Cytoskeleton Flashcards
What are the components of the cytoskeleton?
Actin microfilaments, microtubules, intermediate filaments.
These components are found in all eukaryotes and most animals.
What is the diameter of microtubules?
25 nm.
Microtubules are hollow tubes and fairly rigid.
What is the dynamic property of microtubules?
Dynamic instability.
Each microtubule can switch between growing and shrinking.
What are the two ends of a microtubule called?
Plus end = grows rapidly & beta tubulin is exposed
Minus end = grows slowly if at all, prevented from growing by other proteins
Plus and minus does not refer to the charge but rather the growth
What is the role of the gamma-tubulin ring complex?
It acts as a template for microtubule nucleation.
Concentrated on centrosomes (near the middle of the cell) —> Microtubules grow at their plus ends from the y-tubulin ring complexes of the centrosome
What happens to a microtubule if the GTP cap is lost?
The microtubule will depolymerise.
If a new GTP cap forms, the microtubule will start growing again.
What is the importance of GTP hydrolysis in microtubule dynamics?
It regulates the stability of microtubules.
A slow hydrolysis rate gives a switch activity (on when GTP is bound)
What is the function of EB1 protein?
It binds preferentially to GTP-tubulin, marking growing microtubules.
This protein is important for tracking microtubule dynamics.
Fill in the blank: Microtubules are stabilized by binding to _______.
Microtubule-associated proteins (MAPs) or taxol.
MAPs can bind along the microtubule to regulate its stability.
What are the subunits of actin filaments?
Monomeric actin.
Actin filaments are thin, flexible, helical structures with a diameter of 7 nm.
What is the role of ATP in actin filament dynamics?
Actin is an ATPase
Actin filaments have a plus and minus end with ATP-bound actin at the plus end
Actin hydrolyses ATP after assembly
What nucleating proteins are involved in actin polymerisation?
Arp2/3 complex and formin.
These proteins promote nucleation and branching of actin filaments.
What is the diameter of actin filaments?
7 nm.
Actin filaments are 2-stranded flexible helices.
True or False: Microtubules can grow and shrink independently of one another.
True.
This is a characteristic of microtubule dynamics.
What are the three types of motor proteins associated with cytoskeletal filaments?
Myosins, dyneins, kinesins.
Each motor protein has a distinct role in cellular movement.
What is the effect of phalloidin on actin filaments?
It stabilizes actin filaments.
Phalloidin binds to actin and prevents disassembly.
Fill in the blank: Microtubules are assembled from _______ heterodimers.
Tubulin.
Specifically, alpha and beta tubulin form stable dimers.
What happens to microtubules when cells are placed on ice?
Microtubules can depolymerise but cannot grow.
Cold temperatures affect microtubule dynamics.
Structure and dynamics of Microtubules
- Hollow cylinders of 25 nm diameter assembled from tubulin heterodimers
- alpha & beta tubulin form a stable dimer as soon as they are synthesised and don’t come apart again
- These are stacked end to end to give Microtubules
Describe nucleation
- In a test tube, Tubulin + GTP + Mg2+ are used at 37 degrees to form Microtubules
- In a cell, tubulin concentration is too low for polymerisation to occur spontaneously —> cells use a template made of gamma-tubulin and other proteins to speed up polymerisation
Location of Microtubule plus & minus ends
Plus ends are at the cell periphery = grow outwards so plus end is on the outside
Minus ends are at the cell centre at the centrosome
E.g. fibroblasts
Airway epithelial cells
Ciliated cells have an extra set of Microtubules in the cilia which are nucleated by the basal body = grow directly from the basal body
What is dynamic instability?
