Cytoskeleton I – Microtubules Flashcards
Describe the eukaryotic cytoplasm
- densely packed
- filled with organelles
Describe the diffusion of the endocytic and secretory vesicles in the endomembrane system between the cellular interior and periphery
inefficient
Describe the effect of the inefficient diffusion of the endocytic and secretory vesicles in the endomembrane system between the cellular interior and periphery
bad for nutrients and gaseous exchange, and to prevent central mitochondrion oxygen starvation.
How are diffusion constraints solved in eukaryotic cells?
3D cytoskeletal transport network fills the cytoplasm, driving motile intracellular organelles
Describe the secondary functions of the cytoskeleton
- control of cell shape and its structural support
- muscle contraction
Describe cytoskeletal elements
- long, unbranched, one-dimensional protein polymers
- filaments assembly spontaneously
Describe actin microfilaments
- 7nm
- polar
- dynamic
- ATP-powered
- found in all eukaryotes
Describe tubulin microtubule polymers
- 25m
- polar
-dynamic - GTP-powered
- hollow
- thirteen protofilaments
- alpha-beta heterodimer
Describe protomicrotubule polymerisation
- on subunit incorporation, inefficient hydrolysis of GTP to GDP at the GTP binding site by beta-tubulin (alpha-tubulin cannot)
- creates dynamic assembly from subunits: less dynamic alpha tubulin forms the minus end, and more dynamic beta-tubulin forms the plus end
GTP
guanosine triphosphate
Describe the effects of kinetic inefficiency of GTP hydrolysis
- creates a GTP cap at the growing plus end, where new subunits have been incorporated before GTP hydrolysis occurred
- creates a less stable GDP-bound region of GDP-tubulin dimers towards the minus end
- creates a conformational change that favours depolymerisation
What is a GTP cap?
a GTP-bound filament
What happens if the GTP cap at the plus end is maintained?
GDP-bound monomers don’t dissociate
What happens if the GTP cap at the plus end is accidentally lost?
- catstrophe event causes rapid shrinkage as the GDP-tubulin dimers depolymerise
- can be rescued if the GTP cap is regained
Describe dynamic instability
- if polymerisation rate exceeds that of hydrolysis, the microtubule grows
- if the hydrolysis rate exceeds that of polymerisation, the microtubule shrinks
- alternating growth and catastrophe cycles occurring at the plus end of the microtubule
- can occur and be visualised both in vivo and in vitro under video microscopy
In vitro, the rate-limiting step of microtubule polymerisation is…
the initiation.
Describe the importance of gamma-tubulin in microtubule formation in Animalia
- forms ring complexes (g-TuRC) with accessory proteins - bind to alpha-tubulins, determining the flament’s orientation.
Describe ring complexes
- serve as structural templates for microtubule nucleations
- anchored to intracellular organelles
- nucleate within the centrosome
Describe the importance of gTuSC in microtubule formation in planta
- exists in gTuSC
- bind proteins of the nuclear envelope and cell edges
gTuSC
gamma tubulin small complices
Describe the guidance of microtubules and associated protein CC1 of the direction of cellulose polymerisation and cellulose microfibril formation in the cell wall.
enzyme complices in the plasma membrane extrude cellulose polymers as they travel along the cortical microtubules
Describe differing microtubule organisation across cell phases in animalia
in interphase, there exist approximately 50 microtubules, originating from the centrosome, containing a pair of centrioles
Describe the effect of gTuRC
- rapid microtubule growth
- microtubules can penetrate through the cytoplasm, with the plus ends at the edges of the cell
- dyanmic instability
- longer, less dynamic and long-lasting microtubules can be observed
Describe catastrophins
regulate dynamic instability at either end by increasing catastrophic frequency and promoting disassembly
Describe longer, less dynamic and long-lasting microtubules
regulated at the growing end by MAPs
MAPs
- Microtubule Associated Proteins
- either suppress catastrophic frequency or enhance growth rate
- allows the cell to explore and sense changes in the cytoplasm or at the plasmamembrane
Describe the function of the microtubules in Animalia
- provide organisation and movement tracks for internal organelles and transport vehicles such as the COP-II vesicles, as well as endosomal movement
- aid chromosomal movement in meta- and anaphase
Describe microtubules bidirectional transport
- two motor types: dyenins an dimer kinesins
- allows stretching and clustering of the ER and Golgi with mass movement of vesicles along the microtubules
Describe dyenins
move towards the minus end
Describe dimer kinesins
usually move towards the plus end
Describe the relevance of dyenins and kinesins to COPI and COPII vesicles
- moved in opposite directions
- dyenins carry COP-II vesicles from the peripheral ER to the Golgi at the centrosome
- kinesins return the COP-I vesicles to the ER in retrograde transport
Describe kinesin mechanics
- walking along microtubules in 8nm steps
- cost of one ATP
- at any point, one kinesin head is attached, holding the cargo to the microtubule
- termed high processivitity
What is important about the 8nm step of a kinesin
it is the distance between one tubulin dimer and its adjacent
How is kinesin attachment mediated?
