Flashcards in 19-21. Cytoskeleton Deck (70):
What is the primary function of actin filaments?
determine cell shape, movement, secretion, and endocytosis
What is the primary function of microtubules?
1. position membrane bound organelles
2. intracellular transport direction
3. form centrioles/mitotic spindle
4. form cilia/flagella
What is the primary role of intermediate filaments?
1. mechanical strength
2. resist mechanical stress
What is the structure of actin filaments?
1. two-stranded helical polymer
2. compact and globular
What is the structure of microtubules?
1. long hollow cylinder
2. formed by tubulin subunits
3. one end attached to microtubule organizing center (centrosome)
What is the structure of intermediate filaments?
2. span cytoplasm and cell-cell junction to provide support
Which surface of cells absorbs nutrients?
What quadruples the surface area in intestinal cells to increase absorption rate?
1. microvilli which form from actin units
What allows cytoskeletal filaments to re-arrange rapidly?
1. non-covalent bonds between protein subunits
What is a protofilament?
1. long linear string of proteins joined end-to-end.
must be stacked next to one another to increase strenght. 1 is weak
A stacked formation of protofilaments is most stable, what is the result of staggered protofilament assembly?
1. flexible, bending and stretching intermediate filaments.
2. yarn-like properties
Polymerization of an actin filament refers to what?
1. assembly of actin or tubulin monomers into a polymer
What is nucleation?
1. the initial step required for polymer assembly to occur.
What units combine to form actin nucleation site?
alpha, beta, gamma
Describe lag phase of filament formation.
1. filament nucleation
2. rate-limiting step and formation of aggregate
Describe the growth phase of filament formation.
1. elongation of polymer
2. rapid addition of monomers to nucleated filament
Describe the steady equilibrium phase of filament formation?
1.addition of new subunits equals rate of subunit disassociation
What is the critical concentration, and when does it occur?
1. occurs during the equilibrium phase (steady state)
2. refers to concentration of monomers in solution
Tubulin is a heterodimer structure of what?
alpha, beta units non-covalently linked
Does tubulin (microtubules) have polarity?
yes. due to alpha/beta subunit addition
Does actin have polarity?
yes, head to tail arrangement causes polarity.
What is the plus-end?
1. fast-growing end
2. consists of beta-tubulin or barbed end
What is the minus end?
1. slow-growing, shrinking end
2.pointed end on tubulin
What will cause treadmilling?
1. plus end addition is faster than minus end.
2. plus end will remain with a triphophate nucleotide and minus end will remain with diphosphate nucelotide
3. polymer remains a constant length as the plus and minus ends change at the same rate.
4. predominates in actin filaments
What type of filaments is treadmilling predominant in?
What is catastrophe?
1. microtubule begins to shrink
What is the cause of catastrophe?
1. nucleotide hydrolysis more rapid than subunit addition, causing the cap to be lost and microtubule fall apart
What is rescue?
1. GTP units add to shrinking end, and can form a cap
What does rescue of a microtubule refer to?
1. returning microtubule back to growth.
What does the loss of GTP on microtubule filaments produce?
1. curving effect on confromation of subunits leading to shrinking
Where is dynamic instability most often associated with?
Describe the building of intermediate filaments.
1. antiparallel pairing of a coiled-coil dimer.
2. forms overall tetramer when two dimers combine
How is an intermediate formed?
1. lateral packing of 8 parallel tetramers
What is keratin and where is it most likely to be found in cells?
1. diverse group of intermediate filaments providing mechanical support
2. found in hair nails
3. at desmosomes, or hemodesmosomes
What are the most important features of cytoskeleton
1. form from protofilaments
2. cytoskeleton filament formation has nucleation as rate limiting step
3. subunits are polarized
4. treadmilling and dynamic instability are results of nucleotide hydrolysis
What is responsible for determining the shape and movement of cells?
actin filaments in the cell cortex
What are lamellipodia?
1. flat protrusive regions of the PM from actin filament rearrangment
What are filopodia?
1. spike-like protrusions from PM via actin filament rearrangment
What is the role of ARP2, ARP3 or the ARP complex?
