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
What is the function of the motor proteins? e.g. Myosin
Transport of vesicles and/or organelles through actin filaments
What are the functions of the actin filaments within muscle cells?
Interaction with Myosin motors allow muscle contraction
What are the functions of the actin filaments in non-muscle cells?
- Form the cell cortex along with myosin and ABP’s - a thin sheath beneath the plasma membrane
- Help to maintain the structure of the microvilli
- Help form contractile bundles (help the cell squeeze into small spaces)
- Help form and maintain lamellipodia and filopodia which are formed during cell migration
- Form the contractile ring along with myosin
Cell migration
- Cell recognises a particular chemical/mechanical signal which requires the cell to move in order to respond to it
- The cell pushes out protrusions at its front (lamellipodia & filopodia) which forms the leading edge
- Actin polymerization occurs within the leading edge.
- Once Produced Actin filaments then organized into parallel bundles or mesh-like network
- These protrusions adhere to the surface
(Facilitated by Integrins which link the actin filaments to the extracellular matrix surrounding the cell) - Cell contraction and retraction of the rear part of the cell
(Facilitated by Interaction between actin filaments and myosin)
Structure of intermediate filaments
- Rope-like with many long strands twisted together and made up of different subunits.
- Intermediate size (8-12nm) between actin and microtubules.
- Toughest of the cytoskeletal filaments
- Form a network: Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes) and surrounding nucleus
Structure of the subunits of intermediate filaments
Each unit is made of:
N-terminal globular head
C-terminal globular tail
Central elongated rod-like domain
Intermediate filament polymerisation
2 intermediate filaments coil together to form stable dimers called coiled-coil dimers
Every 2 dimers associate forming a staggered, antiparallel tetramear
Tetramers bind to each other and twist to constitute a rope-like filament
Types of intermediate filament
Cytoplasmic and nuclear
What are the 3 types of Cytoplasmic intermediate filament?
- Keratins
- Vimentin and Vimentin-realted
- Neurofilaments
What are the types of nuclear intermediate filament?
Nuclear lamins
Name the different types of intermediate filament binding proteins
- Filaggrin (Filament aggregating protein)
- Synamin
- Plectin
- Plakin
What is the role of Filaggrin?
binds keratin filaments into bundles in epithelial cells
What roles do Synamin and Plectin have?
bind desmin and vimentin (Also Link IF to the other cytoskeleton compounds (i.e. actin and microtubules) as well as to cell-cell contact structures (desmosomes)
What is the role of Plakin?
Keep the contact between desmosomes of epithelial cells.
Functions of the intermediate filaments in the cytoplasm
Provide Tensile strength as well as Structural support by:
Creating a deformable 3D structural framework
Reinforcing cell shape and fix organelle localization
Functions of the intermediate filaments in the nucleus
Line the inner face of the nuclear envelope to strengthen it
Also, provide attachment sites for chromatin
Structure of microtubules
Hollow tubes made up from the protein tubulin
25nm thickest of the filaments
Each filament shows structural polarity
Dynamic structure
Polymerisation of the microtubules
Microtubule organizing centres (MTOC) are specialized protein complexes from where assembly of tubulin units starts.
Microtubule organizing centres nucleate and organize microtubules
MTOCs contain gamma- tubulin which combines with other associated proteins to form the -tubulin ring complex
This complex acts as a cap at the minus end of the microtubule while microtubule growth occurs at the plus end
Microtubule growth occurs as a result of tubulin dimers binding to GTP to form complexes
These complexes then assemble onto the plus end of the microtubule
Functions of microtubules
Intracellular transport- act like railway tracks on which molecular motors run
Organises position of organelles
What are the 2 motor proteins involved in transport along the microtubules?
Dynein - Transports things towards minus end of microtubule
Kinesin - Transports things towards the plus end of microtubule
