IV. Cell Biology | 60. Structure of cytoskeleton; structure and function of motor proteins Flashcards
I. Basics
1. What is the cytoskeleton?
a network of proteins that provides the cell with structure and shape
I. Basics
2. What is the function of cytoskeleton?
Its functions in the regulation of:
- Cell movement
- Internal transport and signaling
- Cell division
I. Basics
3. What are the components of cytoskeleton?
The components of the cytoskeleton are the filaments:
(1) microtubules
(2) intermediate filaments
(3) actin filaments
I. Basics
4. The components of the cytoskeleton are the filaments: (1) microtubules (2) intermediate filaments (3) actin filaments
=> What are the features of these filaments?
- Each type of cytoskeletal filament is constructed from smaller protein subunits
- Small subunits can diffuse rapidly, while assembled filaments cannot
- Cytoskeletal polymers are held together by weak non-covalent interactions
- Different cytoskeleton-associated accessory proteins regulate the spatial
distribution and the dynamic behavior of the filaments - Formation of cytoskeletal polymer: nucleation, elongation, steady state
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
1. What is the structure of actin filaments?
- It is 7-9nm in diameter, makes up 20% of protein in a cell
- Composed of G-actin monomers (globular actin subunits), which are tightly associated with ATP/ADP
- G-actin subunits assemble in a head-to-tail fashion to form a right handed helix called filaments or F-actin
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
2. What are the features of the G-actin subunits?
- G-actin subunits assemble in a head-to-tail fashion to form a right handed helix called filaments or F-actin
- The G-actin subunits are asymmetrical, and they all point in the same direction when assembled, giving the actin filament a polarity with a minus end and a plus end
+) Minus end = pointed end, grows slower (ADP-bound actin)
+) Plus end = barbed end, grows faster (ATP-bound actin)
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
3. Where are actin filaments usually found?
Actin filaments are usually found in a network, rather than as ‘’free filaments’’ – bound together by cross links which makes them much more stable than a single filament would have been
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
4. What is Nucleation?
Nucleation = formation of a new actin filament from scratch
- For a new actin filament to be formed, the G-actin assemble into an initial aggregate/nucleus, that is stabilized by multiple subunit-subunit contacts and can then elongate rapidly by addition of more subunit
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
5. What is the role of free G-actin?
Free G-actin carry a tightly bound ATP, and hydrolyzes the bound ATP to ADP soon after it is incorporated into the filament.
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
6. What is the consequence of Hydrolysis of ATP to ADP?
Hydrolysis of ATP to ADP in actin filament reduces the strength of binding between monomers
=> reducing the stability of the polymer = nucleotide hydrolysis promotes depolymerization
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7A. When does elongation of actin filaments occur?
Elongation occurs when there is a high concentration of G-actin, and these subunits will be bound to the actin filament at both ends, at a rate faster than ATP hydrolysis, so that the net result is elongation/growth rather than disassembly
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7B. What are the 3 steps of elongation of microfilaments?
1) At intermediate concentration of G-actin, actin monomers add to the plus end at a rate faster than the bound ATP can be hydrolyzed -> the plus end grows
2) But at the minus end, the hydrolysis of ATP to ADP is faster than the addition of new monomers, and since ADP destabilize the structure -> the minus end will shrink
3) When the addition of G-actin on the plus end equals the loss of G-actin on the minus end, a ‘’steady state’’ is reached where there is no net change in length of the actin filament, but the actin filament ‘’moves’’ towards the plus end and away from the minus end = treadmilling (this movement is related to actins role in cell motility).
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7C. In the elongation of microfilaments, what happen at the plus end?
- At intermediate concentration of G-actin, actin monomers add to the plus end at a rate faster than the bound ATP can be hydrolyzed
=> the plus end grows
** When the addition of G-actin on the plus end equals the loss of G-actin on the minus end, a ‘’steady state’’ is reached where there is no net change in length of the actin filament, but the actin filament ‘’moves’’ towards the plus end and away from the minus end = treadmilling (this movement is related to actins role in cell motility).
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7D. In the elongation of microfilaments, what happen at the minus end?
- At the minus end, the hydrolysis of ATP to ADP is faster than the addition of new monomers, and since ADP destabilize the structure -> the minus end will shrink
=> the plus end grows
** When the addition of G-actin on the plus end equals the loss of G-actin on the minus end, a ‘’steady state’’ is reached where there is no net change in length of the actin filament, but the actin filament ‘’moves’’ towards the plus end and away from the minus end = treadmilling (this movement is related to actins role in cell motility).
