The cytockeleton Flashcards
Function of cytoskeleton
• Shape of cell
• Intracellular movement/location of organelles
• Modify cells in response to environmental cues
Cell movement
Contents of cytoskeleton
- Actin filament
- Intermediate filaments
Micro tubules
What does the cytoskeleton provide for?
- Shaping of the cell
- Intracellular movement of organelles
Cell movement
Microtubules associated with the cytoskeleton
Organelle positioning
Intracellular transport
Intermediate filaments associated with microtubules
Mechanical strength
Actin filaments associated with microtubules
Cell shape Organelle shape Cell migration Involved in cell migration Helical polymers Flexible structure: 2D networks, 3D gels
How does the cytoskeleton work
• Cytoskeleton is dynamic
• The various filaments are made of monomers that continually polymerise and depolymerise
1. The cell receives a signal via receptors on the cell membrane
2. Existing filaments in the cell depolymerise to form free monomers
3. The monomers rapidly diffuse
4. The monomers reassemble at a new site
• Accessory protein regulate:
○ Nucleation: The site and rate of filament formation
○De/Polymerisation
○ Function
Structure of actin filaments
○ Twisted chains of globular actin
○ When the G actin join together to form a filament, it is called F-actin
○ Thinnest form of cytoskeleton filaments(7nm)
○ Structural polarity
○ Large number of ABP(actin binding protein) in F-actin
Three isomers of actin
○ α-actin found mainly in muscle cells
○ β-actin in non-muscle cells
γ-actin in non-muscle cells
Polymerization of actin
○ G-actin added at either end of the chain
○ Length is determined by concentration of G actin or presence of ABP
Growth is faster at the barbed length
ABP
Protein binding to monomers
○ They determine if G-actin joins together onto polymer chain
○ Two types:
§ Profilin: facilitates actin polymerization
Thymosin: prevents the addition of actin monomers to F actin.
Proteins that bind to filaments
Actin bundling proteins: F-actin in parallel bundles
Cross linking proteins: F actin maintained in a gel like mesh work
F actin severing proteins: break F actin into smaller filaments
Motor proteins: transport of organelles and vesicles along actin filament
Muscle
Interaction with Myosin allows for muscle contractions
Arranged in a paracrystalline structure and integrated with different ABP
Non-muscle
Cell cortex: thin sheet under the plasma membrane
With myosin: cleavage of mitotic cells(D) (cytokinesis)
Cell migration
- The cell pushes out protrusions at its front
- Actin polymerization
- These protrusions adhere to the surface
- Integrins( link the actin filaments to the extracellular matrix surrounding the cell
- Cell contraction and refraction of the rear part of the cell
Interaction between actin filaments and myosin.
Structure of intermediate filaments
○ Each unit is made up of:
§ N terminal globular head
§ C terminal globular tail
§ Central elongated rod like domain
○ Units form stable dimers
○ Every 2 dimers form a staggered tetramear
○ Twists around each other to form rope structure
list the IFBP
○ Fillagrin: Binds to Keratin filaments into bundles
○ Synamin and plectin: bind desmind and vimentin. This links IF to other compounds like actin and microtubules
Plakins: Keep contact between desmosomes of epithelial cells
Function of intermediate filament in nucleus
○ Present in all nucleated eukaryotic cells
○ Form mesh structure
○ They line the inner face of nuclear envelope to strengthen it provide attachment site for chromatin
○ Disassemble and reform at each cell division as nuclear envelope disintegrates:
§ Very different from the stable cytoplasmic intermediate filaments
Process controlled by post transcriptional modifications
Structure of Microtubules
○ Hollow tubes made up of a protein called tubulin
○ Stiff and thick
○ Ach filament polarized
○ Dynamic:
§ (Dis)assembles in response to cell needs
Polymerization of microtubules
a. Microtubules organising centre: specialised protein complexes where microtubule assembly starts
b. Centrosome is the MTOC of most cells
i. Contains g-tubulin ring that initiates the microtubule growth
c. Heterodimers of a and b tubulin constitute the microtubule
d. It is polarized growth
Functions of microtubules
· intracellular transport
· act like railway tracks on which molecular motors run
· different motors for different cargoes
· directionality of filaments is vital (each motor only moves in one direction)
· organises position of organelles
· hence, provides polarisation of cells
· directionality of filaments is vital
