Cytoskeleton (Lecture 7) Flashcards
What 2 forms are actin microfilaments found in?
monomers (G-actin) or long chains (F-actin)
True or False:
Actin microfilaments are highly conserved among eukaryotes.
True, actin microfilaments are hight conserved among eukaryotes.
What variety of actin microfilaments are found in muscle tissues?
alpha-actin
Which two varieties of microfilaments are found in non-muscle?
beta and gamma actin
What are the 2 drugs that affect actin polymerization?
Cytochalasin and Phalloidin
How does cytochalasin affect actin polymerization?
Blocks the growth of actin filament by binding to the barbed ends, thus blocking elongation which can inhibit movements (e.g: cell division).
How does phalloidin affect actin polymerization?
Phalloidin binds to actin filaments and prevents dissociation.
Which drug that affects actin polymerization of microfilaments can be used in the laboratory to allow visualization of actin filaments with fluorescent dyes?
Phalloidin
Name 3 of the 8 actin-binding proteins?
Spectrin Dystrophin Villin and Fimbrin Calmodulin and Myosin I alpha-Actinin Filamin
Which actin-binding protein is found in RBCs?
Spectrin
Which 2 actin-binding proteins both bond cortical cytoskeleton to the plasma membrane?
Spectrin and Dystrophin
Which 2 actin-binding proteins cross-link actin in microvilli?
Villin and Fimbrin
Which 2 actin-binding proteins cross-links actin to plasma membrane in microvilli?
Calmodulin and Myosin I
Which actin-binding protein cross-links stress fibers and connects actin to protein-plasma membrane complex complexes?
alpha-Actinin
Which actin-binding protein cross-links actin at wide angles to form screen-like gels?
Filamin
Name 4 of the 7 actin-binding proteins that control treadmilling?
Thymosin Profilin Gelsolin Cofilin Arp2/3 Phalloidin Latrunculins
What does thymosin do?
Captures actin monomers and prevents actin monomers from being polymerized.
What does Profilin do?
Binds to actin monomers and prevents monomers from being polymerized.
Facilitates exchange of bound ADP for ATP - which favors polymerization
NOTE: that only ATP-actin monomers can be assembled into F-actin
What is the function of Gelsolin?
Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin
In presence of Calcium ion, fragments actin filament and remains bound to the plus end
What does Cofilin do?
Triggers depolymerization of ADP-bound actin at the minus end
What does Arp2/3 do?
Initiates growth of F-actin from sides of existing filament - causes branching. Thus, changes the shape of the actin.
What does phalloidin do?
It prevents depolymerization by binding to actin filaments.
What does Latrunculins do?
It binds to G-actin and induces F-actin depolymerization.
In what type of cells would you expect to find an abundance of intermediate filaments?
Cells subject to mechanical stress.
Are intermediate filaments found as desmosomes and hemidesmosomes.
YES
**Intermediate filaments strengthen epithelial cells as desmosomes and hemidesmosomes.
What three specific types of proteins make up the cytoskeleton?
1) Microfilaments (actin) (7 nm thick)
2) Intermediate Filaments (8-10 nm thick)
3) Microtubules (25 nm in diameter)
Which two domains of intermediate filaments determine its specific function?
The head and tail domains.
Label, in order, the general structure/assembly of an intermediate filament, starting at the inner must layer.
central rod of 2 polypeptides form a coiled dimer –> dimers associate in staggered antiparallel fashion to form tetramers –> tetramers assemble end to end to form protofilaments –> 8 protofilaments are wound together to form filaments.
Why are intermediate filaments more stable than actin?
Intermediate filaments are more stable than actin and do not demonstrate dynamic behaviors such as treadmilling because dimers form a staggered antiparallel structure that are polymerized and do not have distinct ends, thus treadmilling is not possible.
What are the 2 main functions of intermediate filaments?
1) form a cytoplasmic network in most cells
2) associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell.
How many types of intermediate filament types are there?
there are 6 types
True or False:
Protofilaments of microtubules have a fast-growing plus end and a slow-growing minus end.
True, thus they have polarity.
True or False:
The plus end grows more slowly than the minus end in the presence of low calcium ion concentration.
False, the plus end grow more rapidly than the minus end in the presence of low calcium ion concentration.
What happens if the concentration of tubulin-GTP drops?
GTP at the plus end is hydrolyzed and dimers are lost.
What happens if there are high concentrations of tubulin-GTP?
The dimers are added more rapidly than GTP is hydrolyzed, and the microtubule grows.
What are the 4 factors that inhibit microtubule polymerization?
Colchicines (arrest mitotic cells at meta-phase)
Colcemid
Vincristine (anti-cancer drug)
Vinblastin (anti-cancer drug – used for Hodgkins Lymphoma)
What factor stabilizes microtubules?
Taxol (used to treat breast caner, thus also an anti-caner drug)
How many heads does myosin I have?
1 head
Which myosin class has 2 heads?
myosin II
Kinesin and Cytoplasmic dynein both have how many heads?
both have 2 heads each.
What does the tail of myosin II bind to?
myosin II
What does the tail of both cytoplasmic dynein and kinesin bind to?
Vesicles
What does the tail of myosin I bind to ?
cell membrane
What does the head of myosin I and myosin II bind to?
actin
what does the head of kinesin and cytoplasmic dynein bind to?
microtubules
The direction of the head motion of myosin I and myosin II is towards what?
barbed (plus) end
Does kinesin move towards the plus end or minus end of microtubules? What about cytoplasmic dynein?
Kinesin towards the plus end
Cytoplasmic dynein towards the minus end
