Cell Organisation And Movement Flashcards
Actin and its structure ?
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
Actin treading?
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
Actin treading at a steady rate ?
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
Regulation of filament turnover by actin - binding properties?
Actin -binding proteins regulate the rate of assembly & disassembly of actin filaments as well as the availability of g-actin → polymerasation
Profilin- cycle 1 by actin
Regulation of filament turnover by actin binding protein
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
Cofflin- cycle 2
COFlin fragments ADP - actin filaments region enhancing overall depolymerisation by making more C-) filaments ends
Rest is on the Book → make notes from there
Functions of cytoskeletons?
3 main functions →
Microfilaments microtubules,
Microfilaments
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
Microtubules
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
Intermediate filaments
If subunits don’t bind a nucleotide
Great tensile strength
Assembled onto pre-existing filaments
Less dynamic
Unpolarised
No motors
Cell and tissue integrity
Regulation of cytoskeleton function by cell signaling.
Essential to maintain homeostasis in normal physiology & adaptation to conditions but can be hacked by bacteria and pathogens
Like \ viruses
Cytoskeleton disorders
S several cardiovascular diseases syndrome
Muscular dystrophy, cancer
Blistering skin diseases
Micro filament based structure
Tensile structures
Architecture
→ microvili, filopodia, camellipodia
Movement
→cel division,endocytosis,phagocytosis,muscle contraction, cue migration, pathogenesis
What is actin compromised of?
Actin compromises 10% of the total protein in muscle cells and 1-5% in other cell types
Structure of G-actin?
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 (+).
What are the 3 phases of in vitro g-actin polymerisation?
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.
Actin nucleation by the ARP 2/3 complex
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
Example of actin polymensation for intracelluler movement
Listeria: food borne bacteria
Mild gastrointestinal symptoms in most adults but can be fatal to elderly simmunocompromised
Enters cell & divides in cyptoplasm.
G
Structure T ubulin dinners ‘
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
Tubular subunit organisation in a microtubule: forms a structurally polansed tubes
Structure
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.
Singlet microtobules
Cytoplasm
13 protofilaments - most cyptoptasmi MT’s
Doublet microtubules
An additional wall of 10 protofilaments forms a 2nd tubule (B) in cilia I flagella outer doubles.
Triplet microtubules
2 10 protofilament walls on the 13 - protofilament microtubule in centriole &basal body microtobule organising center’s
How do organelles move along microtubules?
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
How kinesin-1 - catalysed reside trail transport?
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
What are inter mediate laments?
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.
What is the structure of intermediate filaments?
Are assembled from subunit dinners
If proteins
- conserved coiled - core domain
- subfamily specific globular heads
- form parallel dimers through coiled - coil core domains
What are tetramer filament subunit?
Anti-parallel , staggered, side-by-side aggregation of 2 identical dimers
What are protofibril?
End to end and laterally associated tetramers
Mature filaments: consisting of 4 protofibrils with the globular domains forming beaded clusters on the surface
Vimentin and lamin A structures comparison
Lamin structures →camin protein has a nuclear localisation sequence that targets it to the nucleus
How do blistering diseases work?
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
