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Molecular and Cellular Biology (MCB) > MCB 9: Cell shape, Behaviour and Adhesion (Part I) > Flashcards

MCB 9: Cell shape, Behaviour and Adhesion (Part I) Flashcards

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1
Q

What are the three main components of the cytoskeleton?

A
  • microtubules
  • intermediate filaments
  • microfilaments (actin filaments)
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2
Q

What cellular processes do the cytoskeleton play a key role in?

A
  • cell motility (e.g. crawling, swimming)
  • cell shape
  • cell adhesion
  • cell contraction (e.g. muscle)
  • intracellular organelle and vesicle transport (also chromosome movement in cell division)
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3
Q

Which cytoskeleton component is which?

A
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4
Q

How is the cytoskeleton highly dynamic?

A
  • it is made up of soluble subunits that polymerise to form longer protein filaments
  • it responds to extracellular or intracellular stimuli in order to assemble or disassemble
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5
Q

Describe the properties of the cytoskeletal filaments below and how this affects their stability

  • a linear string of subunits (a single protofilament)
  • multiple protofilaments
  • long linear subunits (e.g. intermediate filaments) with lateral bonds
A

A linear string of subunits (a single protofilament):

  • only one bond has to be broken
  • not as stable

Multiple protofilaments:

  • more stable as you have to break multiple bonds

Long linear subunits:

  • there is end-to-end and lateral bonding between staggered filaments
  • very strong and high tensile strength
  • has rope-like properties
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6
Q

What does this graph show us about the mechanical properties of the different cytoskeletal filaments?

A

Microtubules:

  • readily deform (bend) but break under minimal force

Actin filaments:

  • resistant to deformation
  • break under moderate force

Intermediate filaments:

  • deform readily with increasing forces
  • resist breaking
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7
Q

Are cytoskeletal filaments polar? Why or why not?

A
  • they are polar
  • one end is the plus end and the other end is minus-end
  • this is because the protein subunits themselves have different ends
  • when they polymerise, these ends can change shape
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8
Q

How does the polarity of cytoskeletal filaments affect the rate of subunit addition?

A
  • the rate of subunit addition at the plus end is faster
  • it is slower on the minus end
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9
Q

Describe cytoskeleton polymerisation and depolymerisation due to nucleotide binding and hydrolysis

A

Polymerisation:

  • microtubules and microfilaments can only be polymerised by triphosphate-bound monomers
  • diphosphate monomers must exchange the diphosphate for a triphosphate in order to be capable of polymerising
  • an NTP (nucleotide triphosphate) cap is formed during elongation which is stable

Depolymerisation:

  • with time, the subunit’s own NTPase activity converts cap subunits to NDP (nucleotide diphosphate) forms which are less stable
  • then a shortening phase occurs, where subunits ate lost from the less stable NDP end
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10
Q

Describe the biochemical properties below of microtubules

  • subunit composition
  • polymer filament polarity
  • subunit nucleotide binding
  • enzyme activity of subunits in filaments
A
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11
Q

Describe the biochemical properties below of microfilaments (F-actin):

  • subunit composition
  • polymer filament polarity
  • subunit nucleotide binding
  • enzyme activity of subunits in filaments
A
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12
Q

Describe the biochemical properties below of intermediate filaments:

  • subunit composition
  • polymer filament polarity
  • subunit nucleotide binding
  • enzyme activity of subunits in filaments
A
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13
Q

When does the cytoskeleton shrink?

A
  • when the loss of subunits on one end is greater than the rate of addition on the other end
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14
Q

When does the cytoskeleton elongate?

A
  • when the rate of addition of subunits on one end is greater than the rate of loss at the other end
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15
Q

When does the cytoskeleton ‘treadmill’?

A
  • when the rate of loss of subunits on one end is the same as the rate of addition on the opposite end
  • net ‘displacement’ of exerting force
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16
Q

Describe how polymerisation of cytoskeletal subunits is enhanced by ‘seeding’ with pre-formed filaments

A

First graph:

  • when you have soluble subunits in solution, they don’t initiate polymerisation easily, so there is a long lag phase
  • once oligomers (a short polymerised group) are produced, there is exponential growth of the rate of polymerisation until it reaches equilibrium

Second graph:

  • when short preformed filaments are added, the rate of polymerisation rapidly increases immediately with no lag phase
  • polymerisation reaches a steady state when subunit addition = subunit loss
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17
Q

What is the critical concentration (Cc) ?

