The cytoskeleton Flashcards
(30 cards)
What is the cytoskeleton and What does the cytoskeleton do ?
6 things
A 3D transport network that fills the cytoplasm
Drives movement
Intracellular organelle movement
Cell motility
Muscle contraction
Provides structural support for the cell
Controls cell shape
What are the 3 classes of cytoskeletal element?
What do all three have in common
Actin – Microfilaments
Tubulin – microtubules
Cytokeratin – intermediate filaments (only in animals)
All three have long unbranched one dimensional protein polymers
How do filament properties determine the dynamics
Microfilaments - polar ATP dynamic
Microtubules - polar GTP dynamic
Intermediate filaments - apolar less dynamic
What are the features of the apolar intermediate filaments
Apolar protein filaments
Homo or heterodimer subunits (keratins, vimentin, destine, lamin)
Antiparallel tetramer alignment
Assembly by annealing from small subunit aggregates (ULF)
How do intermediate filaments provide structural support
Dynamic exchange of subunits along the filament length
Extensible - can extend 3.5 times its length, high tensile strength, resistant to compression, twisting and bending
Main functions? Structural support (main one) determination and maintenance of cell and nucleus shape
What do nuclear lamins do ?
Lamins form lattice like nuclear lamina at the interface between inner nuclear envelope and chromatin.
Provides chromatin anchorage surface
What happens when there is a lack of lamin A?
blocks DNA replication
affects cell division
reduces RNA Pol II progression
reduces heterochromatin content
What is meant by spontaneous filament assembly?
Polar filaments can polymerise and depolymerise rapidly
What is the structure of microtubules
Microtubules are polymers of the protein tubulin
Tubulin is an alpha-beta heterodimer
13 protofilaments form the long hollow structure
How does tubulin polymerisation work?
Each monomer has a binding site for GTP
Beta-tubulin is an GTPase and can slowlyyy (inefficiently) hydrolyse GTP to GDP
Alpha-tubulin CANNOT hydrolyse GTP
Microtubules are dynamically assembled from subunits
Minus end (alpha tubulin) less dynamic, plus end (beta tubulin ) more dynamic
gives polarity to filaments
What is the kinetics of GTP hydrolysis in MT filaments
How does MT grow/shrink according to polymerisation
GTP cap at a growing plus end
Slow hydrolysis of GTP after polymerisation causes a conformational change at the tip, favouring depolymerisation
GDP-bound monomers don’t disassemble IF the GTP cap is maintained at the plus end
If polymerisation exceeds rate of hydrolysis, then MT grows
If hydrolysis exceeds polymerisation, then vice versa
** As long as GTP cap is present, then the filament is held together
What is dynamic instability
Growth happens at one end
Dynamic instability is caused by kinetics of GTP hydrolysis
???
In vivo microtubule nucleation
In vitro the initiation is the rate limiting step in polymerisation
Role of gamma tubulin
gamma tubulins provide a platform for nucleation and bind alpha tubulin
Talk about microtubule organisation in interphase animal cells
clue words:
Interphase
Centrosome
Gamma-tubulin ring complexes
Plus ends
In interphase cells there are around 50 microtubules
Originate from the centrosome, or MTOC which contains a pair of centrioles
In MTOC, gamma-tubulin ring complexes nucleate rapid microtubule growth
the MTs penetrate all of the cytoplasm, lying with their plus ends at the edges of the cell
MT dynamics in interphase animal cells
Some MTs are stabilised and long lasting.
Most MTs are constantly growing out of the MTOC, and depolymerising again, with a lifetime of only a few minutes - dynamic instability
This apparently wasteful activity provides the cell with a way to explore and sense changes in the cytoplasm or at the plasma membrane
How can MT stability be regulated by other proteins?
MAPs and catastrophins
Microtubule Associated Proteins can stabilise the growing end . MAP - stabilisation - frequency of catastrophes suppressed and/or growth rate enhanced.
Catastrophins can promote disassembly even from GTP capped ends . Catastrophin - catastrophe - frequency of catastrophe increased.
What do microtubules in animal cells do
Provide tracks for organisation and movement of internal organelles and transport vesicles.
Interphase
Movement of COPII vesicles from ER to Golgi
Movement of vesicles through the Golgi to PM
Movement through the endosomal system
Mitosis
Chromosome movement
How can MTs direct cargo to different directions?
Bi-directional transport by two motor types
Dyneins walk to minus end
Kinesins walk to plus end (usually)
what directions are COPI and II vesicles moved in, and by what proteins?
Minus end directed dyneins carry COPII vesicles from ER to Golgi at the MTOC
Plus end directed kinesins return COPI vesicles to the ER
Describe how kinesins walk along microtubules
e.g. how does ATP binding lead to movement
Kinesins are dimers
Kinesins walk along MTs in 8nm steps, the distance between one tubulin dimer and the next
One step costs one ATP
One head is always attached, holding the cargo to the MT
Kinesin thus shows high processivity
The ATP binding causes a large conformational change, advances by one step and throws the second head forward
Hydrolysis relaxes the changes and causes release from MT
How does dynein transport vesicles
Dynein uses ATP, moves by different mechanism
Binding of dynein proteins to transport vesicles involves accessory proteins
Dyneins and kinesins progress may be likened to that of a trudging postman
Microtubules are 150 times more rigid than microfilaments. what does this enable?
Can transmit compressive as well as tensile forces, can push and pull compartments (not just movement ).
What are the three types of stable spindle microtubules
Kinetochore microtubules - contact’s the chromosomes and pull sister chromatids to opposing poles
Overlap microtubules - interacting from the two poles, position and move spindle poles
Astral microtubules - (equivalent to interphase cytoplasmic MTs) position and move the spindle poles
How do kinetochore spindles work?
Kinetochore spindles pull the chromatids
The kinetochore is a structure at the centromere of a chromosome, spindle attachment site.
1) Active minus end directed movement by motor proteins on spindle
2) subunits are lost from both ends of the microtubules, causing the length of the kinetochore microtubules to decrease significantly (gradually get shorter, pulling motion)
