MCB 9: Cell shape, Behaviour and Adhesion (Part I) Flashcards
What are the three main components of the cytoskeleton?
- microtubules
- intermediate filaments
- microfilaments (actin filaments)
What cellular processes do the cytoskeleton play a key role in?
- 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)
Which cytoskeleton component is which?
How is the cytoskeleton highly dynamic?
- 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
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 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
What does this graph show us about the mechanical properties of the different cytoskeletal filaments?
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
Are cytoskeletal filaments polar? Why or why not?
- 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
How does the polarity of cytoskeletal filaments affect the rate of subunit addition?
- the rate of subunit addition at the plus end is faster
- it is slower on the minus end
Describe cytoskeleton polymerisation and depolymerisation due to nucleotide binding and hydrolysis
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
Describe the biochemical properties below of microtubules
- subunit composition
- polymer filament polarity
- subunit nucleotide binding
- enzyme activity of subunits in filaments
Describe the biochemical properties below of microfilaments (F-actin):
- subunit composition
- polymer filament polarity
- subunit nucleotide binding
- enzyme activity of subunits in filaments
Describe the biochemical properties below of intermediate filaments:
- subunit composition
- polymer filament polarity
- subunit nucleotide binding
- enzyme activity of subunits in filaments
When does the cytoskeleton shrink?
- when the loss of subunits on one end is greater than the rate of addition on the other end
When does the cytoskeleton elongate?
- when the rate of addition of subunits on one end is greater than the rate of loss at the other end
When does the cytoskeleton ‘treadmill’?
- 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
Describe how polymerisation of cytoskeletal subunits is enhanced by ‘seeding’ with pre-formed filaments
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
What is the critical concentration (Cc) ?
- the concentration of monomer subunits when polymerisation is at a steady state
- the concentration is different for plus and minus ends
Describe the composition of microtubules
- 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
Describe tubulin and microtubule structure with the help of the diagrams
- beta-tubulin is the plus end
- alpha-tubulin is the minus end
Which form are alpha/beta subunits of microtubules in individually and after polymerisation?
- 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)
Describe nucleotide binding and hydrolysis in microtubules
- 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
What is this diagram of a microtubule describing?
- when the GTP cap is lost, the microtubule is susceptible to unravelling and depolymerisation
What is dynamic instability of microtubules?
What affects their stability?
- the rapid growing and shrinking of microtubules as a result of GTP-cap status
- other factors include:
- microtubule-binding proteins
- local influences
- signalling events
Briefly describe centrosomes and how microtubules are related
- 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
How are microtubules arranged within a cell?
- microtubules radiate out of the matrix surrounding the centrioles
What do gamma-tubulin complexes do for the formation of microtubules?
- they nucleate microtubules, seeding the formation of microtubules
What is a Microtubule Organising Centre (MTOC)?
- 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
What do microtubules act as tracks for?
- microtubules act as tracks for cargo-carrying molecular motors
What is dynein?
- 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
What are cilia and flagella?
Describe the microtubule arrange of cilia and flagella
Describe dynein activity in cilia and flagella
What are kinesins?
- another family of microtubule motors
What determines the direction of microtubule motors?
- 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
How does cell division depend on microtubules?
Give an example of how defective microtubules can cause conditions
Where are intermediate filaments found?
- 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
Describe how intermediate filaments are assembled
- 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
Give two examples of diseases that occur if IFs are defective and why
- 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
What are some major types of intermediate filament proteins in vertebrate cells?
How can IFs help with the diagnosis of different cancers?
- 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
Describe the intermediate filaments in epithelial layers
- 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
What is the nuclear lamina made of?
- the nuclear lamina is a meshwork of lamin intermediate filaments on the internal surface of the nuclear envelope
What is the role of nuclear lamins in mitosis?
- 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
Why are there few natural, structural mutations of actin?
- 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
What is the structure of actin microfilaments?
- 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
What is the Arp2/3 complex?
- Arp stands for actin-related protein
- the complex plays a major role in the regulation of actin microfilaments
How does Arp2/3 regulate actin microfilaments?
- 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
Where does Arp2/3 bind to the actin subunits and how does the ‘daughter’ filament grow?
- it binds to the ‘mother’ filament with the ‘daughter’ filament growing out at a 70 degree angle
What is another way cells initiate the polymerisation of actin?
- 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
Observe these actin-binding proteins and their roles
What is filamin and what can it do?
- 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
What is the role of actin in single-celled and complex organisms?
What other proteins does actin work with to play its role?
- actin is the key for motility of mant organisms
- microfilaments work with myosins to provide the apparatus for cell contractility
What are myosins?
- the family of myosins have many different functions
- but their common property is acting as motor proteins with actin microfilaments
What is the structure of skeletal muscle fibres?
How does myosin allow contraction?
