Module 3 Flashcards
Cytoskeleton
- provide cells with structure (so they keep their shape, internal organisation and polarity)
- helps cells move and change shape
- needs to “strike a balance” between the two above
- needs to be dynamic
- provides ‘tracks’ for motor proteins to transport stuff from one part of the cell to another
Main components of cytoskeleton
microfilaments
microtubules
intermediate filaments
Microfilaments
- Polymers of actin
- Extend throughout the cytoplasm
Microtubules
- Polymers of tubulin
- Radiate out from the centrosome
towards the cell periphery
Intermediate filaments
- Made from a myriad of units
depending on the cell type - Extend through the cytoplasm
or nucleus (depending on the
type)
Examples of actin structures
Microvilli
Cell cortex
Contractile ring
stress fibres
Stiffness of cytoskeleton components
highest to lowest:
- microtubules
- intermediate filaments
- microfilaments (actin)
Actin polymerisation: the steps
Nucleation
- g-actin
- nucleus
Elongation
- f-actin
Steady State
- - and + end
Rate-limiting step in actin polymerisation
nucleation
Formins
Regulation of actin filament assembly
assemble unbranched filaments
- 1 Formin dimer binds to two G-actin subunits (early stage of nucleation)
- By rocking back and forth, additional subunits are added
Arp2/3 complex
nucleate branched filaments
- 1 Arp2/3 complex binds to the
side of actin filaments (F actin)
and generates a branch - Arp2/3 needs an additional
protein: WASp (verprolin in
yeast) - results in nucleation of
actin filaments.
Actin at steady state
Treadmilling
- Actin is constantly polymerising
and depolymerising within the
cell. - ATP-G actin subunits are always
added to the + end. After
binding ATP is hydrolysed to ADP
and subunits are lost at the -
end of the growing filaments -
treadmilling - Addition at the + end is faster
than the – end, resulting in
treadmilling - relative rate a + end is much greater than at neg end.
Profilin
controls treadmilling
- Catalyses the exchange of ADP/ATP i.e. promotes the formation of ATP-G-Actin, providing a greater supply for binding to (+) end
Cofilin
controls treadmilling
- Binds along the filament, destabilises ADP-actin in filaments, enhancing disassembly at (-) end
Thymosin B4
controls treadmilling
- Sequesters away ATP-G-actin. Acts as a buffer for supply of ATP-G-Actin to (+) end
Actin microfilament motor proteins
Myosins
Myosins
Myosin I found in the cell periphery
Myosin II, found in muscle and
non-muscle cells - required for
cytokinesis and focal adhesion
(in non-muscle cells)
Myosin V, required for organelle transport (e.g. melanosomes are transported from melanocytes to
neighbouring keratinocytes.)
Myosin II motor protein (power stroke)
myosin head contains motor. binds ATP
- myosin head binds ATP, releases actin
- head hydrolyses ATP to ADP +P, rotates to ‘cocked’ state
- cocked state binds actin
- P is released, myosin head moves along filament: the power stroke
- head remains bound to actin while ADP is present. when ADP is exchanged for ATP, myosin head is released and cycle starts again.
The sarcomere
- The sarcomere is a unitary, contractile unit
- Each sarcomere contains two types of filaments: thick filaments of Myosin II and thin filaments of actin
- Z Disc – proteins that anchor thin filaments
- I band – thin filaments only
- A band – overlapping thin and thick filaments
- H zone – thick filaments only
The sliding filament model
- The decrease in sarcomere length is due to decreases in the width of the I band and H zone, with no change in the width of the A band.
- The thick and thin filaments slide along one another, increasing their overlap and so pulling the Z discs
closer together. - Release of Ca2+ from the Sarcoplasmic reticulum* triggers contraction
- Reuptake of Ca2+ into Sarcoplasmic reticulum relaxes muscle
Microfilament motors: Regulation by calcium
Tropomyosin (TM) and troponin (TN) are accessory proteins bound to actin thin filaments.
In the absence of calcium, TM and
TN molecules block the interaction
of myosin with F actin
In the presence of calcium, TN
induces TM to move to a new site,
exposing the myosin-binding sites
on actin
Actin filaments vs microtubules
Similarities
* Both actin filaments and microtubules are polymers of a single type of monomer
* Both have a distinct (+) and (-) end
* Both are continuously assembled and disassembled (polymerisation and depolymerisation)
* Both are involved in maintaining cell shape and in cell movements
Differences
* Microtubules are assembled at a specific site in the cell
* Microtubules are larger in diameter than actin microfilaments
* Microtubules are more rigid than actin microfilaments
* Microtubules can grow very long to 100s of µm or even mm (nerve cell axons)
`Microtubule assembly
- Assembly of the α/β-tubulin dimer happens at the (+) ends of microtubules.
- GTP/GDP regulates assembly and disassembly
- Both α- and β-tubulin dimers can bind GTP but
hydrolysis only happens in the β-tubulin - GTP-to-GDP hydrolysis only happens once the
tubulin is bound to the microtubule - The ring of γ -tubulin at the PCM stabilises the (-) end and prevents disassembly
- GTP-bound β-tubulins promote assembly
- GDP-bound β-tubulins promote disassembly
- The GTP cycle is essential for the dynamic instability of the microtubule (switching between catastrophe aka rapid shrinkage and rescue aka growth)
centrosome
a complex structure - generally lies next to the nucleus
- consists of two centrioles surrounded by pericentriolar material - PCM
- centrioles are always found in pairs and at right angles to each other
