11 The Cytoskeleton Flashcards
What are the 3 main functions of the cytoskeleton
- to maintain cell shape
- to provide the cell strength
- to provide mechanisms which allow movement within the cell and movement of the cells themselves
name the 3 different types of cytoskeletal filaments
- Actin filament
- Microtubules
- Intermediate filaments
- Summarise Actin filaments and its role
- double stranded, composed of globular actin protein subunits
- regulates cell shape and locomotion (movement of cell)
- Distinct at cell poles, cause formation of microvilli
Summarise Microtubules and their role
- long hollow cylinders, composed of globular protein tubulin
- regulate cell transport (movement of things within the cell)
- Run from apical to basal cell pols providing a transport network
Summarise Intermediate Filaments
- many types of IF, composed of different protein subunits
- provide mechanical strength
- anchor across the cell into desmosomes
Describe the structure of Actin filaments
- Composed of a series of monomers called globular actin (G-actin)
- each globular actin has a large cleft where ATP binds
- 2 polymer chains of globular actin twist around each other in an a-helix structure
- monomers have a plus end and a minus end, which all point in the same direction giving polarity
Explains the process of actin filament nucleation/building
- 2 ATP bound G-actin monomers bind to each other weakly
- the addition of a third ATP-G-actin improves stability of the structure forming an oligomer.
- The oligomer acts as a platform or nucleus for more subunits to join
What is the critical concentration when referring to formation of actin filaments
- critical concentration is when the concentration of G-actin monomers and F-actin are in equilibrium (are not changing)
- This is because the rate of G-actin dissociation and association is equal
What is the equation that determines the critical concentration
Cc = Koff/Kon Koff = rate of dissociation of G-actin from F-actin Kon = rate of association of G-actin into F-actin
What happens if the Critical concentration is a different concentration at the plus end to the positive end and the G-actin conc. is between these 2 values
- then G-actin will associate at one end and dissociate at the other
- this is called treadmilling
Why is treadmilling important and useful
- it is essential for cell motility and changing cell shape
What are actin motor proteins
- This is when F-actin forms interactions with myosin motor proteins
- Myosin binds to F-actin and hydrolyses ATP, releasing energy to pull the F-actin along, the Myosin stays stationary and then binds further down the F-actin at the next binding site
- Like pulling on rope
What is used to prevent treadmilling (binding and dissociating of G-actin monomers to F-actin)
- Actin binding proteins can bind to the plus and minus end of F-actin preventing any association and dissociation
Describe the structure of Microtubules
- Largest cytoskeleton filament
- made of 13 repeating protofilaments longitudinally parallel
- these 13 protofilaments make a hollow tube structure with a lumen
- Each protofilament is made of repeating alpha-beta tubulin heterodimers
- Each heterodimer has a negatively charged alpha tubulin and a positively charged beta tubulin
Why is the structure of a microtubule so strong
- strong forces of interactions between lateral alpha-alpha tubulin and beta-beta tubulin
- and also the vertical interactions between alpha-beta tubulins
- the overall helical lattice structure
Explain nucleation of microtubules
- microtubules nucleate from the microtubule organising centre (MTOC)
- the MTOC is also called the centrosome in interphase
- the negative end of the microtubule is anchored to the centrosome
Describe a centrosome
- Made of 2 distinct features: A pair of centrioles, and perinuclear material
- centrioles are organised structures consisting of 9 sets of triplet microtubules
- centrioles are oriented at 90 degrees to each other
perinuclear material is amorphous material containing Y-tubulin ring complexes
What is a Y-tubulin ring Complex
- This is the site of nucleation of microtubules
- It is made from multiple copies of Y-tubulin and other proteins
Describe the dynamics behind microtubules
- Short life span of 10 mins
- Similar to F-actin, Critical conc. is lower at the positive end so grows longer and shrinks at the negative end (treadmilling)
- alpha tubulin binds at the positive end with GTP, if no GTP binds or the GTP dissociates then the microtubule becomes unstable and fully dissociates
- If GTP binds again stability is restored and the microtubule can begin growing again
What 2 microtubule proteins are associated with transporting things about the cell
- Kinesin and Dynein
In which direction do microtubule motor proteins move things within the cell
- Kinesin motors/transports things along the microtubule towards the positive end (anterograde)
- Dynein moves things along the microtubule towards the negative end towards the centre (retrograde)
Describe the process behind how Kinesin carries cargo along the microtubule
- Kinesin is anterograde, moves towards positive end of microtubule
- ‘Hand over hand’ mechanism
- The lagging head is ATP bound to the microtubule and is strongly dissociated, hydrolysis of this ATP causes the lagging head to dissociate from the microtubule
- The leading head binds to ATP which causes a conformational change on shape of kinesin in the neck, This causes the lagging head to step forward in front
- this repeats
Describe the shape of Kinesin
- Leading head and Lagging head
- proteins linking the 2 heads form a neck
- The neck binds to the cargo
What are the main features of Intermediate filaments
- Provide much greater tensile strength than F-actin and microtubules
- Non-polar
- No associated motor proteins as they are not involved in cell movement or transport
- made from many types of proteins
Describe the hierarchy of structure in intermediate filaments
- A helical monomer
- 2 coiled monomers form a dimer
- 2 staggered dimers form a staggered tetramer
- 8 tetramers laterally associated make a short thick unit of the filament
What forms the cytoskeleton
- Actin filaments
- microtubules
- Intermediate filaments
features of which actin and microtubules share
- polar and highly dynamic
where do microtubules nucleate
- centrosomes
Name the 3 types of junctions