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
8. What is the role of ARP (actin-related proteins) complex?
ARP (actin-related proteins) complex is responsible for branching of actin
=> can attach minus end of an actin filament to another actin filament, and mediate growth at the plus end
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9A. How is actin behavior regulated?
Actin behavior is also regulated by proteins that can bind to the actin monomer of filaments (Thyiomosin, Profilin, Cofilin)
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9C. What is Thyiomosin?
It is a protein that bind G-actin in a way that the actin monomers cannot associate with either plus or minus ends of the filament
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9D. What is Profilin?
Profilin: binds to the actin monomer in a way that it blocks the site that would normally associate with the F-actin minus end, while leaving the part binding the plus end exposing = promoting growth at plus end
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9E. What is Cofilin?
A protein that increases dissociation at the minus end
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
10. What are capping proteins?
Capping proteins are proteins that bind the ends of a filament and thereby stabilize it
=> prevent it from quickly depolymerizing once their ATP is hydrolyzed
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
11. How is Actin filament network made?
Actin filament network is made by cross-linking, either by bundling proteins that cross-link actin filaments into a parallel array (tight meshwork), or by gel-forming proteins (gelsolin) which hold two filaments at large angles (looser meshwork).
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12A. What are the 3 bundling proteins that involve in Actin filament network?
- Fimbrin
- α-actinin
- Filamin
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12B. What is the role of Fimbrin?
Fimbrin packs actin very closely, preventing binding of other proteins such as myosin
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12C. What is the role of α-actinin?
α-actinin: cross-links filaments into loose bundles, allowing binding of myosin + formation of contractile actin bundles
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12D. What is the role of Filamin?
Filamin promotes formation of loose and viscous gel by binding 2 filaments at almost same angles
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
13. What is the role of Severing proteins
- Severing proteins break an F-actin into many smaller filaments, generating a large number of new filament ends.
- The newly formed ends can nucleate filament elongation => accelerate assembly of new filaments
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
14A. What are the 2 major Severing proteins?
- Gelsolin
- Cofilin
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
14B. What is the role of Gelsolin?
Gelsolin: interacts with side of F-actin until thermal fluctuation creates a small gap, into which gelsolin can insert itself and break the filament
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
14C. What is the role of Cofilin?
Cofilin: binds along the length of F-actin
=> twists it more tightly + creates mechanical stress that weakens contact between actin subunit
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
1. What are the features of intermediate filaments?
- 10nm in diameter (intermediate in size)
- Prominent in the cytoplasm of cells that are subjected to mechanical stress
- Elongated proteins with a central α-helical part (rod domain), that will form parallel coiled-coil dimers
- Dimers then assemble into tetramers with another dimer, but this assembly is anti-parallel -> intermediate filaments do not have polarity = no difference between the two ends
- The tetramers are staggered, so that the ends of the 2 coiled dimers are not at the same location
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
2. How are intermediate filaments formed?
- The tetramers then pack together laterally to form the filament
-> one filament = 8 strands of tetramers that form the rope-like filament
=> α-helical monomer -> coiled-coil dimer -> staggered tetramer of 2 anti-parallel dimers -> 8 strands of tetramers twisted into a rod-like filament
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
3. What are the functions of intermediate filaments?
- Cell shape (high tensile strength)
- Cell-cell junctions
- Nuclear lamina (anchor proteins, chromosomes)
- Anchoring of organelles
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
4A. The intermediate filaments can be easily bent
=> T/F?
True!
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
4B. The intermediate filaments can be easily bent
=> Explain why and consequences
The intermediate filaments can be easily bent, are difficult to break and can be stretched to over 3 times their length
=> allows cells to withstand mechanical stress, they deform under stress but will not rupture
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
5A. What is the most diverse intermediate family?
Keratin is the most diverse intermediate family, found in epithelial cells (skin, hair, nails)
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
5B. Where can you find keratin?
Found in epithelial cells (skin, hair, nails)
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
5C. Explain the role of keratin as intermediate filaments
Keratin is the most diverse intermediate family, found in epithelial cells (skin, hair, nails)
=> cross-linked keratin network held together by disulfide bonds can survive death of their cells, forming a tough covering on the outer layer of skin, hair etc.