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Cell Biology and Biochemistry > 11 The Cytoskeleton > Flashcards

11 The Cytoskeleton Flashcards

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1
Q

What are the 3 main functions of the cytoskeleton

A
  • to maintain cell shape
  • to provide the cell strength
  • to provide mechanisms which allow movement within the cell and movement of the cells themselves
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2
Q

name the 3 different types of cytoskeletal filaments

A
  • Actin filament
  • Microtubules
  • Intermediate filaments
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3
Q
  • Summarise Actin filaments and its role
A
  • double stranded, composed of globular actin protein subunits
  • regulates cell shape and locomotion (movement of cell)
  • Distinct at cell poles, cause formation of microvilli
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4
Q

Summarise Microtubules and their role

A
  • 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
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5
Q

Summarise Intermediate Filaments

A
  • many types of IF, composed of different protein subunits
  • provide mechanical strength
  • anchor across the cell into desmosomes
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6
Q

Describe the structure of Actin filaments

A
  • 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
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7
Q

Explains the process of actin filament nucleation/building

A
  • 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
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8
Q

What is the critical concentration when referring to formation of actin filaments

A
  • 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
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9
Q

What is the equation that determines the critical concentration

A 
Cc = Koff/Kon
Koff = rate of dissociation of G-actin from F-actin
Kon = rate of association of G-actin into F-actin
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10
Q

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

A
  • then G-actin will associate at one end and dissociate at the other
  • this is called treadmilling
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11
Q

Why is treadmilling important and useful

A
  • it is essential for cell motility and changing cell shape
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12
Q

What are actin motor proteins

A
  • 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
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13
Q

What is used to prevent treadmilling (binding and dissociating of G-actin monomers to F-actin)

A
  • Actin binding proteins can bind to the plus and minus end of F-actin preventing any association and dissociation
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14
Q

Describe the structure of Microtubules

A
  • 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
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15
Q

Why is the structure of a microtubule so strong

A
  • 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
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16
Q

Explain nucleation of microtubules

A
  • 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
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17
Q

Describe a centrosome

A
  • 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
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18
Q

What is a Y-tubulin ring Complex

A
  • This is the site of nucleation of microtubules

- It is made from multiple copies of Y-tubulin and other proteins

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19
Q

Describe the dynamics behind microtubules

A
  • 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
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20
Q

What 2 microtubule proteins are associated with transporting things about the cell

A
  • Kinesin and Dynein
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21
Q

In which direction do microtubule motor proteins move things within the cell

A
  • 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)
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22
Q

Describe the process behind how Kinesin carries cargo along the microtubule

A
  • 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
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23
Q

Describe the shape of Kinesin

A
  • Leading head and Lagging head
  • proteins linking the 2 heads form a neck
  • The neck binds to the cargo
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24
Q

What are the main features of Intermediate filaments

A
  • 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
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25
Q

Describe the hierarchy of structure in intermediate filaments

A
  • 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
26
Q

What forms the cytoskeleton

A
  • Actin filaments
  • microtubules
  • Intermediate filaments
27
Q

features of which actin and microtubules share

A
  • polar and highly dynamic
28
Q

where do microtubules nucleate

A
  • centrosomes
29
Q

Name the 3 types of junctions

A
  • anchoring junctions
  • occluding junctions
  • communicating junctions
30
Q

what is the job of an anchoring junction

A
  • to anchor the cytoskeleton between 2 cells or between cells and the extracellular matrix
31
Q

what is the job of an occluding junction

A
  • to prevent the passage of ions and small molecules between cells, typically tight junctions in vertebrates
32
Q

What is the job of communicating junctions

A
  • to directly connect the cytoplasm of 2 adjacent cells
33
Q

Describe the structure of anchoring junction

A
  • 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
34
Q
  • what 2 superfamilies do these transmembrane adhesion proteins that create anchoring junctions belong to
A
  • Cadherins

- Integrins

35
Q

whats the difference between cadherins and integrins

A
  • Cadherins are for Cell-Cell attachment

- Integrins are for Cell-extracellular matrix attachment

36
Q

What is known about the binding of cadherins

A
  • 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
37
Q

Describe the mechanism by which Cadherins become rigid in order to bind to other cadherin proteins

A
  • 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
38
Q

How are Cadherins bound to the cytoskeleton

A
  • 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
39
Q

How do Adheren junctions form from just a couple of cadherin proteins from adjacent cells binding

A
  • 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
40
Q

What is an adhesion belt?

A
  • The actin and myosin belt like feature that runs in the cell along the periphery where adheren junctions have formed
41
Q

how can the adhesion belt be used to alter tissue shape

A
  • When myosin pulls the F-actin in the adhesion belt the belt contracts
  • This causes a drawstring effect pinching off a sheet of tissue
42
Q

In which cells are desmosomes mostly found

A
  • in cells under a lot of mechanical stress like cardiac cells
43
Q

Describe where desmosomes are found and the structures present

A
  • 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
44
Q

what is the extra cellular matrix and and why is it important

A
  • its an intricate network of proteins carbs and water

- it provides support for cells, its an important regulator of cellular signalling through cell junctions

45
Q

What are the 3 major macromolecules in the extra cellular matrix

A
  • Glycosaminoglycans (GAGs)
  • Fibrous Proteins
  • Glycoproteins
46
Q

What are glycosaminoglycans

A
  • Oligosaccharides covalently bonded to a protein
47
Q

Name the 2 types of cell-Matrix junctions

A
  • Focal adhesions

- Hemidesmosomes

48
Q

What type of proteins do Cell-matrix junctions use

A
  • Integrins
49
Q

Describe the structure of an Integrin

A
  • 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
50
Q

Describe focal adhesions

A
  • 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
51
Q

Name a protein and its amino acid sequence that integrins bind to in focal adhesions

A
  • fibronectin

- special sequence of RGD

52
Q

Describe a hemidesmosome

A
  • 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
53
Q

Explain what occurs in the mechanism of integrin switching

A
  • 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
54
Q

When might integrins switch from active to inactive

A
  • When the cell wants to migrate to a different location
55
Q

What are occluding junctions and their purpose

A
  • 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
56
Q

How are tight junctions formed?

A
  • 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
57
Q

What is a communicating junction

A
  • Junctions which provide channels that can connect the cytoplasm of 2 adjacent cells
58
Q

Describe the structure of a communicating junction in a vertebrate

A
  • 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
59
Q

How do Gap/communicating junctions alter their permeability

A
  • 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
60
Q

What are the types of cell-cell junctions

A
  • Anchoring junctions (Adherens, desmosomes)
  • Occluding junctions (tight junctions)
  • Communicating junctions (gap junctions)
61
Q

What types of cell-matrix junctions are there

A
  • focal adhesions

- hemidesmosomes

Cell Biology and Biochemistry (9 decks)

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