role of the cytoskeleton Flashcards
what is the cytoskeleton?
dynamic scaffold inside all eukaryotic cells that is responsible for cell shape and motility as well as transport and organization
how does the cytoskeleton assist neuronal function?
- cytoskeletal function
- protein localisation
- intracellular transport
- vesicle fusion and recycling
what are cytoskeletal networks made up of?
actin
microtubular networks
intermediate filaments
where is actin found?
around the plasma membrane to provide structural strength
what mechanism are microtubules important for?
transportation
what is the importance of intermediate filamemts?
give cell overall shape and strength
what are microfilaments made of?
actin
what is the size of actin?
~5nM in diameter, thin fibres found throughout cells
what are the two forms of actin?
monomeric G-actin and polymeric F-actin (filamentous)
G-actin description
ATP-bound (G-actin has a binding site for ATP)
can use ATP to make F-actin
ATP-bound G-actin has a higher affinity for the filament and polymerizes more readily at the plus end
F-actin description
F-actin is a double-stranded helix, where each strand is composed of G-actin monomers
has polarity, with a plus (+) end (fast-growing, barbed end) and a minus (−) end (slow-growing, pointed end)
action of actin
found extensively just below the plasma membrane and
forms thin filament in muscle fibres – binds myosin molecules
during contraction
what is the size of microtubules?
~20nm diameter thick fibres
what are microtubules formed by?
polymerisation of alpha and beta tubulin which takes the form of a spiral walled tube, forming networks throughout the cytoplasm
what are microtubules crucial for?
transport of vesicles and organelles through out the cell
forms the mitotic spindle
what is the energy source of microtubules?
GDP and GTP
what is the size of the intermediate filaments?
~10nm medium fibres
formation of intermediate filaments
monomers form dimers by sense-sense double helix
tetramers are formed by two dimers wrapping round each other to form sense-antisense double helix
tetramers coalesce and wind round each other to form the final filaments (c.f rope) which creates a very stable structure for mechanical strength to cell structure
neurofilaments (neuronal intermediate filamemts)
three protein subunits: NF-L (light), NF-M and NF-H
NF-M and NF-H can be heavily phosphorylated and are found mostly in axons
importance of NF-H and NF-M in neurofilaments
provide mechanical strength to axons – particularly highly expressed in motor neurons which have very long axons
what does phosphorylation in neurofilaments control?
phosphorylation controls axonal diameter – high
levels of phosphorylation = large diameter axon
actin and microtubule distribution in axons
microtubules are found throughout the axon
actin is found in the growth cones
actin and microtubule distribution in dendrites
microtubules are found throughout the dendrites
actin is found in the dendritic spines
actin polymerisation
at the end of the growth cone/spines, actin is perpendicular
as it grows it extends forward and pushes the membrane forward
monomers at the end are recycled
this is how actin drives growth
cytoskeletal organisation of dendritic spines
actin filaments are
found in dendritic
spines but not
microtubules
spine head (actin parallel to membrane), spine shaft and dendritic shaft (contains mictotubules)
microtubules act as a…
motorway network in both dedrites and axons
MAPS (microtubule associated proteins)
polymeric assembly of microtubules is stabilised by MAPS
axon MAP= tau
dendrite MAP= MAP2
protein localisation due to the cytoskeleton
dendrite= postsynaptic receptors and ion channels
axon hillock= sodium channels
NOR= sodium and potassium channels
pre-synaptic bouton= calcium channels and and autoreceptors
axonal localisation- actin, spectrin and ankyrin G
spectrin= dimer formed by two monomers in a double helix, protein binds ankyrin G (anchor protein, binds to cytoskeleton and ion channels)
1. leads to clustering of channels
2. generates high concentrations of
Na+ channels at the axon hillock
3. localises Na+ and (some) K+ channels at nodes
dendritic actin organisation
- actin is organised in short longitudinal strands
- spectrin is likely to be much less dense
- much lower degree of ion channel clustering
- ‘hot spots’, esp near branch points
however there are regions that are similar to axons= branching points
post synaptic density anchoring
contains LOTS of proteins
some of the most
important are the PDZdomain proteins e.g. GRIP, PICK1
MAGUKs=membrane
associated GUanylate
kinase e.g.PSD95
PSD is ultimately anchored to the actin cytoskeleton
anchor proteins at the
post synaptic membrane
weiss and hiscoe (1948)
1st evidence for movement in axons
* ligated peripheral nerves
* swelling of axons observed proximal to constriction site
* accumulated material moved along axon when constriction
was removed
axoplasmic flow
refers to the movement of cytoplasm within the axon of a neuron
slow rate (1 - 2 mm/day)
structural proteins (e.g. tubulin, neurofilaments)
what is axonal transport (bidirectional) driven by?
fast axonal transport (400mm/day) supported by ATP hydrolysis
anterograde transport
- begins in cell body
- cargo transported out to the axon terminus / processes
- powered by a kinesin
retrograde transport
- begins at axon terminus
- cargo transported back through the axon to cell body
- powered by MAP-1C (microtubule associated ATPase) – a dynein
kinesin description
heterotetramer
comprising of 2 heavy
and 2 light chains (double helix)
heavy chains contain a
globular head (ATPase
domain)
globular heads act as
motors when attached
to microtubules
travel towards the + end
have vesicle attached at end which carries receptors
what are microfilaments?
made of actin protein subunits that form long, thin, flexible helical structures
mechanical support, facilitate cell movement, muscle contraction etc
what are intermediate filaments?
made up of fibrous proteins that assemble into rope-like structures and coiled into a helical structure
provide structural stability and anchor organelles
what are microtubules?
hollow tubes made up of tubulin protein subunits (alpha and beta tubulin dimers)
have rigid and have dynamic properties meaning they can grow and shrink
provide structural support and intracellular transport
dynein description
complex of heavy,
intermediate and light
chains
heavy chains contain a
globular head (ATPase
domain)
globular heads act as
motors when attached
to microtubules
travel towards the - end
what are kinesin and dynein?
two types of motor proteins that move along microtubules
these proteins convert chemical energy stored in ATP into mechanical work to facilitate intracellular transport
