Cytoskeleton 1 Flashcards
The cytoskeleton is involved with:
- cell shape
- cell’s ability to migrate
- formation of mitotic spindle
- chromosome separation during anaphase
- intracellular transport
- exo/endocytosis
What are the three cytoskeletal components?
- actin (5-9nm)
- intermediate filaments (10nm)
- microtubules (25nm)
all of these components can interact with one another
Cytoskeletal structures are:
- non-covalent polymers of smaller protein subunits.
- dynamic and adaptable.
What regulates the sites and states of cytoskeleton assembly?
accessory proteins in response to intra- and extracellular signals
Intermediate filaments roles:
- major components of nuclear (lamins) and cell structure
- roles in:
- mechanical support (skin)
- cell migration and movement
- cytoarchitecture
- signaling
Intermediate filaments protects cells from:
- mechanical stress; they are stress absorbers
- viscoelastic filaments within cells and at junctions between cells and with extracellular matrix.
Skin, hair, and nails are all composed of what types of filaments?
intermediate filaments
Keratins, nuclear lamins, and neurofilaments are all what types of filaments?
intermediate filaments
Basic structure of intermediate filaments:
- two-chained coiled coil that assembles to form tetramer.
- tetramer forms higher order assemblies, 10 nm filament.
- N-terminal and C-terminal ends are globular; coiled coil region interrupted by linker domains.
Steps in formation of an intermediate filament:
- alpha-helical region in a monomer forms coiled-coil dimer with another alpha-helical region of a monomer.
- coiled-coil dimer forms staggered tetramer with another coiled-coil dimer.
- two staggered tetramers are packed together.
- eight-tetramers twist into a rope-like filament.
Intermediate filaments assemble as _____ tetramers:
- antiparallel
- non-polar filaments
- reason why there are no motors
- not involved in directional movement
- non-polar filaments
What two filaments are polar structures?
actin and microtubules
Actin filaments (F-actin) are polymers of:
- globular protein actin (G-actin) that contains a bound nucleotide (ATP or ADP).
General characteristics of actin filaments:
- polymers of actin
- polar
- helical
- plus-end (fast growing) and minus-end (slow-growing)
- only two actin genes in genome
Plus end and minus end of actin filaments:
- have nothing to do with charge; names based on assembly kinetics:
- plus end = fast-growing
- minus end = slow-growing
What is the rate-limiting step of actin polymerization?
- nucleation
Actin monomers have ATP attached. What is this ATP used for?
- NOT required for polymerization
- the bound ATP influences the stability of the filament ends.
What kind of proteins regulate actin polymerization and growth?
- capping proteins
- (actin filament can only grow one way)
- severing proteins
- (cuts actin filaments)
- cross-linking proteins
- (attaches actin filaments)
Stability of actin filaments and microtubules is determined by:
- the nucleotide at the plus-end of the filaments
Actin filament and microtubule parallels:
- nucleation-polymerization from monomeric proteins
- polar structures
- grow from plus-ends
- plus-end determines stability
- regulated by binding proteins
Are actin and microtubule structures related?
no
The monomer building block of microtubules is:
- tubulin heterodimer.
- two subunits:
- alpha-tubulin
- beta-tubulin
- two subunits:
Basic microtubule structure:
- Polymers of alpha/beta-tubulin arranged in tubules with 13 protofilaments.
- POLAR
Microtubules are used for:
- vesicular and organelle transport
- formation of mitotic spindle, cilia and flagella
- formation of centriole and basal bodies
Primary cilium:
- a non-motile cilia composed of microtubules
- one per cell
- sensory organelles involved in signalling pathways
- NO DYNEIN (the motor of cilia)
Centrosome:
- microtubule organizing center
- forms around two centrioles
- nearly all microtubules project from centrosomes
- plus-end projects to cell periphery
- minus-end at centrosomes
- POLARITY
Centrioles are duplicated during what phase of the cell cycle?
S-phase
Formation of the mitotic spindle and chromosome separation is dependent on:
microtubule polarity
How is polarity formed in actin filaments and microtubules?
- minus end addition is slow
- GTP/ATP hydrolyzed to GDP/ADP
- plus end addition is fst
- GTP/ATP hydrolysis cannot keep up
- GTP/ATP remains on plus-end cap
The nucleotide at the plus end affects microtubule growth and stability. How so?
- plus end GTP-cap stabilizes the microtubule.
- microtubule grows
- loss of GTP-cap destabilizes microtubule.
- microtubule collapses
- regain of GTP-cap stabilizes microtubule.
- microtubule growth recurs
Mictroubule catastrophe and rescue:
-
catastrophe = loss of GTP-cap on plus end and microtubule collapse
- when GTP-hydrolysis catches up to the growing end.
- rescue = regain of GTP-cap on plus end and microtubule regrowth.
Microtubule associated proteins (MAPs):
- Regulate state of microtubule assembly
- Stabilize or destabilize plus or minus end
- Bind to the side: stabilize by side binding or bundle formation
- Sever
gamma-tubulin ring complex location and function:
- found at the minus end of microtubules at the centrosome
- nucleates minus end
- stabilizes minus end
Tau:
- a microtubule side-binding protein
- Alzheimer’s disease = tau in neurofibrillary tangles/aggregates.
+tip proteins:
INHIBIT MICROTUBULE CATASTROPHE
- bind to and track with the plus end of a growing microtubule.
- stay with the plus end as it is growing.
- interact with actin skeleton
- transport materials to cell periphery
- connects to kinetochore in mitosis
Phalloidin:
- actin filament toxin
- binds and stabilizes actin filaments
- found in death angel mushroom
Colchicine:
- microtubule toxin
- deploymerizes microtubules
Taxol:
- microtubule toxin
- binds and stabilizes microtubules
- widely used as an anti-cancer drug
Cell migration is involved in:
- pathfinding and targeting of neurons
- chemotaxis (neutrophils to infection)
- tissue formation, repair, remodeling (wounds)
- cancer metastasis
Cellular movement and intracellular transport can be driven by:
- both polymerization and motors
Neutrophils chase bacteria during infection via:
- chemotaxis
- actin polymerization at the leading edge
- myosin-II dependent contractions of the tail
How does actin polymerization alone provide the force for cell movement?
- actin filaments push against cell membrane:
- elongation/polymerization at plus/barbed end
- nucleation of more actin filaments
- formation of branches
Arp2/3 complex:
- Nucleates filaments from the sides of actin filaments, making complex branched structures.
- branches grow at plus end and can push against cell membrane.
- involved in cell movement
Arp2/3 is activated by:
- Downstream rho family of small GTPases signaling cascades.
- localizes activation at the cell membrane
- catalyzes cell movement
Arp2/3 activation is involved in actin polymerization for cell movement in what situations?
- neutrophil migration to infection
- wound healing
- cancer metastasis
- endocytosis
Listeria:
- food-borne pathogen
- contains proteins homologous to Arp2/3 activating proteins
- hijacks cell’s Arp2/3 machinery causing actin polymerization on listeria molecule tails.
- listeria molecules protrude from one cell and infect another.
