The cytoskeleton Flashcards
What is the cytoskeleton? What are the 3 polymers it’s composed of?
- The cytoskeleton is the skeleton of the cell.
- It is a complex network made of 3 different polymers:-
- Microtubules
- Intermediate filaments
- Actin filaments
What is the function of the cytoskeleton?
The cytoskeleton provides for:—
- Shaping of the cell
- Intracellular movement of the organelles - (such as transportation of vesicles into and out of a cell, chromosome manipulation during mitosis and meiosis).
- Cell movement
What are the general features of microfilaments / actin filaments?
- Helical polymers made of actin.
- Flexible and an be organised into 2-D networks and 3-D gels
- Important as they keep maintain the cell shape and some organelles shapes
- They participate in cell migration
What are the general features of intermediate filaments?
- Made of heterogenous (similar) groups of filamentous proteins
- They have a rope like structure
- Give mechanical strength to the cell
What are the general features of microtubules?
- Hollow tubes made of tubulin
- They are rigid, long and straight
- They help in organelle positioning, Intracellular transport and cell movement.
What do the microfilaments / actin filaments provide to the cell?
They provide:-
- Cell shape
- Organelle shape
- Cell migration
What do the intermediate filaments provide to the cell?
They provide:-
- Mechanical strength
What do the microtubules provide to the cell?
They provide:-
- Organelle positioning
- Intracellular transport
Describe the structure of actin filaments.
[Actin:- It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous).]
- Actin filament consists of two twisted strands of monomers of the protein actin (G-actin, approx. 43KDa). This chain constitutes the filamentous form (F-actin).
- The actin filaments are the thinnest out of the three cytoskeleton filaments (7nm in diameter).
- They have structural polarity:- The chemical reaction favours that they add the monomers to one end and that end is called the plus end. The unfavourable end is called the minus end.
- The reaction is controlled by a big group of proteins called the actin binding proteins (ABP) - They help to decide: what structure it needs in the filament, wants to put filaments together and wants to depolarise them.
- There are 3 isoforms of G-actin with different isoelectric point:
- Alpha - actin is found mainly in muscle cells.
- Beta - actin and gamma - actin found in non-muscle cells.
How does actin get polymerised?
- Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.
What are the two factors that determine the length of the actin filament?
The length of the filament is determined by:-
- The concentration of G-actin.
- Presence of Actin Binding Proteins (ABPs).
What are the 2 ABPs that control the G-actin levels?
G- actin levels are controlled mainly by 2 ABPs:-
- Profilin: facilitates actin polymerisation
- Thymosin 4: prevents the addition of actin monomers to F-actin
What are the 4 proteins that bind to actin filaments?(ABP)
- Actin bundling proteins
- Cross-linking proteins
- F-actin serving proteins
- Mtor proteins (myosin)
What do Actin bundling proteins do?
They keep F-actin in parallel bundles (as in microvilli observed in epithelial cells).
What do cross-linking proteins do?
They maintain F-actin in a gel-like meshwork (as seen in the cell cortex, underneath the plasma membrane).
What do F-actin serving proteins do?
They break the F-actin into smaller filaments. You can only remove monomers from the minus end but if you cut them you end up having more ends where actin can be depolymerised.
What do Motor proteins (myosin) do?
They transport of vesicles and/or organelles through actin filaments. Microtubules are the main route to transport vesicles but when they reach the top of the plasma membrane they jump to actin filaments where the motor proteins helps to transport vesicles.
How are actin filaments arranged in the skeletal muscle and what is their function?
- They are arranged in a para-crystalline array integrated with different ABPs.
- Interaction with myosin motors allow muscle contraction.
What is the function of actin in non-muscle cells?
- Cell cortex: forms a thin sheath beneath the plasma membrane.
- Associated with myosin form a purse string ring result in cleavage of mitotic cells.
How does cell migration occur?
- The cell detects that it needs to go to a certain point.
- The cell pushes out protrusions at its front (lamellipodia and filopodia)
- Actin polymerisation occurs
- These protrusions adhere to the surface.
- Integrins- link the actin filaments to the extracelluar matrix surrounding the cell
- Cell contraction and retraction of the rear part of the cell
- Interaction between actin filaments and myosin occurs during this.
Describe the structure of intermediate filaments.
Each unit is made up of:-
- N-terminal globular head (amine).
- C-terminal globular tail (carboxyl).
- Central elongated rod like domain.
- Two filaments make a dimer.
- Two dimers make a tetramer.
- Multiple tetramers bind to each other and twist to make a rope-like filament unit.
- They are the toughest of cytoplasmic filaments (resistant to detergents, high salts, etc.)
- Its diameter is ~8 - 12 nm wide.
What are the types of intermediate filaments?
What do intermediate filament binding proteins do? (IFBP)
- They are mainly linkers of intermediate filament structures.
- IFBP stabilise and reinforce IF into 3D networks.
What are the functions of the 3 IFBP?
- Fillagrin :- Binds keratin filaments into bundles
-
Synamin and Pectin :- (i) Bind desman and vimentin.
(ii) Link IF to the other cytoskeleton acompounds (i.e. actin and microtubules) as well as to cell-cell contact structures (desmosomes). - Plakins :- Keep the contact between desmosomes of epithelial cells.
What are the functions of the IF in the cytoplasm?
- In the cytoplasm they provide:-
- tensile strength: this enables the cells to withstand mechanical stress (to stretch)
- Structural support by:
- Creating a deformable 3D structural framework
- Reinforcing cell shape and fixing organelle positions
What is the function of the IF in the nucleus?
- It forms a mesh-like structure.
- It lines the inner face of the nuclear envelope to:
- Strengthen it.
- Provides attachment sites for chromatin.
- It is able to disassemble and reform at each cell division as nuclear envelope.
Describe the structure of microtubules.
- They are hollow tubes made up from the protein tubulin.
- They are ~25nm in diameter and are relatively stiff.
- Each filament is polarized - meaning it has a positive and negative end.
- It is a dynamic structure :-
- Assemble and disassemble in response to cell needs.
- Tubulin in cell is roughly 50:50 as free or in filament.
- Assemble and disassemble in response to cell needs.
Describe the polymerisation of microtubules.
- Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts.
- The centrosome is the MTOC in most of the cells which contains gamma-tubulin ring that initiates the microtubule growth.
- Heterodimers of alpha and beta tubulin constitute the microtubule.
- It is a polarised growth (i.e. there is an end that grows faster (+end) than the other (- end).
What are the functions of microtubules?
- Intracellular transport :-
- They act like railway tracks on which molecular motors run
- Directionality of filaments is vital (each motor only moves in one direction)
- Organises position of organelles :-
- This causes polarisation of cells (The polarisation refers to differences in shape, structure and function).
- Rhythmic beating of cilia and flagella :-
- There are 9 microtubule pairs around 2 central microtubules (axoneme).
- Bending of cilia & flagella is driven by the motor protein Dynein.
What controls the assembly of the axoneme?
The basal body, at the base of the tubule, controls the assembly of the axoneme.
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