1.nucleates actin filaments at the (-) end
2. but it requires activation factor (inactive proteins without)
What is the benefit of having ARP complex and activating protein present?
allows growth/elongation by bypassing the normal nucleation step (rate-limiting)
-- used by listeria for movement
When is the ARP complex most efficient at elongation?
- 70 degree angle from existing filament, forming cross-linking
What is the specific role of formin with actin filament growth?
1. facilitates straight, un-branched growth
2. bind to (+) end, provides growth from this region
What is the role of thymosin in actin polymerization?
1. binds to increase soluble monomers
2. unable to associate with actin filament, increases cytosolic concentration of actin monomers
What is the specific role of profilin with actin polymerization?
1. binds to actin monomer, causing reduced affinity for thymosin
2. allows monomer assembly into an actin filament
What two proteins competitvely compete for binding with actin monomers?
1. thymosin (polymerization inhibitor)
2. profilin (polymerization activator)
Both MAP2 and Tau help stabilize microtubules from degradation. What is the difference between the two?
1. MAP2: only has one point of contact, which increases the space in between adjacent tubules.
2. Tau: two tubule binding points that reduce amount of free protein, allowing for tighter packing.
What is a key protein in erythrocyte membrane skeleton assembly that prevents actin filaments from interacting with other proteins?
Cofilin binds to ADP regions of actin filament to promote degradation how?
1. induces twisting of filament to weaken interactions between actin subunits
2. tropomyosin offers protection against cofilin
What protein is able to increase the rate of microtubule shrinkage?
What mechanism does kinesin-13 use to cause catastrophe in microtubules?
1. lower activation energy barrier of the protofilaments, causing the protofilament to spring apart and curve, allowing it to fall away as a monomer
Alpha-actinin and fimbrin are both actin filament cross-linking proteins. How do they differ from one another?
1. alpha-actinin has loose bundle formation to allow for myosin II to form contactile fiber
2. fimbrin: packs tightly together to exclude myosin II
What do cells require in order to extend membrane projections allowing them to crawl across solid surfaces?
1. filamin which forms an actin gel
What proteins are required to enhance the attachment between actin and PM?
ERM family (ezrin, radixin, moesin)
What is the purpose of the head and tail regions of motor proteins?
1. head region: binds to certains tracks and moves along them
2. tail region: binds cargo for transportation
Where does ATP bind MyosinII?
N-terminal region followed by coiled-coil region for heavy chain dimerization
Where can the large amount of myosin heads be found?
on the tail region where tail-tail overlapping occurs
Myosin II uses ATP hydrolysis to produce what?
1. movement toward (+) end, causing contraction
Contain tail binding site for membrane enclosed organelles
- motor domain at N-terminus and migrate to (+) end of microtubules
motor domain migrates to (-) end of microbtubules
1. two types
- cytoplasmic dyneins
- axonemal dynein
What is cystoplasmic dyneins used for?
1.vesicle trafficking and golgi apparatus localization
2. 2 motor domains
What are axonemal dyneins used for?
1. rapid microtubule movement of cilia/flagella.
2. 2-3 motor domains
3. faster than cytoplasmic dyneins
What are the different stages of cell crawling?
1. polarization and protrusion
2. Adhesion and traction
What occurs in the polarization/protrusion phase of cell crawling?
1. actin-rich regions extend in front of cell.
front and back differ in make-up
What occurs in adhesion/traction phase of cell crawling?
1.adherence to ECM, providing traction for movement over material
What occurs in re-traction phase of cell crawling?
1. adhesions disassemble at rear. majority of cell is drawn forward
1. migrating fibroblasts
3. long core of actin filament
1. epithelial cell, fibroblast, neurons
2. 2-D, sheet
3. cross-linked mesh of actin
1. amoeba and neutrophil formation
2. 3-D with actin filament gel protrusion
Activation of Rac-GTP from PI3kinase acts on what messengers to induce formation for cell migration?
1. activate WASP--> lamellipodia formation
2. activates PAK--> filamin but inhibiits myosin light chain kinase (reduce myosin activity)