Each Microtubule can switch between growing and shrinking
Each one grows and shrinks independently of its neighbours
Importance of tubulin as a GTPase
In the Microtubule, GTP is gradually hydrolysed to GDP
GDP tubulin cannot polymerise (in the wrong shape)
GTP tubulin can polymerise
Cytosolic pool contains unassembled GDP tubulin
Difference between GTP and GDP tubulin dimers
GTP-tubulin dimers bind more tightly to each other than GDP-tubulin dimers because their shape is slightly different
Importance of GTP cap
If a GTP cap is present, the Microtubule will continue growing
If the cap is lost, Microtubule will depolymerise = catastrophe
If a new cap forms, Microtubule will start growing again
What proteins can interact with Microtubules and control their behaviour & organisation?
- Stabilising proteins
- Nucleating proteins (gamma tubulin ring complex)
- Catastrophe inducing motor protein (e.g. kinesin-13)
How can Microtubules be stabilised?
- By binding Microtubule-associated proteins (MAPs) along the Microtubule = prevents it from being able to disassemble
- By binding the drug taxol = used to treat some cancers
How can Microtubules be depolymerised experimentally?
- By putting cells on ice —> Microtubules can depolymerise but can’t grow
- Using drugs that prevent disassembly —> Nocodazole (binds dimers & prevents them assembling) or Colcemid and Colchicine (bind to the MT end, preventing further assembly)
What are actin filaments?
Major component of muscle and are found in contractile bundles & non-contractile bundles in almost all other cell types
E.g. stress fibres, involved in cytokinesis, epithelial cells, motile cells (fibroblasts)
Structure of actin filaments
Polymers assembled from monomeric actin
Thin, flexible, helical filaments
7 nm in diameter
Role of capping proteins in actin polymerisation
Capping proteins often bind to the minus end, preventing depolymerisation
Actin vs Microtubule polymerisation
Disassembly occurs from different ends (minus end in actin
Treadmilling occurs in actin polymerisation (loses & gains subunits at the same time)
Actin —> requires Actin + ATP + Mg2+ at 37 degrees
Microtubules —> requires Tubulin + ATP + Mg2+ at 37 degrees
Natural small molecules that alter actin polymerisation
- Phalloidin stabilises actin filaments
- Cytochalasin caps filament ends, preventing actin polymerisation from existing ends
- Latrunculin binds to actin monomers, preventing actin polymerisation
Why is only 50% of the actin polymerised in the cell?
Many proteins bind actin filaments and alter their organisation and dynamics
Use of nucleating proteins in actin polymerisation
Promote polymerisation in specific regions
Cell uses them to control where polymerisation happens
Proteins that alter actin filament lengths or dynamics
- Severing protein
- Capping protein (stop polymerisation but also stabilise ends)
Proteins that alter actin filament organisation
- Cross linking protein (in cell cortex) = generate mesh works
- Bundling protein (in filopodia) = give parallel aligning in filaments
Role of myosin motor protein in actin polymerisation
Control or drive movement along actin filaments
What is the role of the Arp2/3 complex in migrating cells?
Main nucleator of actin filaments in lamellipodia
Arp = actin-related protein
The complex
- binds to the side of existing actin filaments
- nucleates assembly of new actin filaments
- prevents disassembly at the minus end (acts as cap)
- causes branching & drives polymerisation of actin network
How does actin polymerisation drive membrane protrusion?
By pushing the plasma membrane forward due to the growth of actin filaments
Actin polymerisation at the leading edge of cells, such as in lamellipodia, is crucial for cell migration
What is the function of formins in cell migration?
Actin-nucleating proteins attached to the plasma membrane that add actin monomers to the plus end of actin filaments to form filopodia
What are lamellipodia?
Protrusions at the leading edge of migrating cells formed by actin filament dynamics
Lamellipodia play a key role in cell movement
What prevents actin filaments from sliding back during cell migration?
Interactions with other actin filaments via cross-linking proteins
This anchoring is crucial for maintaining the structure of filopodia
What are focal contacts?
Adhesive structures that connect protrusions to the surface during cell migration
They contain trans-membrane proteins called integrins
What is the role of myosin II in cell migration?
