IV. Cell Biology | 60. Structure of cytoskeleton; structure and function of motor proteins Flashcards
1
Q
I. Basics
1. What is the cytoskeleton?
A
a network of proteins that provides the cell with structure and shape
2
Q
I. Basics
2. What is the function of cytoskeleton?
A
Its functions in the regulation of:
- Cell movement
- Internal transport and signaling
- Cell division
3
Q
I. Basics
3. What are the components of cytoskeleton?
A
The components of the cytoskeleton are the filaments:
(1) microtubules
(2) intermediate filaments
(3) actin filaments
4
Q
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?
A
- 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
5
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
1. What is the structure of actin filaments?
A
- 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
6
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
2. What are the features of the G-actin subunits?
A
- 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)
7
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
3. Where are actin filaments usually found?
A
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
8
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
4. What is Nucleation?
A
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
9
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
5. What is the role of free G-actin?
A
Free G-actin carry a tightly bound ATP, and hydrolyzes the bound ATP to ADP soon after it is incorporated into the filament.
10
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
6. What is the consequence of Hydrolysis of ATP to ADP?
A
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
11
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7A. When does elongation of actin filaments occur?
A
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
12
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7B. What are the 3 steps of elongation of microfilaments?
A
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).
13
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7C. In the elongation of microfilaments, what happen at the plus end?
A
- 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).
14
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
7D. In the elongation of microfilaments, what happen at the minus end?
A
- 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).
15
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
8. What is the role of ARP (actin-related proteins) complex?
A
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
16
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9A. How is actin behavior regulated?
A
Actin behavior is also regulated by proteins that can bind to the actin monomer of filaments (Thyiomosin, Profilin, Cofilin)
17
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9C. What is Thyiomosin?
A
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
18
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9D. What is Profilin?
A
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
19
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
9E. What is Cofilin?
A
A protein that increases dissociation at the minus end
20
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
10. What are capping proteins?
A
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
21
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
11. How is Actin filament network made?
A
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).
22
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12A. What are the 3 bundling proteins that involve in Actin filament network?
A
- Fimbrin
- α-actinin
- Filamin
23
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12B. What is the role of Fimbrin?
A
Fimbrin packs actin very closely, preventing binding of other proteins such as myosin
24
Q
II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
12C. What is the role of α-actinin?
A
α-actinin: cross-links filaments into loose bundles, allowing binding of myosin + formation of contractile actin bundles
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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
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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
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II. Structure of the cytoskeleton - ACTIN FILAMENTS (MICROFILAMENTS)
14A. What are the 2 major Severing proteins?
1. Gelsolin
2. Cofilin
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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
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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
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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
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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
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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
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III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
4A. The intermediate filaments can be easily bent
=> T/F?
True!
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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
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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)
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III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
5B. Where can you find keratin?
Found in epithelial cells (skin, hair, nails)
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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.
38
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
6. What is the cause of the disassembly of intermediate filaments?
Protein phosphorylation induces the disassembly of intermediate filaments (+ nuclear lamin)
- Phosphorylation -> disassembly
- Dephosphorylation -> assembly
- Phosphorylation occurs during cellular re-organization such as
division, migration and differentiation
39
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
7. How are intermediate filaments assembled and disassembled?
Protein phosphorylation induces the disassembly of intermediate filaments (+ nuclear lamin)
- Phosphorylation -> disassembly
- Dephosphorylation -> assembly
- Phosphorylation occurs during cellular re-organization such as
division, migration and differentiation
40
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
8A. What are the 4 types of intermediate filaments?
1. Nuclear IF
- Lamin A, B and C in nuclear lamina (inner lining of nuclear envelope)
2. Epithelial IF
- Keratin in epithelial cells (hair, nails)
3. Vimentin-like IF
- Vimentin + desmin in muscle cells
4. Axonal IF
- Neurofilament proteins in neurons (determine axon
diameter => speed of electrical impulses)
41
III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
8B. What is nuclear IF?
Nuclear IF
- Lamin A, B and C in nuclear lamina (inner lining of nuclear envelope)
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III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
8C. What is Epithelial IF?
Epithelial IF
- Keratin in epithelial cells (hair, nails)
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III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
8D. What is Vimentin-like IF?
