The cytoskeleton Flashcards
Why does a cell need a cytoskeleton?
Cell needs cytoskeleton to keep shape and modify it in response to environmental cues (dynamic structure)
What are the 3 polymers that make up the cytoskeleton?
- Microtubules
- Intermediate filaments
- Actin filaments
What roles do the microtubules have within the cytoskeleton?
Involved in Organelle positioning and intracellular transport
What roles do the intermediate filaments have within the cytoskeleton?
Give cell Mechanical strength
What roles do the actin filaments have within the cytoskeleton?
Involved in keeping and changing cell shape; Organelle shape and cell migration
Why is the the cytoskeleton describes as dynamic?
Because polymers which make up cytoskeleton are constantly being reorganised into monomers and then re-formed back into polymers in different areas of cell in response to chemical signals
What is it that gives the cytoskeleton its dynamic structure?
Facilitated by its organisation - made up of polymers which are made up of monomers
What features of the monomers also facilitate the cytoskeleton’s dynamic structure?
Monomers very abundant within the cell
Monomers aren’t covalently linked (making it easier for them to be reorganised into polymers)
What are the regulatory processes that occur within the cytoskeleton?
- Site and rate of filament formation (nucleation)
- Polymerization / depolymerization
- Function
Key characteristics of actin filaments
Helical polymers made of Actin
Flexible, organised into 2-D networks and 3-D gels
Key characteristics of intermediate filaments
Heterogeneous group of filamentous proteins
Rope-like structure
Key characteristics of the microtubules
Hollow tubes made of Tubulin
Rigid, long straight
Structure of the Actin filaments
Twisted chain of units (monomers) of the protein G-actin (Globular – actin)
This chain constitutes the filamentous form (F-actin).
Thinnest class of the cytoskeleton filaments (7 nm)
Associated with a large number of actin-binding proteins (ABP)
Structural polarity of Actin filaments
Show structural polarity because the chemical reaction that causes monomers to be added to filament favours adding monomers to one of the ends (plus end) compared to other end (minus end)
What are the 3 isoforms of G actin?
Alpha actin
Beta actin
Gamma actin
What are the 2 features that determine the length of the actin filament?
- Concentration of G-actin.
2. Presence of Actin Binding proteins (ABPs)
What are the 2 actin binding proteins involved in Actin polymerisation?
- Profilin
2. Thymosin beta 4
What is the role of profilin in Actin polymerisation?
Profilin catalyses the exchange of actin-bound ADP to actin-bound ATP thus forming the profilin-ATP-actin complex
(ADP-actin complex doesn’t polymerise into actin filament very well while ATP-actin is readily polymerising)
This complex is then fed into actin filament via certain proteins where the profilin will release the ATP-actin complex into the actin filament
This allows the ATP-actin complex to bind to the actin filament
What is the role of Thymosin Beta 4 in Actin polymerisation?
prevents the addition of actin monomers to F-actin by binding to the G-actin monomers thus preventing profilin from binding to them. (This stops actin filament from constantly growing)
What are the 2 actin binding proteins involved in the organisation of the actin filaments?
- Actin bundling proteins
2. Cross-linking proteins
What role do Actin bundling proteins play in the organisation of actin filaments?
Percolate between the actin filaments and keep F-actin in parallel bundles
What role do Cross-linking proteins play in the organisation of actin filaments?
Maintain F-actin in a gel-like meshwork
What is the function of the F-actin severing proteins?
Break F-actin into smaller filaments
What is the advantage of breaking up F-actin into smaller filaments?
Increases the number of ends that actin can be removed from (depolymerised) as normally actin could only be removed from minus end). This speeds up process of actin depolymerisation