CBG Lecture 37:Cytoskeleton Flashcards
what are the main roles of the CSK in eukaryotes
support of cell shape: microvilli, cell cortex
intracellular traffic - guides moving vesicles
cell division - mitotic spindle
motion - cell migratoin, muscle contraction
within a cell, where is microtubules/actin
microtubles arranged from centre outwords
actin generally on periphary/circumference
how does CSK help in support
microvilli
cell cortex
how does CSK help in intracellular traffic
guides moving vesicles
how does CSK help in cell division
separation of spindle poles
how does CSK help in motion
filapodia
flagella
what is CSK essential for
wound healing
sperm-egg fusion
muscle function
what are the 3 distinct subregions of CSK
- microtubules:detemine organell position
- intermediate filaments:give mechanical strength
- microfilaments - actin filaments: give cell surface shape
what is a microfilament, and its role
actin
gives cell surface shape
what do intermediate filaments do
give mechanical strength
what do microtubules do
determine organelle position
what do motor proteins do
move organelles along the filaments of move the filaments themselves
what do all 3 filaments (microfilaments,intermediate filaments, microtubules) have in common
all form as helical assemblies of subunits that self-associate using a ocmbo of end to end and side to side protein contacts
discuss structure of intermediate filaments
rope like and hard to break but easy to bend
give structure of microtubules
strong rigid hollow tubes
give structure of microfilaments (actin filaments)
thinnest of the 3, hard to stretch but easy to break
whatis base unit of mictrotubules
alpha/beta tubulin heterodimer hollow cylindrical polymer polar filament stiff/rigfid GTP hydrolysis has major effect on microtubule dynamics
what is outer diameter of microtubules
25nm
what do microtubules typically have one end attacxhed to
a single MTOC (micrtubule organising centre) called a centrosome
talk about microtubule dynamics
beta tubulin binds GTP during polymerization, this molecule hydrolysed to GDP
GDPbound form of tubulin is unstable in the microtubule and causes microtubules to shrink between shrinking (catastrophe) and growing (rescue) phases
alpha tubulin is ALWAYS in its GTP-bound state and has a structural role
GTP hydrolysis puts the lattice under stress
when cap is lost microtubule depolmerises and peels apart
discuss microtubule organisation
centrosomes are MTOCs - microtubule organisation centres
gamma tubulin ring complex (gamma-TURC) within MTOC nucleates microtubule assembly
grow from + end
centrosome (interphase) spindle poles (mitosis) are MTOCs
what does gamma TURC do
within MTOC and nucleates microtubule assembly
name an MTOC in mitosis
spindle poles
name an MTOC in interphase
centrosome
how can microtubules be used to treat disease
controlling microtubule dynamics can treat gout
colchicine - affects tubulin dynamics by inhibiting polymerization and therefore relieves gout joint pain
name drugs that prevent polymerization of tubulin
Colchicine was first known drug
Taxol is moder drug that inhibits mitosis and used to treat some breast/ovarian cancers
what is movement along mictubules (MT) based on
the action of motor proteins that utilize energy derived from ATP hydrolysis to produce force and movement
such proteins: kinesin, dynein
what do kinesin and dynein do
move cargo along microtubule tracks
what are kinesins
transport cargo towards + end MT via a processive movement - lots of contact, never dissociates from MT
eg. carries NTs from Golgi to terminal branches of aoxn, generally carries AWAY from centre
discuss structure of kinesins
2 heavy chains wound round eachother in coiled coil and two light chains
the globular head domains of the heavy chains bind MTs and are the motor domains of the molecule
what end does kinesin transport cargo to
kinesin transports cargo to the + end of the MT
what are dynein proteins
motorprotien which requires qaccessory proteins to make a dynactin complex
in what direction does dynein transport cargo
form axon BACK to cell body - eg when no NT left
generally TOWARD centre
outline structure of dynein
2/3 heavy chain in association with multiple light and intermediate chains
the globular head domains of the heavy chains are the motor domains
outline “kinesin walking”
when one of the two kinesin heads encounters a MT it bind tightly
MT binding causes ADP release from attached heads
ATP then rapidly enters the empty nucleotide binding site - nucleotide exchange triggers the neck linker to zipper onto catalytic core
this action throws the second head forward and brings it near next binding site on MT
the attached trailing head hydrolyses ATP and releases Phosphate
as the neck linker unzippers from the trailing head, the leading head exchanges its nucleotide (ADP for ATP) and zippers its neck linker onto catalytic core
what does processive movelment along MT mean? what does this
kinesin does this
it means step by step
describe intermediate filaments
provide mechanical strength - dont serve as tracks for motor proteins, less dynamic
various subunits
no polarity
do intermediate filaments have polarity
no
give some examples of intermediate filaments
keratins
lamins (nucleus)
what filaments are lamins an example of
intermediate filaments
what residues are keratins rich in
cysteine- form disulfide bonds
which filaments hve subunits that DONT bind nucleotides
intermediate filaments
what are actin filaments
microfilaments
what do microfilaments do
determine the shape of the cells surface and necessary for whole cell locomotion
serve as tracks for motor protein myosin
line the perimeter of cell and give shape to microvilli
what is the base unit of microfilaments
an actin monomer G actin
G actin assembles into a linear filament of F actin - 2 helices
F actin ha spolarity
in microfilaments, where is the nucleotide binding site
on - end as it is polar
at what end to microfilaments grow faster
positive ends
name some cellular structures uysed by cells to explore territory and pull themselves around
lemellipodia
filopodia
what is actin filament made of
2 parallel protofilaments that twist round eachother in a right handed helix
what is profilin
recharges ADP-actin
what is cofilin
binds ADP-F-actin and destabilizes the filament, untwist
how are actins organised
by crosslinking proteins
filaments attach laterally at the (+) end to the PM
what is the term given to actin for how it grows
treadmilling
what is treadmilling
actin dynamics - made possible by the nucleoside triphosphate hydrolysis that follows subunit addition
what gives directionality to actin dynamics
filament growth from + end, ADP actin disassembles from - end
profilin recharges and allows regrowth
cofilin binds ADP-F-actin and destabilises the filament causing untwisting
discuss movement along actin filaments
myosin motor proteins move along actin tracks
myosin structure: head domain - motor
neck domain -lever arm
tail domain -cargo binding
which part of myosin is the cargo binding, and lever arm
`cargo binding - tail domain
neck domain -lever arm
how is myosin movement powered
ATP hydrolysis
what is function of myosin 2
muscle contraction
move towards + end
what is function of myosin 4
`transport vesicular cargo
move towards - end
endocytosis
which myosin involved in muscle contraction
myosin 2
which myosin involved in vesicular cargp transport
myosin 4
outline myosin movement
1`.prestroke ADP-Pi form
- actin binding triggers release Pi, converts to post stroke ADP form
- ADP dissociates, ATP binds triggers detachment
what is power stroke
generation of tensile force
what od myosin motor proteins do
control actin dynamics during muscle contraction
what makes up individual muscle fibres
laterally aligned muscle fibres
what are myofibrils made up of
sarcomeres which are assemblies of actin and myosin microfibrils
what does myosin do
binds and hydrolyses ATP which drives its movement along actin filament
what are actin filaments anchored by in the sarcomere
anchored by plus end to Z disc
what happens in the sliding filament mechanism
- contraction begins with the head of myosin molecules bound to actin on actin filament
while still bound to actin, the myosin head fibres, pulling the actin filament along with it - this causes actin filament to slide by myosin filament
myosin head then releases from the actin and unflexes - change powered by ATP hydrolysis
this frees the myosin head to bind with a different actin molecule
