Lecture 14: Cytoskeleton Flashcards
what is the function of the cytoskeleton 3
provides structural support
maintains cell shape
enables cellular movement and organisation
what is the cytoskeleton composed of 3
3 major types of protein:
actin microfilaments
microtubules
intermediate filaments
what other processes is the cytoskeleton involved in 4
cell division
intracellular transport
signal transduction
cell adhesion
describe microtubules 3
cylindrical, tube like
diameter of 25nm is biggest component
made of a-tubulin and b-tubulin protein subunits
what does a-tubulin and b-tubulin make together
heterodimers
how do these heterodimers work 2
assemble head to tail creating protofilaments
13 parallel protofilaments associate laterally to form a microtubule
explain polarity within microtubules 2
microtubules exhibit polarity with a “plus” and “minus” end
the plus end is the site of rapid growth, whereas the minus end is more stable
what regulates the rapid assemble and disassemble of microtubules
GTP hydrolysis GTP -> GDP + Pi
what is the function of microtubules 3
intracellular transport
maintenance of cell shape
formation of the mitotic spindle during cell division
explain the role of microtubules in the formation of the mitotic spindle
non-dividing cells = microtubules radiate from microtubule organising center (MTOC) near the nucleus
minus end anchors at MTOC and plus end extends towards end of cell
during mitosis centrosomes duplicate and move to ends of cell with spindle microtubules extending from them
some spindle microtubules attach to kinetochores of xsomes
others interact with microtubules from the opposite centrosome
microtubule dynamics facilitate xsome alignment and segregation = equal distribution of genetic material to daughter cells
what is dynamic instability
a process characterised by alternating phases of polymerisation and depolymerisation
what is dynamic instability regulated by in microtubules
GTP and GDP bound tubulin at the end of the microtubules
GTP-bound b-tubulin is incorporated into the growing end and the GTP is hyrdolysed into GDP
presence of GTP bound tubulin caps promotes growth
what are intermediate filaments
rope like structures
second largest component
diameter of 10nm
what are intermediate filaments composed of 6
various proteins divided into 6 classes
type 1 - acidic keratin
type 2- basic keratin
type 3 - vimentin, desmin, glial fibrillary acidic protein
type 4 - neurofilaments
type 5 - nuclear lamins
type 6 - nestin
what does each intermediate filament protein have
conserved central a-helical rod domain flanked by non helical head and tail domains
how do intermediate filaments assemble
assemble into parallel dimers through coiled-coil interactions between their a helical rod domains
these dimers arrange antiparallel to form staggered tetramers/ protofilaments
8 protofilaments form a cylindrical intermediate filament with non- polar, elongated structure
give 2 distinct differences between intermediate filaments and microtubules and actin filaments
do not exhibit polarity
are not directly involved in intracellular transport or motor protein based movements
what is significant about the intermediate filaments
their unique structure allows them to withstand significant tensile forces and maintain cell integrity under mechanical stress
what is the function of intermediate filaments 7
1 maintain cell shape and integrity by distributing mechanical stress evenly preventing deformation and damage to cellular components
2 cell to cell and cell to extracellular matrix adhesion by linking cytoplasmic structures to cell junctions and focal adhesion
3 nuclear positioning and anchoring, nuclear lamins provide structural support to nucleus and connecting it to cytoskeleton
4 muscle cells desmin connects myofibrils to sarcolemma - maintains structural integrity of muscle fibers & force transmission
5 nerves - neurofilaments - structural & functional properties of neurons. GFAP - glial fibrillary acidic protein for glial cells
6 regulation of cell signalling pathways - interact with signaling molecules and influencing their localisation and activity
7 involved in various cellular processes, including cell migration
describe actin microfilaments
thinnest component
diameter of 7nm
what are actin filaments composed of
globular actin (G-actin) protein subunits
assemble into helical polymer called filamentous actin (F-actin)
how do actin filaments exhibit polarity
they have a “barbed” plus end where subunits are added rapidly
“pointed” minus end more stable
explain G-actin
monomeric form of actin
highly conserved protein