Pulls the rear of the cell forward using contractile actin bundles
Myosin II interacts with actin filaments to facilitate movement
What are intermediate filaments?
Cytoskeletal components that provide structural support and integrity to cells
Found in animals
Where are keratin filaments found?
In epithelial cells
Keratin filaments are a type of intermediate filament
What is the diameter of intermediate filaments?
10 nm
Intermediate filaments are thicker than actin filaments but thinner than microtubules
What diseases are associated with mutations in nuclear lamins?
Progeria and other diseases
Progeria is a syndrome of premature aging
What is the function of desmin in muscle cells?
Maintains organisation within muscle cells
Desmin filaments in adjacent muscle cells are linked via desmosomes
Expressed in cardiac, smooth and skeletal muscle
Fill in the blank: Intermediate filaments are _______.
Not polarized
Unlike actin filaments and microtubules, intermediate filaments do not have a plus or minus end
What is the role of plectin?
Links intermediate filaments, actin filaments, microtubules, and desmosomes
Plectin mutations can lead to severe diseases, including skin disruption, muscular dystrophy & neurodegeneration
True or False: Intermediate filaments rapidly grow and shrink like actin filaments.
False
Intermediate filaments are stable and do not undergo rapid dynamics
What is a LINC complex?
A structure that links cytoplasmic filaments to nuclear lamins
It involves SUN and KASH proteins
What role do neurofilaments play in neurons?
Strengthen neurons, especially in long axons (over 1 metre long)
What does cell migration involve?
- Myosin motors
- Cell adhesion to the surface
- Actin filament dynamics and organisation
How does actin polymerisation drive cell migration?
- pushes the plasma membrane forward
- the distribution/activity of actin associated proteins controls actin dynamics —> whether filaments assemble or disassemble and where
- actin filaments disassemble at the rear end of the lamellipodium
What are filopodia?
Extend by actin polymerisation pushing on the plasma membrane
Play a key role in guiding the migrating cell by probing the environment and establishing new contacts with the surrounding ECM
Why don’t filaments slide back?
They are anchored by interactions with other actin filaments via cross-linking proteins
Mechanism of animal cell migration
- Cell pushes out protrusions at the leading edge of the cell
- Protrusions adhere to the surface
- Rear of the cell is pulled forward
How do protrusions adhere to the surface?
- via focal contacts containing trans-membrane plasma membrane proteins called integrins
- contractile actin bundles (stress fibres) attach to focal contacts
Types of intermediate filaments
Cytoplasmic
- found in most animals but not arthropods or hydra
- e.g. keratin filaments in epithelial cells
- e.g. vimentin & vimentin related IFs (like desmin) in connective tissue cells, muscle cells and glial cells
- e.g. neurofilaments in nerve cells
Nuclear
- found in all animals
- e.g. nuclear lamins in all nucleated animal cells
Nuclear lamins
Intermediate filaments underlie the nuclear envelope in nucleated animal cells, forming the nuclear lamina
Properties of intermediate filaments
- 10 nm in diameter
- do not bind nucleotides (e.g. ATP and GTP)
- strong, rope-like and durable
- stable (do not grow and shrink rapidly)
- some disassemble during cell division (nuclear lamins, vimentin filaments) and reassemble in telophase
- disassembly is triggered by phosphorylation
Role of intermediate filaments
- Strengthen cells
- Provide protection against stretching (epithelial and muscle cells)
- Keratin filaments in adjacent cells are linked via desmosomes
- Results in strength across the epithelial sheet
Desmosome mutation causes cells to pull apart
What can keratin mutations cause?
Blistering
Intermediate filament assembly
- alpha helical region of a monomer joins with another IF protein
- forms a coiled-coil dimer
- this forms a staggered tetramer of 2 coiled-coil dimers = antiparallel
- eventually forms 8 tetramers
- final formation —> 8-stranded flexible helix, 10 nm long
Intermediate filaments are symmetrical and NOT polarised like actin filaments or Microtubules
