Cytoskeleton & Actin Flashcards
What are the THREE filament systems in the Cytoskeleton?
- Microfilaments (Actin)
- Microtubules
- Intermediate Filaments
Which motor protein is comprised of Microfilaments (Actin)?
Myosin
Which motor proteins are comprised of Microtubules?
- Kinesin
- Dynein
Does Actin bind ATP or GTP?
ATP
Does αβ-Tubulin bind ATP or GTP?
GTP
What is the atomic mass of G-Actin (Actin Monomer)?
43kDa
TRUE OR FALSE:
G-Actin is a highly conserved sequence
True
How many Actin genes are present in humans?
6
* 4 muscle
* 2 non-muscle
What is the structure of G-Actin?
- 4 subunits
- ATP binding cleft on one side
What is the structure of Actin Filaments (F-Actin)?
2 strands of subunits twisted into a helical filament
How many subunits do helical repeats have and what is its length?
- 28 subunits
- Spans 72nm
TRUE OR FALSE:
ATP-binding clefts on F-Actin point in opposite directions?
False.
They all point in the SAME direction
Which end of F-Actin is the ATP-binding cleft?
(-) end
TRUE OR FALSE:
G-Actin can self assemble?
True.
Self assemble in the presence of MG++ and physiological ionic strength
How many G-Actin molecules are needed to “seed” or nucleate actin polymerisation?
3
Which end does Actin polymerisation occur at?
Both ends
Which end of Actin is ATP-Actin addition fastest?
(+) end
Why does ATP-Actin addition occur faster at the (+) end?
Critical concentration is LOWER for the (+) end
What is Threadmilling in regards to Actin?
Net addition of actin at (+) end and loss at (-) end
How is Threadmilling powered?
ATP Hydrolysis
* After addition to (+) end, ATP is hydrolysed to ADP + Pi
Which end of the Actin filament has ATP-Actin and which end has ADP-Actin?
- (+) end has ATP-Actin
- (-) end has ADP-Actin
What is the function of Monomer Binding Proteins in Actin Filaments?
Binds soluble actin and either enhances or decreases its availability to form filaments
What is the function of Filament Stabilising or Destabilising Proteins in Actin Filaments?
Regulates filament stability
What is the function of Capping Proteins in Actin Filaments?
Bind to the ends of actin filaments and block assembly / disassembly
What are TWO examples of Capping Proteins in Actin Filaments?
- CapZ binds (+) end
- Tropomodulin binds (-) end
What is the function of Actin Filament Nucleating Proteins in Actin Filaments?
Catalyse the formation of new filaments
How many Myosin genes are there in the human genome?
40
Which Myosin protein forms bipolar filaments?
Myosin II
What is the function of Formins in Actin Filament assembly?
Assembly of unbranched filaments
What is the function of Arp2/3 complexes in Actin Filament assembly?
Assembles highly branched filament networks
How do Formins work?
- Formin dimer nucleates filament assembly by binding two actin monomers
- Remains bound to the (+) end (preventing CapZ binding) even as futher monomers are added
How many subunits does Actin Related Proteins 2 & 3 (Arp 2&3) have?
Seven subunits
How does the Actin Related Proteins 2 & 3 (Arp 2&3) work?
- Arp 2/3 binds to the sides of existing actin filaments
- Upon activation, Arp2/3 adopts a conformation that resembles the (+) end of an actin filament
- Acts as a nucleus for filament assembly
- (-) end remains attached to Arp 2/3
- (+) end grows at 70° angle
Where does cell migration occur?
- Wound healing
- Embryonic development
- Cancer mestastasis
What are the FOUR stages in cell migration?
- Extension
- Adhesion
- Translocation
- De-adhesion and Endocytic Recycling
What are Lamellipodia?
Sheet like protrusions
What are Filopodia?
Spiky / finger like protrusions
What happens at the Leading Edge in cell migration?
Actin polymerisation mediated by Arp 2/3 that pushes the plasma membrane forward
What THREE small GTPases play a key role in the regulation of the actin cytoskeleton?
- Rho
- Rac
- Cdc42
