Cytoskeleton, cell adhesion and motility

Question 1

what is the cytoskeleton

Answer 1

a network of protein filaments that extend throughout the cell

Question 2

what does the cytoskeleton do

Answer 2

determines cell shape, polarity, cell movement - extra and intracellular, adhesion and tissue structure

Question 3

what does the cytoskeleton comprise of

Answer 3

actin (microfilaments) - 7nm
intermediate filaments - 10nm
microtubules-20nm

Question 4

what are actin filaments called

Answer 4

F-actin (filamentous actin or microfilaments)

Question 5

what are actin filaments

Answer 5

polymers of individual actin proteins called G proteins

Question 6

how much of total protein does actin consist of

Answer 6

5%

it is the most abundant protein in all cells

Question 7

what is the structure of an actin filament

Answer 7

polarised double helix

- 13 actin subunits for every complete turn of the helix -37nm

Question 8

what gives the actin filament polarity

Answer 8

asymmetric shape if an actin monomer gives polarity to the filament + and - ends

Question 9

How do actin filaments grow?

Answer 9

• requires ATP to be bound to the actin monomer (G actin)
• filaments are very dynamic
• monomers can be added and removed from both ends of the filament
• monomers add much more rapidly to the + end of the filament
• once incorporated into the filament the ATP is hydrolysed into ADP

Question 10

where do actin filaments turn over rapidly

Answer 10

cell migration

• neural crest cells
• wound closure

Question 11

major functions of actin

Answer 11

1. Mechanical support
2. cell shape changes and maintenance
3. cell motility

Question 12

actin in cochlea in the ear

Answer 12

• held rigid as actin fills it

- not dynamic but static

Question 13

actin in RBC

Answer 13

RBC have concave shape as actin binds to spectrin and creates tension to pull in centre of RBC to inc SA

Question 14

what type of actin proteins are there

Answer 14

• membrane attachment proteins
• severing proteins
• cross linking proteins
• capping proteins
• side binding proteins
• motor proteins
• actin bundling proteins
• actin sequestering proteins

Question 15

structure of intermediate filaments

Answer 15

• rope like polymers of individual intermediate filament proteins
• 10nm diameter
• heterogenous family of proteins

Question 16

what is the function of intermediate filaments

Answer 16

some functions rather obscure
mainly mechanical
- used to anchor cells at some cell junctions
- support nuclear structure and protects chromatin

Question 17

formation of the intermediate filament polymer

Answer 17

1. intermediate filament protein
2. helical dimer
3. two dimers combine and form a tetramer (the fundamental unit of the IF)
4. tetramers link in a staggered formation and end - to - end to form the filament

Question 18

are intermediate filaments very dynamic

Answer 18

• grow slowly and don’t change much

- likely to be involved in static processes e.g. maintaining shape

Question 19

microtubules structure

Answer 19

• long, relatively stiff hollow tubes

- approx. 20-25 nm dia

Question 20

what is microtubule polymer made up from

Answer 20

• polymer built from monomers of tubulin
• 13 monomers seen in each cross section
• tubulin monomer contains of one molecule of alpha tubulin and one of beta

Question 21

why do microtubules have polarity

Answer 21

made from asymmetrical monomer
- one end stops with alpha (-
)
and one with beta (+)

Question 22

assembly and disassembly of the microtubule

Answer 22

GTP rich end (+ve end) can extend and collapse within seconds

• most growth occurs at + end and also most monomers are lost from this end
• • end much less dynamic
• Gtp bound monomers assemble onto filament
• GDP bound monomers dissociate rapidly

Question 23

how are microtubules polymerised in centrosomes in humans

Answer 23

• minus end remains close to centrosome

- plus end points towards the cell periphery

Question 24

cell shape and orientation

IF and microtubules

Answer 24

IF - stabilise shape of axons

MT - stabilise shape of platelets and axons

Question 25

microtubules - anchoring organelles

Answer 25

microtubules organise the ER of the cell, they hold together ER and their lipid channels

Question 26

IF - anchoring organelles

Answer 26

IFs form a meshwork around the cell nucleus to hold it into position

Question 27

actin filaments - anchoring organelles

Answer 27

required to hold synaptic vesicles close to the presynaptic membrane
- they are tethered to the actin cytoskeleton

Question 28

cytoskeleton - anchoring cells

Answer 28

essential to anchor cells to eachother and to extracellular matrix at cell junctions

Question 29

cell motility - actin based movements example

Answer 29

e.g. migration of neutrophils to site of infection by chemotaxis

Question 30

actin based cell movement - four steps

Answer 30

1. cell pushes out protrusions at the leading edge. Actin filament polymerisation provides the force for membrane protrusion
2. protrusions adhere to the surface on which the cell is moving through focal adhesions. F- actin connects to the focal adhesions to provide a contractile force for the cell
3. the rest of the cell pulls against the anchorage points to drag itself forward
4. actin depolymerises at the rear

Question 31

microtubule based movements - cilia and flagellae

Answer 31

• movement is initiated by the microtubule associated protein - dynein - a minus end directed protein
• microtubules slide along one another cause the cilium to bend

Question 32

movement if intracellular contents and organelles - microtubules

Answer 32

• movement of organelles e.g. synaptic vesicles along axons to synapse
• two motor proteins kinesin and dynein move cargo like vesicles along microtubules
• kinesin moves towards + end (cell periphery)
• dynein moves towards - end (near nucleus)