Cytoskeleton - Microtubules Flashcards
What is the cytoskeleton?
As eukaryotic cells are too large - sometimes diffusion isn’t possible so we need motors and tracks
Cytoskeletal systems are dynamic and adaptable and are made up of proteins only nm in size - which is helpful for disassembly/diffusion
What are the type of cytoskeletal filaments?
Three types of cytoskeletal filaments:
Actin filaments (microfilaments): 7nm in diameter
Polymers of actin, tracks for myosin (molecular motors)
This defines the shape of the cells surface, are needed for whole-cell movement and drives the pinching in telophase (forming a belt - contractile ring)
Found beneath the nuclear lamina
Microtubules: 25nm in diameter (largest)
Polymers of alpha-beta tubulin dimers
Tracks for kinesin and dynein (molecular motors)
Intermediate Filaments:
10nm in diameter
No motors associated with these filaments and non-polar = less dynamic
Strong rope like fibres - forms the nuclear lamina
What are accessory proteins?
All these filaments interact with hundreds of accessory proteins - these are essential for the controlled assembly of the cytoskeletal filaments
They regulate the length and stability of the cytoskeleton so they can form a variety of high-order structures - specifically regulating the spatial distribution and dynamic behaviour
They can: determine sites for new filaments, regulate partitions within a filament, alter filament kinetics, harness energy to generate force and link filaments to one another or other cell structures
What are motor proteins?
They bind to polarised cytoskeletal filament and use energy from ATP to move along it
They differ in direction, binding filament and cargo e.g. Membrane bound organelles like mitochondria
The direction of sliding depends on the polarity of the track
Describe the cytoskeleton in bacteria in most archaea?
FtsZ - homolog of tubulin, which polymerises into filaments - Z-ring (half-life of few mins)
The Z-ring generates a bending force to drive membrane invagination in telophase
MreB and Mbl - homologs of actin, found in rod or spiral shaped dynamic cells that move around the circumference of the cell
They serve as a scaffold to direct the synthesis of the peptidoglycan cell wall
Mutations cause extreme cell shape abnormalities
Caulobacter crescentus, harbours an intermediate homolog crescentin - that influences the crescent/sickle shape of this bacteria
Describe microtubules?
They are dynamic tubes growing from the centromere and then they collapse back
They are like highways along which motors carry membranes
They have moving vesicles along them, in both directions
Negative end is towards the centre and the positive end is towards the periphery of the cell (showing directionality)
○ Positive end = fast growing - B tubulin
○ Negative end = slow growing - a tubulin
They are the biggest type of filament in a cell - seen by immunostaining during interphase
What is the composition of microtubules?
They are made from tubulin
There are two types of tubulin: alpha and beta
There are around 6-7 genes for each alpha and beta tubulin (in mammals) = diverse types of tubulin
They are formed from the polymerisation of tubulin proteins - therefore the ‘building blocks’ are an alpha-beta tubulin complex (heterodimer)
The subunits of microtubules are asymmetrical and bind to one another head-to-tail = they all point in one direction
What is a protofilament?
Protofilament - a ‘stack’ of heterodimers forming a vertical line
Each protofilament is slightly offset creating a slightly helical structure
There are 13 protofilaments in a microtubule (diameter of 25 nm)
What is the centrosome?
The microtubules are nucleated at the centrosome in cells
It comprises of barrel shaped structures surrounded by ‘fuzzy material’ = pericentriolar matrix, containing various proteins that helps anchor the microtubules
Gamma tubulin rings - initiates microtubule polymerisation
The centrosome has a ‘mother’ (maternal) and ‘daughter’ centriole
A pair of centrioles each has ninefold symmetry
Only the maternal centriole has two sets of extra appendages - distal and subdistal
Subdistail seems to anchor microtubules
What is a property of microtubules?
They polymerise and depolymerise
GTP-tubulin subunits add to the fast growing plus end of microtubules
If the GTP tubulin ‘CAP’ is lost, the microtubule will start to depolymerise:
This is called ‘catastrophe’ (it happens rapidly)
Note both α and β tubulin bind GTP, but only β tubulin hydrolyses GTP
This subunit is exposed at the ‘plus’ (fast growing’ ends)
How can microtubules be post-translationally modified?
The C-terminal tail can be polyglutamylated, or polyglyclated, or the C-terminal tyrosine can be removed
C-terminal tails on the outside of the microtubule ‘tube’
This affects binding of MT proteins & their behaviour (& stability)
What are the main functions of microtubules?
