Cell Organisation And Movement

Question 1

Actin and its structure ?

Answer 1

Actin is highly conserved and abundant eukaryotic cell protein
Cells assemble diverse structures of actin filaments for different functions
G-actin reversible assembles into polarised F-actin filaments :
Composed of 2 protofilaments in which the actin subunits all oriented in same direction
Protofilaments are wound around each other to form a helix with the actin nucleotide biding site exposed on the - end of each

Question 2

Actin treading?

Answer 2

Actin filament assembly-disassembly at each end
-Rate of ATP g actin assembles is 10x faster as + end then -
Rate of ATP gactin disassembly similar at the 2 ends

Question 3

Actin treading at a steady rate ?

Answer 3

On the + lower Cc end is faster than actin ATP hydrolysis in the filament giving rise to a filament with a short region of ATP -actin and regions of adp- pi-actin and ADP actin towards the-ve end
- treadmilling filaments can do work in vivo

Question 4

Regulation of filament turnover by actin - binding properties?

Answer 4

Actin -binding proteins regulate the rate of assembly & disassembly of actin filaments as well as the availability of g-actin → polymerasation

Question 5

Profilin- cycle 1 by actin
Regulation of filament turnover by actin binding protein

Answer 5

Profiling binds to ADP - G actin opposite the nucleotide-binding cleft opening the cleft & catalysing the exchange of ADP for ATP.
-profilin binding sterically blocks ATP - G actin assembly on the filament (-) end but allows the unblocked g-actin monomer end to assemble onto the filament end (+ ).
- ATP - G actin - profilin complex assembly on the ( +) end dissociates profilin to interact with another ADP-a actin

Question 6

Cofflin- cycle 2

Answer 6

COFlin fragments ADP - actin filaments region enhancing overall depolymerisation by making more C-) filaments ends

Question 7

Rest is on the Book → make notes from there

Question 8

Functions of cytoskeletons?

Answer 8

3 main functions →
Microfilaments microtubules,

Question 9

Microfilaments

Answer 9

Actin binds to ATP ‘
Form rigid gels, network,linear bundles
Regulated assembly from large numbers of locations
Highly dynamic
Polarised
Tracks for myosin
Contractile machinery and network at the cell cortex

Question 10

Microtubules

Answer 10

Aß- Tublin binds to GTP
Rigid and not easily bent
Regulated assembly from small number of locations
Highly dynamic
Polarised
Tracks for kinesics and dyneins
Organised and long range transport of organelles

Question 11

Intermediate filaments

Answer 11

If subunits don’t bind a nucleotide
Great tensile strength
Assembled onto pre-existing filaments
Less dynamic
Unpolarised
No motors
Cell and tissue integrity

Question 12

Regulation of cytoskeleton function by cell signaling.

Answer 12

Essential to maintain homeostasis in normal physiology & adaptation to conditions but can be hacked by bacteria and pathogens
Like \ viruses

Question 13

Cytoskeleton disorders

Answer 13

S several cardiovascular diseases syndrome
Muscular dystrophy, cancer
Blistering skin diseases

Question 14

Micro filament based structure

Answer 14

Tensile structures
Architecture
→ microvili, filopodia, camellipodia
Movement
→cel division,endocytosis,phagocytosis,muscle contraction, cue migration, pathogenesis

Question 15

What is actin compromised of?

Answer 15

Actin compromises 10% of the total protein in muscle cells and 1-5% in other cell types

Question 16

Structure of G-actin?

Answer 16

Divided by a central cleft into 2 approximately equal sized lobes t four subdomains I, IV
ATP/ADP binds at the bottom of the cleft &contacts both lobes
Mg2+ is a necessary cofactor
- G actin monomer polymerises into long helical structure f-actin structure (microfilament)
Structure: filament is composed of 2 helically wound strands with repeating 28 subunits
Polarity: the filament end with an exposed binding cleft is the (-) end, the opposite end is the (+).

Question 17

What are the 3 phases of in vitro g-actin polymerisation?

Answer 17

Necessary conditions:actin monomersbuffered ATP, low strength cations including mg2+
Nucleation (lag) phase: formation 3ATP - G actin “nucleus” (more stable than 2 actin associations) because of extra bands- initiates formation of a filament.
- elongation phase:actin subunits rapid assemble on each end of a filament.
- steady state phase: g-actin monomers exchange with subunits at the filaments ends but no net change in the total length of filaments.

