Cytoskeleton Flashcards
Describe microfilaments (actin)
They are highly conserved in eukaryotes.
They bind to specific transmembrane proteins directly or indirectly.
What are G-actin and F-actin?
G-actin is actin as a monomer and F-actin is actin polymerized.
What is the first step of actin polymerization?
Nucleation- ATP binds tightly to G-actin, G-actin binds tightly with 2 other G-actin to form a trimer.
Do all actin filaments display polarity and why or why not?
Yes they do because all actin monomers are oriented in the same direction.
Which end of the chain can actin monomers be added?
Either side, but the barbed side is 5-10x faster than the pointed side.
Once the G-actin + it’s bound ATP is added to the chain what happens to the ATP?
It is hydrolyzed to ADP
What determines the rate of actin polymerization?
The concentration of actin monomers in the cytoplasm.
Which one dissociates more readily from the filament? ADP actin or ATP actin?
ADP actin
What does very low G-actin concentration encourage?
The disassembly of actin filaments.
What does intermediate G-actin concentration encourage?
Treadmilling
What is Treadmilling?
An equilibrium state where as one G-actin is removed, another is added resulting in zero net growth.
What does high concentration of G-actin encourage?
The addition of G-actin at both ends, resulting in growth.
Once actin microfilaments are polymerized, what do they form?
A double helical chain.
What are cytochalasins?
They block the elongation of actin by binding to barbed ends which inhibits cell movement (mitosis)
What is phalloidin?
It prevents dissociation by binding to actin filaments.
Can be labeled with fluorescent dyes so you can see actin filaments.
Even though actin has the same structure in all cells…
It has many different functions and their functions are determined by binding proteins.
Arrange the following in ascending order:
Microtubukes, microfilaments, intermediate filaments
Microfilaments, intermediate filaments, and microtubules
What is the role of the actin binding protein spectrin?
Found in RBCs, binds cytoskeleton to cell membrane.
What is the role of the actin binding protein dystrophin.
Binds the cytoskeleton to the plasma membrane.
What do the actin binding proteins villin and fimbrin do?
Cross-link actin in microvilli
What are the roles of the acting binding proteins calmodulin and myosin 1?
Cross-link actin to the plasma membrane in microvilli.
What is the role of the actin binding protein alpha actinin?
Cross-links stress fibers and connects actin to protein-plasma membrane complexes.
What is the role of the actin binding protein filamin?
Cross-links actin at wide angles to form screen like gels.
What is the role of thymosin is controlling treadmilling?
Captures actin monomers and prevents them from polymerizing.
How does profilin control treadmilling?
Binds to actin monomers preventing them from being polymerized.
They also facilitate the exchange of ADP for ATP so that the monomer can be added to the chain.
How does gelsolin control treadmilling?
Destabilizes F-actin and caps actin filaments preventing loss and addition of G-actin.
In presence of calcium ion, fragments actin filaments and remains bound to the plus end.
How does cofilin control treadmilling?
Triggers depolymerization of ADP bound actin at the minus end.
How does Arp 2/3 control treadmilling?
Initiates growth of F-actin from sides of existing filament causing branching.
How does latrunculins control treadmilling?
Binds to G-actin and induces F-actin depolymerization.
What is the role of intermediate filaments?
The cell undergoes mechanical stress and they provide strength to the cells. (Nucleus, neurons, and macula)
What is the structure of intermediate filaments?
They have a common monomer..a helical rod with heads and tails.
The heads and tails determine the function.
How do intermediate filaments form a coiled dimer?
The central rod of 2 polypeptides come together aligned head to head and tail to tail.
How do dimers associate and why is this important?
They associate into a staggered antiparallel fashion to form tetramers.
This doesn’t allow them to have distinct ends and makes them more stable than actin-no treadmilling.
What do tetramers assemble to form?
Protofilaments
8 protofilaments form a filament.
How do intermediate filaments work in the cytoplasm?
They connect with other cytoskeleton elements for form structure within the cell.
What are the types of intermediate filaments and what is each associated with?
Type I- acidic keratin Type II- neutral and basic keratin Type III- vimentin,desmin,glial fibrillary acidic protein, peripherin. TypeIV- neurofilaments Type V- nuclear lamins Type-VI- nestin
What are microtubules composed of?
Tubulin dimers alpha and beta unit.
Protofilaments are longitudinal rows of Tubulin dimers.
13 protofilaments arranged parallel to form a cylinder with a hollow core = microtubule
Protofilaments have a fast growing plus end and a slow growing minus end? True or False?
True
Explain treadmilling in microtubules
GTP bound Tubulin dimers bind to the plus (growing end) end.
* the plus end grows much faster in low Ca conc
After polymerization, the GTP is hydrolyzed to GDP.
Tubulin becomes less stable and dimers dissociate at the minus end.
What happens when there are high levels of GTP-tubulin? And what happens when there are low levels?
The dimers are added more rapidly than the GTP is being hydrolyzed and dimers are dissociating. The reverse happens at low levels.
What factors inhibit microtubule polymerization?
Colchicines
Colcemid
Vincristine - anti cancer drug
Vinblastin- used for Hodgkin’s lymphoma
What factors stabilize microtubules?
Taxol- used for breast cancer
