SGT5: Cytoskeleton Flashcards
1
Q
What are the functions of the cytoskeleton?
A
- Provides shape, support, and resistance to external forces to allow cell migration, division and maintenance
- Enables dynamic changes in cell structure from environmental changes
- Attaches cells together connections to the external environments, generating
co-ordinated forces for movement
2
Q
What types of cytoskeletal filaments do you know?
A
- Actin filaments
- Microtubules
- Intermediate filaments
3
Q
What are actin filaments?
A
- Composed of actin
- Involved in cell shape, motility and division, mainly in muscle contraction and intracellular transport).
- Forms using ATP not GTP through polymerisation of actin
- Has polarity with a positive/plus (‘barbed’) end where new monomers are added to the filament and a negative/minus (‘pointed’) end where disassembly occurs
- 2 types of actin - G-actin which is globular, monomeric and F-actin, which is filamentous, polymerised to form filaments arranged in a twisted chain
4
Q
What are intermediate filaments?
A
- Provide tensile strength to cells to maintain integrity, including keratins
- Form rod-like structure which are really stable
4
Q
What are microtubules?
A
- Tubulin subunits form a dimer of alpha and beta
- Form the spindle fibres in cell division
- Provide structural support for cilia and flagella and act as tracks for motor proteins like kinesis and dynein in transport)
- Uses GTP to form the dimer
- Has both a positive and negative end due to differences in polarity
5
Q
What proteins controlling the cytoskeletal architecture do you know?
A
- Tubulin (dimers of alpha and beta)
- Microtubule-associated proteins – interact with microtubules to stabilise them and control organisation, linking them to other cell components
- Actin-binding proteins such as formin, profilin, Arp2/3 in nucleation, ADF/cofilin in capping and GTPases (cdc42, Rho, Rac) as regulatory proteins.
- Keratin, vimentin and lamin form intermediate filaments
6
Q
What are 3 examples of GTPases?
A
- Cdc42
- Rho
- Rac
7
Q
What protein is used in actin nucleation?
A
Arp2/3
8
Q
What protein is used in actin capping?
A
ADF/cofilin
9
Q
How is actin organised in a cell? What actin structures do you know?
A
- Stabilise, organise and modulate actin filaments, forming structure like microvilli and enable cell movement
- Filopodium – tight parallel bundles
- Stress fibres – hold the cell in place and are contractile bundles
- Mesh-like networks but allows the shape of the cell – cortical actin
10
Q
What controls actin polymerisation?
A
- ATP is bound and hydrolysed in actin polymerisation
- ATP-bound G-actin can polymerise into F-actin, and subsequent ATP hydrolysis destabilises F-actin, enabling actin filament turnover
- 3 GTPases – Rho, Rac and Cdc42
- Stress fibres are formed between focal adhesions using Rho GTPases
- Cdc42 aids in the formation of filopodium
- Rac GTPases form lamellipodium
- Activate monomer binding proteins – sequester and release
- Activate polymer binding proteins – bundling, cross-linking, severing and contracting
11
Q
How do cells move?
A
- Stress fibres form between focal adhesions
- Back and top edges of the cell are rounded, leading edge does not contact substratum.
- Protrusion – requires actin polymerisation, membrane insertion or protrusion. The back of the cell remains the same. Focal adhesions form at the front of the cell
- Front becomes rounded and the back becomes elongated.
- Translocation – formation of new adhesions, actin-myosin contractility of stress fibres and traction
- Formation of retraction fibres at the back of the cell, using actin-modulating proteins to degrade the actin stress fibres at the back – detachment – actin polymerisation generates cellular movement.
- Actin-binding proteins regulate the polymerisation and organisation of actin filaments, enabling the cell to push its membrane forward in the direction of movement
12
Q
What is responsible for the movement of vesicles and organelles in cells?
A
- Conventional kinesins are primarily involved in transporting vesicles and organelles along microtubules toward the cell periphery
- Cytoplasmic dynein transports organelles and vesicles along microtubules
- The cytoskeleton and motor proteins primarily – work symbiotically
- Motor proteins use myosin interacting with actin and dyneins and kinesins interact with microtubules
13
Q
Explain how the motor proteins function in cells?
A
- Motor proteins are enzymes that convert chemical energy, like ATP, into mechanical energy to produce movement within the cell
- ATP hydrolysis causes a conformational change in the motor protein’s structure, which is then transmitted as movement along cytoskeletal filaments
- The tail domain of motor proteins binds to specific cargo, allowing the motor protein to transport various vesicles, organelles, or other cellular materials
- Motor proteins have inherent directionality toward either the plus or minus end of filaments, ensuring targeted transport within cells
14
Q
What are processive motor proteins?
A
- A processive motor protein is a motor that coordinates its heads to stay attached to the filament continuously, allowing smooth movement, as seen with kinesin and some myosin’s.
- Make many steps before detaching form the track
- Single motor molecule is sufficient to transport cargo over a significant distance
- In most cases these are dimeric, moving ‘hand-over-hand’
- Many become non-processive monomers
15
Q
What are non-processive motor proteins?
A
- A non-processive motor protein is a motor that does not coordinate its heads and may detach from the filament intermittently, such as myosin in muscle contraction.
- Dissociate from track after each step
- Uncoordinated attachment/detachment
- Can still move loads over long distances
- Co-operate in large numbers
16
Q
What is the structure of myosin?
A
- Head domain: binds to F-actin and ATP, uses ATP hydrolysis to generate ‘force’
- Neck domain: acts as a linker and works as a lever to transport the force generated by the head
- Tail domain: mediates interactions with cargo molecules or other myosin tail regions.
17
Q
How does myosin move?
A
- ATP binds to myosin and the myosin head group is released
- ATP is hydrolysed and the myosin head group ‘cocks’
- Pi is released and the myosin head group binds to actin
- ADP is released and the myosin ‘powerstroke’ occurs – moves everything along and returns to the correct orientation
18
Q
What are the structure of kinesins?
A
- Head domain: binds to microtubules and to ATP, uses ATP hydrolysis to generate ‘force’
- Neck domain: acts as a linker to transduce the force generated by the head
- Tail domain: mediated interactions with cargo molecules and kinesin regulatory chains
19
Q
What are the structure of dyneins?
A
- Two identical heavy chains (>500 kDa) forming the head domains
- These head domains interact with the microtubules and contain the ATPase motor domain
- Stalk and several intermediate chains and light chains
- Cargos are attached via a complex of accessory proteins
20
Q
How do cytosolic dyneins move?
A
- ATP bound state (stalk not attached to microtubules)
- ATP hydrolysis causes stalk to attach to microtubules
- Release of ADP and Pi, binding the dyneins to the microtubule
- Causes a powerstroke causing a conformational change that moves the dynein along the microtubule