A
  • the concentration of monomer subunits when polymerisation is at a steady state
  • the concentration is different for plus and minus ends
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18
Q

Describe the composition of microtubules

A
  • made of alpha and beta tubulin protein heterodimers subunits
  • they bond end-to-end to form linear protofilaments
  • 13 protofilaments associate laterally to form microtubules
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19
Q

Describe tubulin and microtubule structure with the help of the diagrams

A
  • beta-tubulin is the plus end
  • alpha-tubulin is the minus end
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20
Q

Which form are alpha/beta subunits of microtubules in individually and after polymerisation?

A
  • individually, they are in the GTP form
  • after polymerising, GTP hydrolysis converts the subunits to the GDP form, which more readily detach from the molecule

(both alpha and beta-tubulin bind to GTP but alpha-tubulin can be ignored because it is not hydrolysed)

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21
Q

Describe nucleotide binding and hydrolysis in microtubules

A
  • GTP forms of the alpha/beta subunits are present as a GTP cap at the end of the microtubules
  • the subunits will hydrolyse the GTP to GDP to form a less stable GDP cap
  • this will shrink the microtubule
  • rescue may occur, adding new GTP forms
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22
Q

What is this diagram of a microtubule describing?

A
  • when the GTP cap is lost, the microtubule is susceptible to unravelling and depolymerisation
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23
Q

What is dynamic instability of microtubules?

What affects their stability?

A
  • the rapid growing and shrinking of microtubules as a result of GTP-cap status
  • other factors include:
  • microtubule-binding proteins
  • local influences
  • signalling events
24
Q

Briefly describe centrosomes and how microtubules are related

A
  • microtubules originate at centrosomes
  • a centrosome is made up of a pair of centrioles (cylindrical cell structure made from bundles of microtubule triplets), which are arranged at right angles of each other and surrounded by a specialised matrix
  • this matrix contains gamma-tubulin complexes that act as nucleation (seed) sites for microtubule assembly
  • centrosomes replicate in mitosis to form the spindle
25
Q

How are microtubules arranged within a cell?

A
  • microtubules radiate out of the matrix surrounding the centrioles
26
Q

What do gamma-tubulin complexes do for the formation of microtubules?

A
  • they nucleate microtubules, seeding the formation of microtubules
27
Q

What is a Microtubule Organising Centre (MTOC)?

A
  • it is the centrosome, usually close to the nucleus
  • microtubules generally organise their microtubules from a single region of the cytoplasm
  • see images for more detail
28
Q

What do microtubules act as tracks for?

A
  • microtubules act as tracks for cargo-carrying molecular motors
29
Q

What is dynein?

A
  • a molecular motor
  • allows vesicular cargoes to be carried along microtubule tracks
  • part of a complex multi-protein assembly
  • can also be used for bending in cilia and flagella
30
Q

What are cilia and flagella?

A
31
Q

Describe the microtubule arrange of cilia and flagella

A
32
Q

Describe dynein activity in cilia and flagella

A
33
Q

What are kinesins?

A
  • another family of microtubule motors
34
Q

What determines the direction of microtubule motors?

A
  • the direction in which microtubules walk is influenced by the +/- polarity of microtubules
  • in general + end tends to be oriented towards the periphery of a cell
  • dyneins: take cargo from the + to - ends of microtubules
  • so transports material from the periphery to the centre
  • kinesins: take cargo from the - to + end of microtubules
  • so transports from the centre to cell periphery
  • occasionally organelles being transported are seen to switch directions, suggesting that both motors are present
35
Q

How does cell division depend on microtubules?

A
36
Q

Give an example of how defective microtubules can cause conditions

A
37
Q

Where are intermediate filaments found?