- anchoring junctions
- occluding junctions
- communicating junctions
what is the job of an anchoring junction
- to anchor the cytoskeleton between 2 cells or between cells and the extracellular matrix
what is the job of an occluding junction
- to prevent the passage of ions and small molecules between cells, typically tight junctions in vertebrates
What is the job of communicating junctions
- to directly connect the cytoplasm of 2 adjacent cells
Describe the structure of anchoring junction
- these are huge transmembrane adhesion proteins
- the span across the cell membrane and bind 2 cells together or a cell to the extracellular matrix
- One end is bound to the cytoskeleton of the cell
- the other end is bound to another cell anchoring junction or the matrix
- what 2 superfamilies do these transmembrane adhesion proteins that create anchoring junctions belong to
- Cadherins
- Integrins
whats the difference between cadherins and integrins
- Cadherins are for Cell-Cell attachment
- Integrins are for Cell-extracellular matrix attachment
What is known about the binding of cadherins
- C-terminus binds to the cytoskeleton
- N-terminus binds to another cadherin
- Cadherins only bind to other of the same types of cadherins
- This is called homophilic bonding
Describe the mechanism by which Cadherins become rigid in order to bind to other cadherin proteins
- flacid cadherins are no use for binding
- Ca2+ ions attach at the hinge regions of cadherins, making the cadherins stick out and rigid
- This means that homophilic bonding between 2 cadherins knobs and pockets is more likely
How are Cadherins bound to the cytoskeleton
- Cadherins are transmembrane proteins so they pass into the cell
- adaptor proteins assemble on the C terminus of the cadherin and mediate the binding to F-actin
- Cadherin - adaptor protein- F-actin
How do Adheren junctions form from just a couple of cadherin proteins from adjacent cells binding
- Actin and cadherin recruitment expands the junction, meaning more cadherins begin binding from each cell making the junction larger and stronger
- Actin is then remodelled and myosin recruitment expands the junction further until it is large enough to be called an adhesion belt
What is an adhesion belt?
- The actin and myosin belt like feature that runs in the cell along the periphery where adheren junctions have formed
how can the adhesion belt be used to alter tissue shape
- When myosin pulls the F-actin in the adhesion belt the belt contracts
- This causes a drawstring effect pinching off a sheet of tissue
In which cells are desmosomes mostly found
- in cells under a lot of mechanical stress like cardiac cells
Describe where desmosomes are found and the structures present
- Desmosomes is a large overall structure where 2 cells are linked via non-classical cadherins
- The cadherins are connected to a dense plaque of adaptor proteins in the cell which are connected to intermediate filaments
what is the extra cellular matrix and and why is it important
- its an intricate network of proteins carbs and water
- it provides support for cells, its an important regulator of cellular signalling through cell junctions
What are the 3 major macromolecules in the extra cellular matrix
- Glycosaminoglycans (GAGs)
- Fibrous Proteins
- Glycoproteins
What are glycosaminoglycans
- Oligosaccharides covalently bonded to a protein
Name the 2 types of cell-Matrix junctions
- Focal adhesions
- Hemidesmosomes
What type of proteins do Cell-matrix junctions use
- Integrins
Describe the structure of an Integrin
- Composed of 2 glycoprotein subunits
- Both alpha and beta subunits are transmembrane
- Both subunits have a small intracellular C-terminus and large extracellular N terminus
Describe focal adhesions
- the part of the integrin outside the cell binds to specific amino acid sequences in extracellular matrix proteins
- The intracellular domain of integrins bind to adaptor proteins (Talin and Vinculin) which link to F-actin
Name a protein and its amino acid sequence that integrins bind to in focal adhesions
- fibronectin
- special sequence of RGD
Describe a hemidesmosome
- A junction which connects the ECM to intermediate filaments of a cell
- use integrins like focal adhesions
- rely on specific integrins called alpa6,beta4 integrins
Explain what occurs in the mechanism of integrin switching
- Integrins switch between their active and inactive state to unbind the cells from the ECM
- when inactive: the outside N-terminals fold together so they cant bind to matrix proteins, while the cytoplasmic tails become hooked to prevent cytoskeletal binding
- When active: cytoplasmic tails unhook to expose themselves to adaptor proteins, while the N terminals unfold and extend to bind to the ECM
When might integrins switch from active to inactive
- When the cell wants to migrate to a different location
What are occluding junctions and their purpose
- these a tight junctions which seal the gaps between epithelial cells like cement between bricks
- They ensure molecules that enter at the cells apical side and leave at the basal side don’t diffuse back where they came from
- They also stop proteins in the membrane from drifting too much
How are tight junctions formed?
- Tight junctions are formed from sealing strands
- sealing strands are thick rows of transmembrane homophilic adhesion proteins (Claudins and occludins)
- Claudins are essential and the main protein
- Occludins are non-essential but help reduce permeability of the tight junction
What is a communicating junction
- Junctions which provide channels that can connect the cytoplasm of 2 adjacent cells
Describe the structure of a communicating junction in a vertebrate
- Connexins are 4 pass transmembrane proteins
- 6 connexins form a connexon
- 2 connexons on adjacent cells connect in parallel when in close enough proximity to form a gap/communicating junction
How do Gap/communicating junctions alter their permeability
- Different connexons regulate different permeability
- Gap junctions are only permeable to small molecules and are gated
- They can be sensitive to voltage, pH, [Ca2+], neurotransmitter, etc
What are the types of cell-cell junctions
- Anchoring junctions (Adherens, desmosomes)
- Occluding junctions (tight junctions)
- Communicating junctions (gap junctions)
What types of cell-matrix junctions are there
- focal adhesions
- hemidesmosomes
