Lecture 7: Cytoskeleton Flashcards
1
Q
List the 3 components of cytoskeleton (and their approximate sizes)
A
- Microfilaments (actin) (7 nm thick)
- Intermediate filaments (8-10 nm thick)
- Microtubules (25 nm in diameter)
2
Q
Describe the characteristics of actin microfilaments
A
- Highly conserved among eukaryotes
- Are 7 nm thick, or at least, filaments may be up to 7 μm in length.
- Filaments are organized into bundles and 3-D networks. (Such as sarcomeric actin bundles)
- Bind to specific transmembrane proteins either directly or indirectly
- Exist as monomers (G-actin) and long chains (F-actin)
- Constitute three major varieties:
- α-actin(found in muscle tissue), β-actin(non-muscle actin), and γ-actin(non-muscle actin)
3
Q
How are actin microfilaments polymerized/nucleated?
A
- Each actin monomer (G actin) has a binding site for ATP, which binds tightly to G-actin.
- Each actin monomer can bind tightly with two other actin monomers to form filamentous actin (F-actin).
- Because all actin monomers are oriented in the same direction, actin filaments display polarity.
- The first step in actin polymerization is nucleation:
- (A trimer is formed.
- Additional actin monomers can then be added to either end. )
- Actin polymerization is reversible.
- ATP-actin associates with the growing (plus or barbed) ends, and the ATP is hydrolyzed to ADP following polymerization.
4
Q
Describe the rate of polymerization and treadmilling.
A
- The rate at which monomers are added to the growing filament is proportional to the cytosolic concentration of actin monomers.
- ADP-actin dissociates more readily from filaments than ATP-actin.
- The rate of addition of new G-actin to actin filaments occurs more rapidly and at a lower concentration at the plus (barbed or polymerization) end.
- The barbed (+) end grows 5-10 times faster than the pointed end.
- The minus end (pointed or depolymerization end) is the slower growing end.
- Very low concentrations of G-actin favor the disassembly of actin filaments.
- Intermediate concentrations favor a dynamic equilibrium between the minus end and the plus end = treadmilling.
5
Q
What exactly is occurring during treadmilling?
Also, review slide 9 for polymerization picture
A
- Treadmilling results in equlibrium and zero net growth.
- Higher concentrations of G-actin favor net addition at both ends and, therefore, growth of the actin filament.
- Actin microfilaments consist of a double helical chain of G-actin subunits.
6
Q
Describe 2 drugs that affect Actin polymerization
A
Cytochalasins:
- Bind to barbed ends
- Block elongation
- Can inhibit movements (e.g., cell division)
Phalloidin:
- Binds to actin filaments and prevents dissociation
- Can be labeled with fluorescent dyes to allow visualization of actin filaments
7
Q
Actin is so versatile because it itself doesn’t dictate it’s function. That’s the job of it’s actin binding proteins it’s associated with.
Name and describe the first 6 actin binding molecule/proteins?
A
- Spectrin: Found in RBCs, Binds cortical cytoskeleton to the plasma membrane
- Dystrophin: Binds cortical cytoskeleton to the plasma membrane
- Villin and Fimbrin: Cross-links in microvilli
- Calmodulin and Myosin I: Cross-links actin to plasma membrane in microvilli
- α-Actinin: Cross-links stress fibers and connects actin to protein-plasma membrane complex complexes
- Filamin: Cross-links actin at wide angles to form screen-like gels.
8
Q
List the “first 3” actin binding proteins that control treadmilling
A
- Thymosin: Captures actin monomers; prevents monomers from being polymerized.
- Profilin: 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.
- 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 plus end.
9
Q
List the other 4 actin binding molecules that control treadmilling
Also, review diagram on Slide 14 - Lecture 7
A
- Cofilin: Triggers depolymerization of ADP-bound actin at the minus end
- Arp2/3: Initiates growth of F-actin from sides of existing filament—causes branching
- Phalloidin: Prevents depolymerization by binding to actin filaments.
- Latrunculins: Binds to G-actin and induces F-actin depolymerization
10
Q
Give the characteristics of intermediate filaments
A
- Are 8-10 nm thick
- Are abundant in cells subject to mechanical stress
- Provide tensile strength in cells such as neurons and muscle.
- Strengthen epithelial cells as desmosomes and hemidesmosomes.
- All have a common monomer consisting of a central α-helical rod flanked by head and tail domains.
- Head and tail domains determine specific functions.
- Intermediate filament functions: Form a cytoplasmic network in most cells, and associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell.
11
Q
Describe intermediate filament assembly
Review slide 18 on Lecture 7
A
- Central rod of two polypeptides form a coiled dimer.
- Rods are aligned tail-to-tail and head-to-head.
- Dimers associate in a staggered antiparallel fashion to form tetramers.
- Because of the antiparallel association of the dimers, polymerized filaments do not have distinct ends.
- Therefore, they are more stable than actin and do not demonstrate dynamic behaviors such as treadmilling.
- Tetramers assemble end to end to form protofilaments.
- Eight protofilaments are wound together to form filaments.
12
Q
Loosely describe the intermediate filament types
A
- Type I and Type II are the keratins.
- Type III is a bunch of different crap.
- Type IV are straight neurofilaments
- Type V is lamins (Not to be confused with laminins. Those are different.)
- Type VI: Nestin - more on this much later.
13
Q
Describe the characteristics of microtubules
Review picture on slide 20
A
- 25 μm in diameter
- Composed of tubulin dimers: Alpha unit + beta unit
- Protofilaments are longitudinal rows of tubulin dimers.
- Microtubules consist of 13 protofilaments arranged parallel to form a cylinder with a hollow core.
- Protofilaments have a fast growing plus end and a slow growing minus end. Like actin microfilaments.
14
Q
Describe treadmilling and dynamic instability in microtubules
A
- Tubulin dimers with GTP bound to the β-tubulin associate with the growing end.
- Plus end: Grows more rapidly than minus end in presence of low calcium ion concentration.
- After polymerization, GTP is hydrolyzed to GDP and the tubulin is less stable.
- Dimers at the minus end dissociate.
- At high concentrations of tubulin-GTP, the dimers are added more rapidly than GTP is hydrolyzed, and the microtubule grows.
- If concentration of tubulin-GTP drops, GTP at the plus end is hydrolyzed and dimers are lost. So a lot like actin. Except uses calcium ions (not sure why) and GTP
15
Q
What are the functions of the cytoskeleton?
A
- Cell movement
- Support and strength for the cell
- Phagocytosis
- Mitotic spindle formation
- Cytokinesis
- Cell-to-cell and cell-to-extracellular matrix adherence
- Changes in cell shape
