) to the centrosome (minus end) on microtubules.
- much larger than kinesins
Myosins:
- move cargo on actin
- most move from the minus end to the plus end
Dynein, kinesin, and myosin are all:
ATPases
“mechanochemical enzymes”
What two motors are related structurally and mechanistically?
kinesin and myosin
however, have different functions
Particular isoforms of cytoskeletal motors …
- only move in one direction on an actin filament or microtubule.
Molecular/cytoskeletal motors are the downstream targets of:
signaling cascades
How do molecular motors function/move on a filament?
- They are enzymes (ATPases)
- Chemical energy from binding/hydrolysis of ATPase leads to:
- → intramolecular conformational change
- → mechanical work
What is the work done by molecular motors?
- generation of tension or movement of an object along an actin filament or microtubule
Do intermediate filaments have motors?
NO
Myosin structure:
tail → neck/hinge → light chains → motor/head
- Globular “head” is the motor domain; contains ATPase.
- Variable “Tail” domain contains coiled coil for dimerization, and/or binds to membrane or target vesicle.
Classification of myosins is based on:
- motor domain (“head”) homology
Kinesin head domains are structurally related to:
myosin and G-proteins
Kinesin Structure:
tail → stalk → neck → motor/head
What portion of the kinesin determines its polarity?
the neck region next to the motor/head domain
What part of the kinesin contains the light chains?
the tail region
Kinesin N-terminal motors have what polarity:
plus-end directed
Kinesin C-terminal motors have what polarity:
minus-end directed
What two molecular motors have large gene families?
myosin and kinesin
Dynein motors:
- ATPase
- minus-end directed
- multi-subunit protein
Dynein is what kind of protein:
- AAA protein
- “ATPase Associated with diverse cellular Activities”
- Typically 6 domains
- ATPase domain connected to a microtubule via a stalk
ATPase NOT DIRECTLY ATTACHED TO MICROTUBULE
Primary ciliary dyskinesia (PCD)
AKA “Kartagener syndrome”
- Dynein associated disease
- respiratory tract infections; male infertility
- dynein is mislocalized in the cell and does not make it to the cilia
Cause of Primary ciliary dyskinesia (PCD)
AKA “Kartagener syndrome”
- mutation in the outer arm ciliary dynein heavy chain leads to:
- dynein outer arms in cilia are missing
- cilia are immotile
- chronic infection of the respiratory tract
- males sterile
What is the rate-limiting step in cytoskeletal motor function?
- product release following hydrolysis
- a phosphate for myosin and kinesin
- Binding to actin or microtubules accelerates the rate-limiting step.
How do cytoskeletal motors work?
- All convert the energy of ATP hydrolysis into mechanical work and heat.
- Binding and/or hydrolysis of ATP causes conformational change in the motor.
What can lead to rigor mortis in muscle?
- depletion of ATP, which causes myosin to remain attached to muscle
What determines the speed of myosin moving on an actin filament?
- rate of myosin ATPase and Pi release
Steps in myosin II moving down an actin filament:
- ATP binds and causes release of myosin motor from actin
- recovery stroke occurs
- ATP hydrolyzed
- myosin re-binds to actin
- phosphate from ATP hydrolysis released
- power stroke
- repeat
Where/when can you regulate myosin II function?
- at the kinetic step when ATP is hydrolyzed
- at the step when a phosphate from ATP hydrolysis is released
What causes strong binding of myosin II to actin, which ultimately leads to the power stroke?
- release of the phosphate residue from ATP hydrolysis
How does the kinesin motor function?
Same as myosin:
- Binding of ATP weakens affinity of kinesin for microtubule.
- After ATP hydrolysis, kinesin binds with higher affinity to microtubule.
- Binding to microtubule accelerates product release and conversion from weak to strong binding.
- Power stroke occurs.
Does kinesin bind to alpha or beta tubulin?
beta tubulin in the microtubules
Myosin substrate, filament, and direction of movement:
- ATP
- actin
- plus-end (mostly)
Kinesin substrate, filament, and direction of movement:
- ATP
- microtubules
- plus-end (mostly)
Dynein substrate, filament, and direction of movement:
- ATP
- microtubules
- minus end
What determines the localization of a cytoskeletal motor?
the tail region of the motor
Melanin transport:
- melanin produced by melanocytes
- melanin placed in vesicles called melanosomes.
- melanosomes transported on microtubules and actin filaments to the periphery of the cell where they are taken up by keratinocytes.
Myosin V is required for transport to the periphery.
What myosin transports melanin from the melanocytes to the keratinocytes?
Myosin V
Besides myosin V, what else is required to move melanin from the nucleus of a cell to the periphery?
- myosin V tail binds melanophilin, which is bound to Rab27aGTP/melanin
Griscelli type 1 and Elejalde syndromes:
- due to mutations in myosin Va, melanophilin or Rab27a
- patients have silvery hair, light skin in child of dark-skinned parents
- severe neurological effects
- melanin in skin is clumped in basal layer
- melanin in hair is clumped in center