A
  • they are found in all animals cells
  • particularly important in cells that require a lot of strength e.g. epithelial cells of the skin
  • due to its high tensile strength so they do not break easily under mechanical stress
38
Q

Describe how intermediate filaments are assembled

A
  • some IFs span the length of the cell, connecting cell-cell junctions called desmosomes
  • each filament is rope-like, made of 8 thinner strands of protein subunits
  • two monomers associated with each other to form a twisted dimer
  • two dimers then line up to form a staggered tetramer, arranged in opposite orientations (amino terminals facing away from each other)
  • tetramers then link together, building up one strand of an IF
  • eight strands stack and twist
39
Q

Give two examples of diseases that occur if IFs are defective and why

A
  • tissues can become damaged by normal mechanical forces
    e. g.
  • severe blistering diseases (epidermolysis bullosa simplex):
  • caused by defective intermediate filaments leading to epidermal fragility
  • see diagram
  • progressive muscle weakening:
  • when muscle intermediate filament, desmin, is defective
  • muscle fibre loss and pathology in skeletal muscles
40
Q

What are some major types of intermediate filament proteins in vertebrate cells?

A
41
Q

How can IFs help with the diagnosis of different cancers?

A
  • as different cell types express different IF types, cancer types can be diagnosed from the cell types from which they were developed
  • cancer cells will retain some of their characteristics
42
Q

Describe the intermediate filaments in epithelial layers

A
  • cytokeratins are a type of IF protein found in epithelial cells
  • there are many types of cytokeratins
  • although the keratins appear connected between cells, they are not actually and they terminate at desmosomes
43
Q

What is the nuclear lamina made of?

A
  • the nuclear lamina is a meshwork of lamin intermediate filaments on the internal surface of the nuclear envelope
44
Q

What is the role of nuclear lamins in mitosis?

A
  • they are a target for the breakdown of the nuclear envelope during cell division
  • some of the enzymes controlling cell division phosphorylate nuclear lamins
  • this breaks down their regular structure and the nuclear envelope fragments
  • condensed chromosomes are now free to attach to their spindle
  • later, when the nuclei of the daughter cells are reforming, the fragments of the nuclear envelope begin reassembling
  • the desphosphorylated lamins then bind to the fragments and cause them to coalesce
45
Q

Why are there few natural, structural mutations of actin?

A
  • actin plays a central role in many different processes
  • if there are mutations, the loss of its function will result in the cell’s death
  • these mutations are unlikely to be inherited
46
Q

What is the structure of actin microfilaments?

A
  • actin microfilaments (F-actin) are polymers of globular actin (G-actin)
  • G-actin binds a molecule of ATP and can hydrolyse it to ADP
  • when actin monomers polymerise to form actin microfilaments, the dumb-bell shape of the molecules causes the monomers to arrange as a single helical filament
47
Q

What is the Arp2/3 complex?

A
  • Arp stands for actin-related protein
  • the complex plays a major role in the regulation of actin microfilaments
48
Q

How does Arp2/3 regulate actin microfilaments?

A
  • the Arp2/3 complex is a controllable nucleating structure of actin polymerisation (a ‘seed’ that can be switched on or off)
  • a signal in the cell switches the inactive Arp2/3 complex to an active state that ‘seeds’ actin polymerisation
49
Q

Where does Arp2/3 bind to the actin subunits and how does the ‘daughter’ filament grow?

A
  • it binds to the ‘mother’ filament with the ‘daughter’ filament growing out at a 70 degree angle
50
Q

What is another way cells initiate the polymerisation of actin?

A
  • another mechanism is through the family of proteins called formins
  • instead of branches forming like in the Arp2/3 complex, formins initiate a straight, linear growth of actin at the + end
51
Q

Observe these actin-binding proteins and their roles

A
52
Q

What is filamin and what can it do?

A
  • filamin is a protein associated with actin
  • it cross-links actin filaments into a three-dimensional network with the physical properties of a gel
  • observe the diagrams
53
Q

What is the role of actin in single-celled and complex organisms?

What other proteins does actin work with to play its role?

A
  • actin is the key for motility of mant organisms
  • microfilaments work with myosins to provide the apparatus for cell contractility
54
Q

What are myosins?

A
  • the family of myosins have many different functions
  • but their common property is acting as motor proteins with actin microfilaments
55
Q

What is the structure of skeletal muscle fibres?

How does myosin allow contraction?

A

Molecular and Cellular Biology (MCB) (13 decks)

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