Question 18

Actin nucleation by the ARP 2/3 complex

Answer 18

ARP 2/3 activation & actin filament nuclea-tion
Step 1: 2NPF’s WH 2 domains each bind on actin monomer
Step 2: binding of 2 np F actin complexes bind to the ARP 213 complex induces a confrontational change that activates the A rp 2/3 complex
Step 3: The activated ARP 2/3 complex binds toside of existing actin filament e binds to the (-) ends of actin subunits transferred from the NPF w domains
Step 4: additional g-actin assemble onto the (+) end of new actin filament, making a 70 angle with old filament

Question 19

Example of actin polymensation for intracelluler movement

Answer 19

Listeria: food borne bacteria
Mild gastrointestinal symptoms in most adults but can be fatal to elderly simmunocompromised
Enters cell & divides in cyptoplasm.

Question 20

G

Question 21

Structure T ubulin dinners ‘

Answer 21

Composedof stably associatedhighly conserved in (euk) & structurally similar alpha - tubulin and ß- tubulin
- most eukaryotes have several genes encoding both dinners t additional genes encoding _ gamma - tubulin - subunit which is involved in Mt assembly
-a tubulin - GTP is never hydrolysed and non-exchangeable
B-tubulin - GDP is exchangeable with GTP, which can be hydrolysed in the site

Question 22

Tubular subunit organisation in a microtubule: forms a structurally polansed tubes
Structure

Answer 22

Dimers are aligned end-end in the same orientation into protofilaments
Protofilaments pack side by side with the same subunit polarity to form the wall of the microbial
Protofilaments are slightly staggered so that a-tubulin in 1protofilaments is in contact with a-tubulin except at the seam, where on a-su bun it contacts ß-subunit
Timer alignment provides structurally polarity to the MT
Subunit are added preferentially at the (+) end where b-tubulin monomer are exposed.

Question 23

Singlet microtobules

Answer 23

Cytoplasm
13 protofilaments - most cyptoptasmi MT’s

Question 24

Doublet microtubules

Answer 24

An additional wall of 10 protofilaments forms a 2nd tubule (B) in cilia I flagella outer doubles.

Question 25

Triplet microtubules

Answer 25

2 10 protofilament walls on the 13 - protofilament microtubule in centriole &basal body microtobule organising center’s

Question 26

How do organelles move along microtubules?

Answer 26

Kinesis & Dyneins:microtubule based proteins
Kinesin(+) end motor superfamily transports organelles, sliding anti-parallel microtubules past each other
Kinesin 1→ highly processive motor as it coordinates ATP hydrolysis through 2 heads so that 1 head is always firmly bound to a microtubule.
Cytoplasmic dyne in is (-) end motor associating with dynactin complex e cargo adaptors to transport cargo

Question 27

How kinesin-1 - catalysed reside trail transport?

Answer 27

Kinesin 1:
Attaches to a vesicle surface receptor
Transports vesicles from the (-) end to the (+) end of a stationary microtubule
ATP hydrolysis motors the run and can take thousands of step
1 foot is always attached

Question 28

What are inter mediate laments?

Answer 28

3rd major filament system of euk cells
1 - subunits are heterogenous
2- great tensile strength (hair trails)
3- No intrinsic polity like microfilament/tubules
4- subunits do not bind a nucleotide
5- No if motors are known
6- dynamic subunits exchange occurs, but if are more stable than microfilamento/ tubules because exchange rate is slower.

Question 29

What is the structure of intermediate filaments?

Answer 29

Are assembled from subunit dinners
If proteins
- conserved coiled - core domain
- subfamily specific globular heads
- form parallel dimers through coiled - coil core domains

Question 30

What are tetramer filament subunit?

Answer 30

Anti-parallel , staggered, side-by-side aggregation of 2 identical dimers

Question 31

What are protofibril?

Answer 31

End to end and laterally associated tetramers
Mature filaments: consisting of 4 protofibrils with the globular domains forming beaded clusters on the surface

Question 32

Vimentin and lamin A structures comparison

Answer 32

Lamin structures →camin protein has a nuclear localisation sequence that targets it to the nucleus

Question 33

How do blistering diseases work?

Answer 33

The 2 outer epidermis layer covering and connecting the soft inner dermal layer by desmosome junction mutant k14 keratin gene →weakening and death of cells at base of epidermis