- Trafficking of cargo (organelles, protein, RNA) in interphase cells
- Mitosis
- Cilium
What is used in trafficking within microtubules?
Kinesin and Dynein are motor proteins that use microtubules as ‘tracks’
The cell body of most motor neurons is in the spinal cord, and the synapses (neuromuscular junctions) can be over 1 metre away
Microtubule motors are important for this
Kinesin takes things out of the microtubule
Dynein will bring things in to the microtubule
Describe Kinesin?
A dimer formed by two heavy chains Motor domain - binds actin and nucleotide (hydrolyses Mg.ATP to generate movement) Tail - Binds cargo It has no lever Mr - around 100 kDa
What is the function of kinesin?
Most kinesins walk to the ‘plus’ (fast growing) ends of microtubules at the cell periphery
Kinesin takes 8nm steps (same size as an αβ-subunit)
Walks straight along a single protofilament
Powered by ATP hydrolysis
The ATPase of kinesin is accelerated when the kinesin binds to microtubules
About 40 different types of kinesins, many specifically for mitosis
Mutations in kinesin-1 would interfere with trafficking organelles
What is the kinesin-1 ATPase cycle?
- Kinesin and ADP binds
The microtubules accelerate ADP release x1000 fold - ADP releases and the neck liner undocks
There is a strong Apo binding state - ATP binds to the kinesin and the neck linker docks
During ATP state there is a strong microtubule binding
Microtubules accelerate ATP hydrolysis x10 fold - ATP hydrolysis results in an ADP.Pi state
Here there is strong microtubule binding - Pi is released - resulting in weak binding
- Kinesin detaches with the ADP
ADP release is the rate limiting step
What is significant about the two kinesin heads?
They are asynchronous (out of sync)
1. Rear head (ATP.Pi): neck linker docked, Front head (ADP) and neck linker undocked
2. ADP is lost by front head and Pi is lost by rear head
’Gating’ behaviour between the two heads for co-ordination
3. Rear head (ADP) detaches, neck linker will undock
4. Front head (was rear head) binds ATP, the Neck linker docks and helps to move rear (ADP) head forward
Powerstroke - this is the docking/undocking of the neck linker
It is the neck linker that is driven by the ATPase state of the motor
Describe Dynein motors?
Minus-end directed microtubule motor
ATPase (4 nucleotide pockets per heavy chain)
Work coupled to release of products of ATP hydrolysis
A member of the AAA+ superfamily of mechano-enzymes
Composition: Heavy chain(s) (>500 kDa each) Accessory chains (I, LI, L- mainly cargo binding) Heavy chain contains the motor domain (ATPase and microtubule-binding domains)
What is the AAA+ superfamily of mechano-enzymes?
AAA+ proteins assemble into ring shaped oligomers: typically homo-hexamers
AAA+ mechanoenzymes perform diverse functions: unfolding/destabilising
Dynein’s contain 6 AAA+ domains in a single polypeptide chain
They convert energy from ATP hydrolysis into force/stepping along microtubules
ATPase kinetic scheme is similar to that of the actin-based motor myosin
What is significant of the iosforms of dynein?
The family tree = all the isoforms that can be in one cell
Only two isoforms of cytoplasmic dynein heavy chains
Only one of these (cytoplasmic dynein 1) is the main trafficker
Multiple functions achieved through light chain/intermediate variation and additional regulatory proteins
What is dynein’s structure?
This is the 9+2 organisation
Each of the 9 outer doublets has rows of dynein arms that reach out to the next doublet and cause it to slide
Driven by ATP hydrolysis
Both stalk and tail flexible
This means distance varies between cargo and MT-binding site
Important to allow Powerstroke of motor without simultaneous movement of cargo
How does the powerstroke of dynein take place?
Tail connects to AAA1 ATPase domain in head via ‘linker’ domain
Linker changes its orientation relative to head and stalk
Linker movement could translate microtubule
We can see the linker’s change in conformation
When you hydrolyse ATP, The linker changes from straight to bent, causing it to come away form AAA4, this part of the molecule pulls on the stalk allowing the microtubule domain to bind = generation of the power stroke
Describe the ATPase cycle for dynein?
- ATP hydrolysis and phosphate release, the linker is straight
- ADP is released, the linker docks and then ATP can bind
- ATP binding, causes the linker to bend
What else does dynein require to work?
Dyactin activates the dynein
Dynactin is 1 mega Da
It binds to the tail of dynein
It makes sure the motor domains stick out correctly, in order to bind to the microtubule
