Cytoskeleton

Question 1

what is the cytoskeleton

Answer 1

network of protein filaments throughout cytoplasm, supports large volume of cytosol, dynamic and responsible for cell shape and movement

Question 2

functions

Answer 2

mitosis
cytokinesis
traffick
support
sperm to swim
muscle contarction
formation of axons/dendrites
cell shape

Question 3

cortical actin

Answer 3

actin filaments found concentrated close to plasma membrane

Question 4

three types of cytoskeletal filament

Answer 4

intermediate filaments
microtubules
actin filaments

Question 5

intermediate filaments

Answer 5

10nm diameter
provide tensile strength
abundant in cells subject to mechanical stress
form a network throughout the cytoplasm, surround nucleus
anchored at plasma membrane at cell junctions
eg keratins

Question 6

3 main classes of intermediate filaments

Answer 6

keratin filaments in epithelial cells
vimentin in connective tissue cells, muscle cells and supporting cells of the nervous tissue (neuroglial)
neurofilaments in nerve cells

Question 7

construction of intermediate filament

Answer 7

from smaller protein subunits
Monomers conisist of globular N (amine) and C (carboxyl) termini, and central rod domain- long alpha helical region in between
two monomers of intermediate filament associate to form a dimer by coiling their alpha helices around each other- coiled-coil conformation
Very strong interaction between monomers because large area in contact with each other
N is start and C end of PP chain

Question 8

what do two dimers align to form

Answer 8

staggered tetramer, N termini antiparallel

Question 9

what happens to tetramers

Answer 9

pack together end to end, N-termini of monomers in one dimer interact with C termini of adjacent monomers in another dimer

Question 10

next step

Answer 10

eight tetramers twisted into rope of diameter approx 10 nm

Question 11

keratins

Answer 11

span interior of epithelial cells

indirectly connected to filaments of other cells through cell-cell junctions (desosomes)

Question 12

desmosomes

Answer 12

cadherins directly make contact with cadherins from another cell
transmembrane proteins that span bilayer and interact with plaque proteins on cytosolic side of membranes
The plaque proteins interact with keratin
keratin-plaque-cadherin-cadherin-plaque-keratin
hence keratin cytoskeletons indirectly connected

Question 13

epidermolysis bullosa simplex

Answer 13

intermediate filament disorder

rare genetic disorder, keratin can’t form normal filaments so skin susceptible to mechanical injury

Question 14

why are cells with defective intermediate filaments more susceptible to mechanical stress

Answer 14

intermediate filaments provide strength and prevent cells rupturing, hold them together when sheet of cells stretched

Question 15

nuclear lamina

Answer 15

network of intermediate filaments inside nuclear membrane

give nucleus its shape

Question 16

how can you show how durable intermediate filaments are

Answer 16

treat with high salt or non-ionic detergent

remain intact

Question 17

actin filament diameter

Answer 17

7 nm

Question 18

monomers that make up actin

Answer 18

more globular
monomers (vs intermediate filament monomers which only have globular ends) associate head to tail
filaments are unstable without associated proteins

Question 19

monomers that form F actin filament

Answer 19

G actin

Question 20

Requirements for formation of F actin

Answer 20

presence of ATP, Mg and K
concentration of G actin must be sufficient for polymerisation
Below critical conc, actin filaments depolymerize

Question 21

Polymerisation of actin

Answer 21

Actin monomers in the cytosol carry ATP
The ATP is hydrolysed to ADP after assembly into filaments
The ADP bound monomer is less stable in the filament
ADP can’t be exchanged for ATP until the monomer disassembles

Question 22

why do actin filaments have polarity

Answer 22

actin monomers all have the same orientation in the filament

minus end has an exposed ATP binding cleft and plus end does not, has a dome

Question 23

What proteins bind to actin and modify its properties

Answer 23

monomer binding proteins
nucleating proteins
cross linking proteins
capping proteins
bundling proteins
motor proteins

Question 24

functions of actin

Answer 24

mechanical strength anc cell shape
cell crawling
muscle contraction
organelle movement

Question 25

Cortical actin

Answer 25

Actin filaments usually concentrated in a layer under the plasma membrane (cortex) linked into a meshwork by actin binding proteins Provide mechanical strength and cell shape

Question 26

cell crawling

Answer 26

filopodia or lammelipodia extend a region of the plasma membrane integrins adhere to extracellular matrix (substrate) cells use internal contractions to pull itself forwards Cytoskeleton underneath plasma membrane is deforming the membrane Will grab onto a suitable substrate via integrins

Question 27

filipodia

Answer 27

finger/needle like projections of the plasma membrane

Question 28

lammelipodia

Answer 28

sheet like projections of the plasma membrane

Question 29

cell crawling further explanation

Answer 29

actin polymerisation at front (plus end) of the cell leads to lamellipodium extension Attachment to extracellular matrix (via integrins) provides an anchor point contraction of the rear pulls the cell forward

Question 30

Motor proteins

Answer 30

Use ATP hydrolysis Myosin are motor proteins that can move along actin filaments Myosin can bind to and hydrolyze ATP This provides energy for their movement along the actin filament from minus end to the plus end

Question 31

movement of myosin along actin filaments

Answer 31

Myosin head lacks ATP at beginning of cycle and is locked onto actin When ATP binds the affinity for actin is reduced, myosin head released ATP hydrolysed causes head conformational change, more upright Then Pi leaves, confromational change and head binds to actin. Then ADP leaves and head conformational change, cocked forwards- power stroke

Question 32

What happens at steady state phase of graph

Answer 32

rate of assembly of new subunits same as rate of disassembly | reached critical concentration

Question 33

drugs that interfere with actin filament assembly

Answer 33

Cytochalasin D binds to plus end of F actin and prevents addition of further G actin Phalloidin from poisonous mushroom Amonita binds F actin and prevents actin filaments from depolymerising