central role in various cellular processes
has ATP/ADP binding sites for interactions
explain F-actin
linear polymer
formed by head to tail assembly of G actin monomers
flexible and dynamic helical filament
assembly process is ATP-dependent
how are actin filaments formed
1 nucleation - assembly of g-actin into a stable nucleus- a trimer = this is the rate limiting step nucleus has high energy barrier
2 addition - g actin monomers bound to ATP associate with plus end extending it
what is treadmilling in the context of actin filaments
the dynamic turnover of actin filaments
addition of g actin monomers at the barbed end is balanced by dissociation of ADP- bound actin subunits from pointed end
maintains constant filament length with directionality
what is the formation of filaments regulated by
actin binding proteins e.g Arp2/3 complex, tropomyosin
explain the function of actin binding protein
organisation, dynamics and function of actin filaments
capping proteins regulate addition or loss of actin subunits ensuring stability
cross link proteins organising them
motor proteins interact with actin filaments to generate force
what are motor proteins
specialised proteins that convert chemical energy-> mechanical
what are the major motor proteins
myosins
kinesins
dyneins
what is the conserved motor domain for in myosins
for actin binding and ATP hydrolysis with a variable tail domain that binds to cellular cargoes to move
what forms the sarcomere
myosin class 2 assemble into bipolar thick filaments which interact with actin
what do myosin 1 and myosin 5 do
vesicle transport
cell migration
cytokinesis
what is myosin function regulated by
phosphorylation
calcium binding
interactions with other proteins such as tropomyosin and troponin
what are the kinesins characterised by
conserved motor domain with a microtubule binding site and ATP binding site which is responsible for the driving movement along microtubules
what does it mean that kinesins are plus end directed motors
they move towards the plus end of microtubules
what causes the walking motion with kinesins
two motor domains alternate between microtubule bound and unbound states while hydrolysing ATP
what are kinesins also involved in
mitosis - formation and positioning of the mitotic spindle
xsome alignment
segregation os ister chromatids to daughter cells
transport of organelles, vesicles
what are dyneins
move laong microtubules and convert chemical energy from ATP to mechanical work for cellular transport and mitosis
are dyneins plus or minus end directed motors
minus end directed motors
what are dyneins classified into
2 major groups:
cytoplasmic dyneins
axonemal
what are cytoplasmic dyneins involved in
intracellular transport and mitosis
what are axonemal dyneins involved in
the beating of cilia and flagella (kinky hehhe)
what does it mean that dyneins are processive motors
they take multiple steps along microtubules without dissociating allowing them to cover long distances
why is dynein mediated transport important
essential for proper positioning and function of organelles like golgi apparatus, endosomes, lysosomes and vesicles
what are focal adhesions
specialised multi-protein complexes that connect actin cytoskeleton to extracellular matrix
faciliate cell adhesion
migration
mechanosensing
what are integrins
transmembrane receptors that play important part in focal adhesions by binding ECM proteins to the actin cytoskeleton
what is the function of focal adhesions
important for mechanotransduction- the process by which cells sense and respond to mechanical forces
they convert mechanical stimuli into biochemical signals that affect cellular processes such as gene xpression migration and differentiation
what are cytoskeletal components regulated by
signaling pathways
what does the Rho signalling pathway do
control actin polymerisation
stress fiber formation
cell migration
what does the cytoskeleton do in cell division 4
organises mitotic spindle
formation and constriction of cleavage furrow during cytokinesis
ensures proper spindle assembly and xsome segregation
movement of xsomes along microtubules
what is the role of cytoskeleton in wound healing
drives cell migration and shape changes necessary for cells to migrate into wounded area
actin polymerisation generate lamellipodia and filopodia which enable cells to move towards the wound
formation and turnover of focal adhesions provide traction for cell migration
cytoskeletal remodeling modulate cell migration and wound healing processes
activate signalling pathways that regulate cell proliferation, migration and differentiation