Vimentin-like IF
- Vimentin + desmin in muscle cells
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III. INTERMEDIATE FILAMENTS - structure of cytoskeleton
8E. What is Axonal IF?
Axonal IF
- Neurofilament proteins in neurons (determine axon
diameter => speed of electrical impulses)
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IV. MICROTUBULES - structure of cytoskeleton
1. What is the diameter of microtubules?
25nm in diameter
46
IV. MICROTUBULES - structure of cytoskeleton
2. What are the functions of microtubules?
- Determine the position of organelles
- Moves organelles and vesicles
- Formation of mitotic spindle:
separation of chromosomes during mitosis
- Movements of cilia and flagella
47
IV. MICROTUBULES - structure of cytoskeleton
3. Describe the structure of microtubules
- Formed from tubulin heterodimers, composed of 2 globular proteins: α- and β-tubulin
- The dimers stack together to form the wall of the hollow
cylindrical microtubule
- Each protofilament has structural polarity with alpha at one end = minus end, and
beta on the other end = plus end
- Their polarity is crucial for their function (ex: would not be able to guide IC transport
without it)
48
IV. MICROTUBULES - structure of cytoskeleton - Polymerization, depolymerization and dynamism of microtubules
4. Explain dynamism of microtubules
Dynamic instability: the microtubule can switch back and forth between polymerization and depolymerization
49
IV. MICROTUBULES - structure of cytoskeleton - Polymerization, depolymerization and dynamism of microtubules
5. What are the steps of microtubules polymerization?
1. Microtubules will grow from γ-tubulin by addition of αβ- tubulin dimers to the plus end
=> start shrinking (lose αβ- tubulin from plus end)cycle repeats itself
2. The dynamic instability allows microtubules to undergo rapid remodeling, which is crucial for their function
3. Microtubules can be stabilized at the plus pole by attaching to a cell structure/molecule = no depolymerization
50
IV. MICROTUBULES - structure of cytoskeleton - Polymerization, depolymerization and dynamism of microtubules
6. What are the steps of microtubules depolymerization?
!!! Microtubules can be stabilized at the plus pole by attaching to a cell structure/molecule = no depolymerization
1. If they do not attach to anything, they retreat = depolymerization
2. Free αβ-tubulin dimers are bound to GTP on the β- tubulin -> GTP will be hydrolyzed into GDP when the αβ- tubulin is connected to a protofilament
3. When the microtubule grows faster than GTP hydrolysis, we get a GTP cap on the microtubule -> GTP-associated dimers form stronger bonds than GDP dimer and pack together more efficiently
4. When the GDP-associated tubulins then attach
-> tip the balance towards depolymerization
-> GTP cap breaks off
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IV. MICROTUBULES - structure of cytoskeleton - Centrosome = microtubule-organizing center (MTOC)
7. What is the role of centrosome?
Centrosomes organize an array of microtubules that radiate out towards the cytoplasm
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IV. MICROTUBULES - structure of cytoskeleton - Centrosome = microtubule-organizing center (MTOC)
8. What is the structure of centrosome?
Centrosome consists of a pair of centrioles, surrounded by a matrix of proteins (matrix consists of ring-shaped structures made from γ-tubulin)
53
IV. MICROTUBULES - structure of cytoskeleton - Centrosome = microtubule-organizing center (MTOC)
9. Describe the nucleation of microtubules
- Centrosome consists of a pair of centrioles, surrounded by a matrix of proteins (matrix consists of ring-shaped structures made from γ-tubulin)
- Each γ-tubulin ring complex serves as a starting point (nucleation site) for the growth of one microtubule (γ-tubulin concentration is highest around MTOC)
- αβ-tubulin dimers add to each γ-tubulin in a specific orientation:
+) each minus end embedded in centrosome
+) growth only occurs at plus end that extends into the cytoplasm
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IV. MICROTUBULES - structure of cytoskeleton - Centrosome = microtubule-organizing center (MTOC)
10. Do centrioles contribute to nucleation of microtubules? Why?
Centrioles do not contribute to nucleation of microtubules, but do act as organizing center in cilia + flagella (called basal bodies there)
