Cytoskeleton and actin microfilaments

Question 1

cytoskeleton summary:
what is it
what is it needed for
what is it made of

Answer 1

Network of protein filaments that provides structure, support and helps with cell movement. Crucial for cell division, shape maintenance and intracellular transport. Consists of microfilaments, intermediate filaments and microtubules

Question 2

subunits of microfilaments (macromolecular polymer)

Answer 2

actin

Question 3

subunits of microtubules

Answer 3

alpha and beta tubulin proteins
form dimers

Question 4

subunits of intermediate dimers

Answer 4

diverse family of proteins

Question 5

how can the polymers form higher ordered structures

Answer 5

pack together and cross link via cytoskeleton binding proteins

Question 6

how are neurotransmitters transported from soma to axon terminals?

Answer 6

motor proteins (kinesin and dynein) move along microtubules
kinesin moves the vesicles anterogradely and dynein moves them retrogradely (back to soma)

Question 7

cytoskeleton involvement in digestive system and how this maintains polarity across epithelial cells

Answer 7

kinesin and dynein transport vesicles loaded with nutrients and sugars along microtubules: kinesin towards the positive apical surface (lumen side) and dynein towards negative basal surface (blood). actin filaments help to anchor and release vesicles at the membrane

Question 8

how cells move

Answer 8

Actin microfilaments are responsible for forming protrusions at the front of the cell, known as lamellipodia or filopodia. Actin filaments rapidly polymerize (grow) at the leading edge of the cell, pushing the membrane forward. This creates the force needed for the cell to extend towards the direction it wants to move. leading edge adheres to the extracellular matrix or other cells via integrins and other adhesion molecules, providing traction.
The cell body and rear end contract via actin myosin interactions, pulling the cell forward.

Question 9

cytoskeleton in muscle contraction

Answer 9

driven by the action of actin microfilaments and myosin that associate with them.
intermediate filaments resist the mechanical stress and tension
microtubules also resist compressive force and generate tracks where substances can be trafficked around the cell

Question 10

cytoskeleton in cell-cell adhesions

Answer 10

cells linked on cytoplasmic face to cytoskeleton, forces transmitted between cells can be felt by neighbouring cells immediately. forces can be transmitted from cells to extracellular matrix
can respond to environmental changes

Question 11

what are stress fibres

Answer 11

contractile bundles of actin filaments and associated proteins (eg myosin II). antiparralel to each other. span across cell and connect to focal adhesions. provide tensile strength and rigidity to resist cell deformation. allow cell movement and signal transduction
important in cytokinesis

Question 12

where are actin microfilaments

Answer 12

underlie membrane
connected to cell-cell and cell-matrix adhesions

Question 13

structure of actin microfilaments

Answer 13

two strands of helical monomers: globular actin and filamentous actin. Polar: barbed (+) end where polymerisation occurs with greater rate, and pointed (-) end

Question 14

G actin mechanism

Answer 14

each G actin monomer has a binding site for one molecule of ATP. ATP-bound G actin ha a higher affinity for the polymer at barbed end. ATP hydrolysis weakens the association so at the pointed end, ADP bound G actin monomers are more likely to dissociate.

Question 15

steady state actin dynamics

Answer 15

rate of addition of monomers = rate of shrinkage
living cells never reach this equilibrium

Question 16

exponential growth phase of actin filaments

Answer 16

not energetically favourable for individual actin monomers to come together and form a filament = low rate
eventually oligomers form and it is now energetically favourable for monomers to add to the pre-existing filament
exponential growth phase until run out of ATP or actin monomers

Question 17

filopodia

Answer 17

higher order structure of actin filament
stiff parallel bundles
can send chemical messages/interact with other cells
sense environment
adhere to matrix via integrins
cell movement
at cell membrane
microvilli

Question 18

lamellipodia

Answer 18

branched (dendritic) network
found at leading edge of cell
cell movement

Question 19

cortex

Answer 19

gel-like mesh network underlying plasma membrane

Question 20

filopodium

Answer 20

thread foot

Question 21

lamellipodium

Answer 21

sheet foot

Question 22

protrusion

Answer 22

force of polymerising F actin pushing on membrane

Question 23

filpodia in neurones

Answer 23

explore and find cells to make contact with
growth cones-actin rich protrusions with long filopodia and enriched in adhesion proteins at the tip

Question 24

interdigitation

Answer 24

epithelial cells form tight sheets where they pack closely with neighbours. extend their filopodia to make contact with another cell and form adhesions on a large surface area

Question 25

structure of growth cones

Answer 25

Large parallel bundles of filopodia interspersed within the cross linked dendritic structure of the lamellipodia

Question 26

role of actin binding proteins

Answer 26

barbed end binding proteins control polymerisation and branching
proteins bind to multiple actin microfilaments and cross link them together

Question 27

AAP 23 complex

Answer 27

binds to the side of a pre-existing actin micro-filament and encourages/nucleates the growth of a new filament from that position. Gives rise to dendritic structure. Grows off in a 70 degree angle

Question 28

role of actin bundling proteins

Answer 28

crosslink microfilaments in filopodia
crosslink cortical meshwork

Question 29

how can some bacteria hijack actin machinery to move around cells

Answer 29

they have proteins on them that acts as actin nucleators
eventually filament punches through the plasma membrane from one cell to another

Question 30

myosins in cell division

Answer 30

cells form contractile ring structure with lots of actin microfilaments in antiparallel bundles and a high concentration of myosin II

Question 31

myosins in cellular transport

Answer 31

Provide contractile force by moving along F-actin filaments
Movement along F-actin is polarised: towards barbed end
Transport cargo along a filament
Slide filament relative to organelle or cell membrane
Slide filaments relative to one another
Bundle F-actin filaments together

Question 32

how do myosins differ

Answer 32

tail domains are variable
actin-binding head domain is highly conserved
monomeric (one head) or dimeric (2 heads)

Question 33

myosin I

Answer 33

one head
associate with plasma or organelle membranes and bind to actin
involved in endocytosis and membrane trafficking

Question 34

myosin II

Answer 34

can form large bundles of thick filaments. Myosin-II has two heads per molecule and forms bipolar filaments that can interact with multiple actin filaments simultaneously.

Question 35

dimeric myosins, eg myosin V

Answer 35

bind to cargo proteins, such as vesicles, and walk along actin filaments, transporting the cargo to different locations within the cell.

Question 36

mechanism of powerstroke

Answer 36

Actin-binding myosin head meets an actin microfilament and tightly associates with one monomer
Myosin head associates with ATP and undergoes a conformational change that weakens the association with the actin microfilament and it can detach
ATP undergoes hydrolysis which powers a conformational change causing the myosin head group to move to the next actin monomer
The ADP bound myosin can make contact with the next monomer along the microfilament and release the ADP so that the cycle can repeat

Question 37

non muscle myosin II

Answer 37

myosin with a long tail region and two head groups in the inactive form. When regulatory region in the neck is phosphorylated, causes a conformational change that extends the tail. Now the dimer can form a tetramer. Now have a double headed myosin structure
The bundles are antiparallel to one another so the movement of the myosin heads will slide the filaments relative to one another to generate contractile force
Drives cell division