Block D Lecture 2 - Proteins of the Cytoskeleton Flashcards
What are 3 properties of cells which depend on the cytoskeleton?
Answers Include:
Correct shape of cells
Cells need to be physically strong (robust)
Cells have to be properly structured internally
Some cells need to be able to change their shape and move
Growth, division and adaptation needs internal rearrangement to occur
(Slide 4)
What is the structure of the cytoskeleton?
It is a system of filaments which themself are a repetitive assembly of a large number of small subunits
(Slide 4)
What are 4 specific functions the cytoskeleton performs to maintain properties of the cell?
Answers Include:
Pulling chromosomes apart at mitosis
Intracellular traffic of organelles
Supports the plasma membrane
Allows swimming or crawling of cells
Contraction in muscle cells
Guides growth of plant cell wall
Maintains cell shape
(Slide 5)
The cytoskeleton has 3 main types of filaments with different mechanical properties and dynamics. What does result in?
Dynamic behaviour which allows an enormous range of structures to be built by the cytoskeleton
(Slide 6)
What are the 3 main filaments in the cytoskeleton, and what properties do they each have?
Intermediate filaments - provide mechanical strength and resistance to shear stress
Microtubules - Controls positioning of membrane-enclosed organelles, and intracellular transport
Actin - Controls the shape of the cell’s surface and whole cell locomotion (the movement of the cell from one place to another)
(Slide 6)
What is shear stress?
The force per unit area exerted by blood flow on the endothelial cells lining blood vessels
(Slide 6)
In addition to filaments, what are 2 other components which are included in the cytoskeleton, and what do these provide?
Accessory proteins - regulate linkage and assembly of the cytoskeleton filaments, and bring the cytoskeleton under the control of extracellular and intracellular signals
Motor proteins - Move organelles and filaments
(Slide 6)
What are the functions of microtubules?
They form a star-like cytoplasmic array and are involved in bipolar mitotic spindle formation (for chromosome segregation).
They also act as motile whips in flagella and cilia and serve as transport tracks
(Slide 7)
What are 3 structures actin forms and what are these essential for?
Answers Include:
Lamellipodia - involved in cell motility and cell adhesion
Filopodia - Involved in cell signalling, sensing the environment and cell to cell communication
Stereocilia - hair cells in the inner ear which function in balance and mechanotransduction, allowing us to hear
Contractile ring - during cytokinesis, the final step of cell division, actin filaments form a contractile ring which is crucial in physically separating the daughter cells
(Slides 8 and 9)
What purpose does actin filaments serve in muscle tissue?
It enables muscle tissue to contract
(Slide 9)
What are 3 structures / functions which intermediate filaments form / have and what do these do?
Answers Include:
They line the inner face of the nuclear envelope - maintaining structure and function of the nucleus
They form twisted strong cables in the cytosol - to provide mechanical support and maintain cell shape and stability
Formation of long robust axons in neurons
Formation of hair and fingernails
(Slide 10)
What are filaments made up of?
Assemblies of small protein subunits (such as actin for actin filaments or tubulin for microtubule filaments) with these subunits being held together by non-covalent weak interactions
(Slide 11)
How can cells undergo rapid structural reorganisation?
As the subunits which make up filaments are small enough to diffuse rapidly throughout the cytosol with disassembly, diffusion and reassembly at a different site allowing rapid structural reorganisation and movement
(Slide 11)
What is a protofilament?
A linear assembly of subunits.
Multiple protofilaments bind to each other generates lateral and longitudinal bonds
(Slide 12)
What is the purpose of protofilaments?
To provide stability and resistance to thermal breakage for a filament
(Slide 12)
Why are the ends of protofilaments kept dynamic and what does this mean?
Keeping the ends dynamic means that the filaments can undergo polymerisation (growth) and depolymerisation (shrinkage). This is done to allow rapid disassembly
(Slide 14)
How are subunits (and protofilaments) arranged in intermediate filaments?
They are elongated and staggered, with a high number of lateral bonds and contacts between α-helical coiled coils
(Slide 13)
What does the arrangement of subunits (and protofilaments) within an intermediate filament enable?
The staggered structure enables intermediate filaments to tolerate stretching and bending - forming a strong rope-like structure
(Slide 13)
What is the first step of new filaments being formed?
Nucleation - when small unstable clusters of monomers come together to form a stable “seed or nucleus” from which the filament can grow
(Slide 14)
Why is nucleation the hardest step of new filament formation?
As smaller aggregates are unstable - leading to a kinetic barrier which means the monomers would rather dissociate than grow.
It is difficult for filaments to grow at this point as the small aggregates don’t reach a large enough size for the polymerisation process to occur efficiency
(Slide 14)
What does the instability of smaller aggregates during nucleation result in?
A lag-phase where no visible growth occurs until a stable “nucleus” is formed
(Slide 14)
What happens in new filament formation after a stable “nucleus” is formed?
The elongation phase occurs, where the filament grows rapidly as monomers add to the end of the filament
(Slide 14)
Why doesn’t filament assembly occur endlessly during the elongation phase of new filament formation?
As there is a point where assembly is balanced by disassembly, forming a steady state / equilibrium.
The distinct concentration of monomers which this occurs at is called the critical concentration (Cc)
(Slide 14)
What is the structure of microtubule filaments?
They are made of α and β-tubulin subunits, which are attached to each other and arranged head-to-tail to make a protofilament.
13 protofilaments form a hollow cylindrical structure.
Each α and β-tubulin also has a GTP attached to them.
Microtubules are attached to the centrosome (microtubule organising centre, MTOC)
(Slide 7 and 17)