55
IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
11. What are Microtubule associated proteins (MAPs)?
They are motor proteins (dynein + kinesins), vesicles + macromolecules along
microtubules
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IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
12. What is the role of Microtubule associated proteins (MAPs)?
MAPs can stabilize microtubules against disassembly (link microtubules to other compounds)
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IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
13. How do MAPs participate in stabilization of microtubules?
Binding of MAP
-> frequency of catastrophes (kinesin) suppressed and/or growth rate enhanced
-> longer + less dynamic microtubules
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IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
13. How do MAPs participate in destabilization of microtubules?
NO binding of MAP
-> frequency of catastrophes increased
-> shorter + more dynamic microtubules
59
IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
14A. What are the 3 types of microtubules?
1. Kinetochore microtubules
2. Interpolar microtubules
3. Astral microtubules
60
IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
14B. What are feature(s) of Kinetochore microtubules?
Chromosomes attached to kinetochore microtubules via the kinetochore complex
61
IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
14C. What are feature(s) of Interpolar microtubules?
Interpolar microtubules interdigitate at the spindle midzone and push the spindle poles apart via motor proteins
62
IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
14D. What are feature(s) of Astral microtubules?
Astral microtubules anchor the spindle poles to the cell membrane
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IV. MICROTUBULES - structure of cytoskeleton - Microtubule associated proteins (MAPs)
14E. Identify
1. Kinetochore microtubules
2. Interpolar microtubules
3. Astral microtubules
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V. Structure and function of motor proteins
1. What is the role of motor proteins?
Motor proteins convert chemical energy to movement:
- ATP hydrolysis -> conformational change -> movement along polarized cytoskeletal filaments
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V. Structure and function of motor proteins
2. What is the general structure of motor proteins?
Cytoskeletal motor proteins have a general structure consisting of
- a globular head that attaches to the cytoskeleton filament and has ATPase activity
- a neck, light chains
- a tail that binds the cargo molecule
66
V. Structure and function of motor proteins
3. Describe force generation by motor proteins
Mechanical cycle of (1) filament binding (2) conformational change
=> force generation (3) filament release and (4) conformational relaxation is linked to chemical cycle of ATP binding and hydrolysis (conformation is dependent on the bound nucleotide)
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V. Structure and function of motor proteins
4. List the cytoskeletal motor proteins with structures that they are associated with
1) actin (myosin)
2) microtubules (kinesin, dynein)
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
5. What are the features of Myosins?
- Myosins use ATP to generate force to contract the muscle.
- It is through the actions of myosins, that the actin cytoskeleton can form contractile structures that cross- link and slide actin filaments relative to one another.
- Have 3 types of myosins: type I, type II, type V
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
6. What are the 3 types of myosin?
Have 3 types of myosins: type I, type II, type V
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
6A. What are the features of Myosin I?
Myosin I proteins are one-headed and have a membrane- binding site on their tails, usually involved in intracellular organization, such as the protrusion of actin-rich structures (microvilli) at the cell surface
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
6B. What are the features of Myosin V?
- Myosin V is a two-headed myosin with a large step size and is involved in organelle transport along the actin filament.
- Both myosin I and V have calmodulin dependent light chains that differ from those of myosin II
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
6C1. What are the features of myosin II?
- Myosin II is the motor protein which generates the force for muscle contraction.
- It is an elongated protein with a heavy chain that has a globular head (ATPase activity), and an elongated tail (alpha-helix) that mediates polymerization.
- The tail of myosin bundles itself together with tails of other myosin filaments and creates the thick filament, which has several hundred myosin heads
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
6C2. What is the general mechanism of myosin II?
- Each myosin binds and hydrolyzes ATP, using this energy to walk towards the plus end of an actin filament resulting in powerful contractions
- The motor proteins use structural changes in their ATP-binding sites to produce cyclic interactions with cytoskeletal filaments
- Each cycle of ATP binding, hydrolysis and release propels them forward in a single direction to a new binding site along the filament
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
7A. How are thin filaments formed?
Thin filaments are formed by aggregation of actin molecules (G-actin) into a two-stranded helical filament called the filaments actin (F-actin)
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
7B. What are the 3 proteins involved in formation of thin filaments?
1. Nebulin
2. Tropomyosin
3. Troponin complex
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
7C. What are nebulin?
