Cytoskeleton I Flashcards
What are the roles of cytoskeleton?
- Establishment of cellular polarity (not necess +/- charge mind you)
- Directional migration
- Formation of bipolar mitotic/meiotic spindle
- Chromosome segregation
- Cytokinesis
Also intracellular transport, exocytosis, endocytosis
What are the three cytoskeletal components & their sizes?
Actin - 5-9mm (10-15% protein in cell)
INtermediate filaments - 10nm
Microtubules - 25nm
What cytoskeleton component composes microvilli projections? Where do they end?
Actin filaments. Come to a terminal web of actin
Where are intermediate filaments located in epithelial cells?
Between desmosomes and basal lamina.
Where are microtubules located in epithelium? What is the polarity of these?
Connecting apical surface to of cell to bottom of cell.
(Minus end (-)) is at apical surface
(Plus end (+)) is at basal surface
How are larger cytoskeletal structures assembled?
Smaller protein subunits
(Non-covalent polymers)
Dynamic
Accessory proteins regulate sites and state of assembly
ADAPTABLE (i.e. Source of something like a nutrient source; dissasembly of filaments and rapid diffusion of subunits; then reassembly at new site)
What are known as the ‘tendons of the cell’?
Intermediate filaments (int. In size betwn actin and microtubules; more stable)
What are intermediate filaments function?
MAJOR components of cytoskeleton and nuclear boundary and functional organization of cellular architecture
Protection from mechanical stress, stress absorbers
(Viscoelastic filaments w/in cell and at junctions betwn cells and with ECM)
Signaling and controlling gene regulatory networks
No known motors
Where are int. Filaments found in the cell?
Surround nucleus and extend to cell periphery
Cell-cell & cell-ecm junctions
Controlled thru phosphorylation
What is basic structure of int filaments?
Two-chained coiled coil that assembles to form tetramer.
Tetramer forms higher order assemblies (10nm filament)
N-term and C-term ends globular.
Coiled coil regoin interrupted by linker domains
How do int. Filaments assemble? Is the structure polar?
Antiparallel tetramers.
Overall structure is not polar (in contrast to actin/microtubules)
Int. filaments only exist as dimers (in an antiparallel fashion)
They dimerize again to form an anti-parallel staggered tetramer.
They dimerize again to form a ropelike int. Filament(10nm thick)
What are actin filaments? (F-actin)
Polymers of globular protein (g-actin)contains bound nucleotide (ATP/ADP)
Is f-actin polar?
YES, they differ in structure and dynamics.
What are the characteristics of the plus end? MInus end?
?
What is shape of filament and how are they modified?
Plus/barbed end -> faster growing end
Minus/pointed end is slower growing
Filament is helical and Actin binding proteins can modify filament dynamics and higer order assemblies
How are polymers of alpha/beta tubulin arranged into tubules?
What is the mictotubule subunit?
13 protofilaments.
Bound GDP/GTP
Polar structures
(Tubulin heterodimer = microtubule subunit)
How are microtubules and actin filaments similar?
- Assembly from globular proteins by a condensation /polymerization mechanism to form polar structure
- Monomer addition preferred at ‘plus end’
- Nucleotide hydrolysis lags behind polymerization leaving an ATp or GTP cap at growing end
- NRG of hydrolysis not required for polymerization
- Nucleotide at plus end determines stability
- Dynamics and state of structure can be determined by actin/microtubular binding protein.
THESE STRUCTURES ARE UNRELATED
At what side is the rate of elongation faster?
“Barbed”/plus end.
Nucleation is rate limiting.
ATP-actin is preferentially added to barbed end
ATP hydrolysis not required for polymerization. Bound nucleotide influences stability of the ends and interactions with other proteins.
Actin filament has ADP-actin except for the extreme barbed end.
What state are soluble subunits in?
T form.
What is the step to take place after polymerization of actin-filaments and mictrotubules?
Hydrolysis of NTP->NDP
Pi release takes place after polymerization
Plus end (+) ->hydrolysis lags behind
Minus end (-) ->Hydrolysis catches up
Are microtubules generally stable?
No, there is dynamic instability
Nucleotide present at plus (+) end affects mictotubule growth and stability
GTP-tubulin cap stabilizes the plus (rescue)
GDP-tubulin destabilizes resulting in rapid depolymerization (catastrophe)
What determines the state and dynamics of actin?
Actin binding proteins
Why are tubulin and microtubule structure important to the cell?
Important for vesicular and organelle transport.
Form the mitotic spindle
Cilia and Flagella
Centriole and basal bodies
What is a primary cilium in a cell?
Non-motile
Usually one per cell
Sensory organelles
Central player in development signaling pathways
What is structural difference from motile and non-motile cilia?
No inner doublet
No arms
What is the centrosome?
Microtubule organizing center.
Organized around a pair of centrioles.
- > centrioles surrounded by pericentriolar material;>100 proteins
- ->contains gammatubulin ring that nucleates 13 protofilaments of microtubules and caps the (minus/-) ends.
Consequence: _plus end oriented towards cell periphery.
When do centrioles duplicate?
Beginning of S phase mitosis
What are MAPs responsible for? Microtubules associated proteinsk
Regulate state of microtubule assembly
(Both stabilize/destabilize plus or minus end)
Bind to side (stabilize by side binding or bundle formation)
Sever
What is a MAP that is implicated in Alzheimer’s diz?
Tau (neurofibrillary tangles).
Binds to side and makes structure very bulky
What proteins inhibit catastrophes in microtubules? What does this allow?
+ tip proteins. Allows microtubules to reach the cell periphery.
Communication and connection with the cell cortex.
- > transport materials to cortex
- > interaction with actin cytoskeleton.
Capture chromosomes during mitosis: association with kinetochore.
What are some toxins that interfere with microtubule/actin polymerization?
- Phalloidin -> binds and stabilizes actin filaments. Death angel mushroom
- Colchicine - depolymerizes microtubules. (Autumn crocus)
3. Taxol - binds and stabilizes microtubules. Pacific yew (natural) Used widely as an anticancer drug
How do neutrophils track down bacteria?
Chemotaxis *involves actin polymerization”
& myosin-II dependent on contractions of tail
What is role of dynamic actin filaments in cellular function?
Actin polymerization alone can drive cell migration.
Certain bacteria can hijack this machinery.
How is actin polymerization enough to force movement?
Needs lots of growing ends to push against the plasma membrane.
- > Nucleate more actin filaments
- > sever existing filaments to creat more barbed ends
- > Form branches from existing actin filaments
Arp2/3 (complex of 7 proteins)
-> nucleates filaments from sides of actin filaments making complex branched structures
What regulates dendritic nucleation?
Rho-dependent signaling cascade (rac is a subclass of rho)
Arp 2/3 is downstream of rho family of gtpases.
Localizes activation at the cell membrane, leading edge, site of protrusion of lamellipodium .
What is Arp2/3 dependent polymerization involved in?
- Neutrophil migration to sites of infection
- Wound healing
- Invasion of metastatic cancer cells
- Certain bacterial infections
- Clathrin-dependent endocytosis
- Et al
How does listeria monocytogenes infect?
Infects intestinal epithelium and hijacks ARP 2/3 dependent actin polymerization machinery
Protein is homologous to WASP/Scar on the bacterial surface activAtes Arp2/3
Actin filaments polymerize forms a comet that propels the bacteria thru the cytoplas.