Nebulin are elongated cytoskeletal proteins that extend along the length of the thin filament and participate in regulating the length
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
7D. How does tropomyosin work?
Tropomyosin form dimers that extend over the entire actin filament and covers the myosin binding sites on the actin molecules
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V. Structure and function of motor proteins - 1-Actin associated motor protein: (MYOSIN)
7E. What are the structure and mechanism of troponin complex?
- A troponin complex consists of 3 troponin molecules (troponin T, I and C), and is present in each tropomyosin dimer
- Binding of Ca2+ to troponin C promotes removal of the tropomyosin dimer on the actin filaments, so that actin-myosin interactions can occur and lead to contractions
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8A. What is the role of kinesins?
Carries membrane enclosed vesicles from the cell body to the axon terminal by walking towards the plus end of the microtubule (anterograde axonal transport)
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8B. What is the structure of kinesins?
Kinesins have 2 heavy chains + 2 light chains, forming 2 globular head motor domains (trailing head + leading head) held together by an elongated coiled-coil responsible for polymerization with other motor proteins
81
V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8C. How many families of kinesins are there?
Have 14 families of kinesins: most of them walk toward the plus end
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8D. What is the mechanism of kinesins?
- The 2 heads of kinesin have nucleotide-binding sites
which regulate the docking and undocking of the heads to the microtubule protofilament -> allows the kinesins to ‘’walk’’ along the filament
- The nucleotide hydrolysis cycle is closely coordinated between the 2 heads, so that the cycle allows one motor protein to dock while the other is bent and moves forward to bind closer to the plus end
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8E. What is the mechanism of leading head of kinesins?
84
V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
8F. What is the mechanism of trailing head of kinesins?
85
V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9A. What are dyneins?
- Family of motor proteins that move toward the minus end of the microtubule (retrograde axonal transport)
- Dyneins are the largest molecular motors and also among the fastest
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9B. What is the structure of dyneins?
Composed of 1, 2 or 3 heavy chains (motor domain) and a variable number of light chains which mediate cargo binding
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9C. What is the role of dyneins?
Involved in movement of organelles toward the cell
center, especially the Golgi apparatus
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9D. What are the 2 types of dyneins?
1. Cytoplasmic dyneins
2. Axonemal dyneins
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9E. What are the features of cytoplasmic dyneins?
Cytoplasmic dyneins are a class of dyneins which are homodimers of 2 heavy chains
- Dynein 1: organelle trafficking towards the centrosome (usually towards the nucleus where the centrosome is typically located)
- Dynein 2: used in transporting material from the tip of the cilia to the base
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V. Structure and function of motor proteins -Microtubule associated motor proteins: (KINESIN + DYNEINS)
9F. What are the features of Axonemal dyneins?
Axonemal dyneins: another class of dyneins that are specialized for the sliding
movement of microtubules that drive the movement of cilia and flagella (beating)
91
VI. Cilia and flagella
1. What is the general structure of Cilia and flagella?
They have a microtubule-based cytoskeleton, called the axoneme
- The axoneme of motile cilia consist of a ring of 9 outer MT doublets together with a central pair of singlets, making the structure (9+2)
- Nexin is connecting the pairs in the dynein head domains, ‘’grabbing’’ adjacent MT doublets
- Cilia: no nexin = cannot bend, but only slide
- Flagella: has nexin = can bend
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VI. Cilia and flagella
2. What is nexin?
- Nexin is connecting the pairs in the dynein head domains, ‘’grabbing’’ adjacent MT doublets
- Cilia: no nexin = cannot bend, but only slide
- Flagella: has nexin = can bend
93
VI. Cilia and flagella
3. Cilia and flagella both contain nexin
=> T/F?
FALSE!!!!
- Nexin is connecting the pairs in the dynein head domains, ‘’grabbing’’ adjacent MT doublets
- Cilia: no nexin = cannot bend, but only slide
- Flagella: has nexin = can bend
94
VI. Cilia and flagella
4. What is the process of movement in cilia and flagella?
1. Dynein is activated by ATP
2. Dynein arm (on one MT doublet) grabs an adjacent
doublet and walks along its length
3. Doublet slide each other and bend
=> Crosslinking proteins limit sliding (nexin)