attachment/detachment hydrolysis cycle
Describe the “morphology” of a kinesin top-down
- cargo-binding tail domain
- stalk
- motor head domain
Describe the similarities between actin and tubulin
- have a plus and minus end
- polarity means they can polymerise and depolymerise rapidly
Describe the intermediate filaments - the basics
- 10n
- apolar
- less dynamic
- extensible (to 3.5x its length)
- high tensile
- proteinous
- protofilament-forming
- found only in animals
Describe the intermediate filaments - the specifics
- can be composed of homo- or heterodimer subunits
- antiparallel tetramer alignment
- assembles by annealing from ULFs
- resistant to compression, twisting and bending
- lacks motor proteins
- capable of exchanging subunits through incorporation and release along its filament length
ULFs
- small subunit aggregates
- unit length filaments
Describe the primary functions of the intermediate filaments
- determination and maintenance of the nuclear and cell shape
- structural support of these entities
Give examples of intermediate filaments
- keratins
- vimentin
- desmin
- lamin
Describe the nuclear lamina - the basics
- important for the determination of nucleus shape, and secondarily for chromatin organisation]
- lattice-like
Describe the nuclear lamina - the specifics
formed by lamins at the interface between chromatin and the nuclear envelope: provides chromatin with an anchorage surface
Describe how kinesin walks along microtubules
- ATP-binding causes large conformational change
- advances by one step and throws second head forward
- hydrolysis relaxes the change and causes release from MT
Describe dynein transport of vesicles - the basics
- uses ATP
- binding of dynein proteins to transport vesicles involves accessory proteins
Describe dynein transport of vesicles - the specifics
- the dynein binds to actin (containing an Arp1 filament, spectrin and ankryin) to form a dynactin complex
- the ankyrin binds to the membrane protein of the vesicle containing the cargo
Compare microtubules to microfilaments
- 150-fold more rigid than microfilaments
- can transmit compressive as well as tensile forces (push as well as pull)
Describe the mitotic spindles
- microtubule filament radiate from the spindle pole bodies
- pull sister chromatids to opposing poles
- position and push spindle poles
Describe the eukaryotic cilia and flagella
- structurally identical cellular extensions
- contain complex arrays of microtubules
Give to flagellated entities
- sperm (1)
- unicellular chlorophyte green alga Chlamydomonas (2)
Give an example of a ciliated organism
- Chromalveolate Paramecium
- covered in short cilia
Describe the basal bodies
- cilia and flagella grow from them
- they originate from centrioles in interphase cells
- found in cilliated/flagellated plants cells
Describe the cilium structure
- basal body extends into axoneme, surrounded by plasma membrane
- 9+2 arrangement of doublet and single microtubules
- central singlet microtubule surrounded by inner sheath, dotted with radial spokes which attach nexin and the outer and inner dyenin arms
- many accessory proteins
Describe the A microtubule
13 protofilaments
Describe the B microtubule
11 protofilaments
Describe the ciliary dyneins
when on A microtubles, they walk along adjacent B microtubules
What do the crosslinks between A and B microtubules result in?
- bending
The A and B microtubules make the
outer doublet microtubule
Describe ciliary dynein action
- not all dyneins can be active at once
- activity is carefully controlled to create the desired motion waveform
- propagation of bending activity down the flagellum leads to a sinusoidal wave form
- causes bending
Describe ciliary dynein
large protein complex with three motor heads
What happens in isolated doublet microtubules?
dyenin produces microtubule sliding using ATP
What happens in a normal flagellum?
dyenin causes microtubule bending
nexin
- links protein filaments
Describe the cilia role in motility
- short
- aymmetric beat
- force perpendicular to long axis
Describe the flagellum role in motility
- longer
- (usually) symmetric sinusoidal beat
- force (usually) parallel to long axis: generated along whole length
Describe the role of the cilia in signalling and sensing
- evolved as a sensory structure to detect extracellular mechanical and chemical signals
- receptors, ion channels and transporter proteins localise to the cilium
Describe human sperm cells
- motile, sensory
Describe olfactory neurones
- non-motile, sensory
- lacks central MT pair (9+0)
Describe the rod cells of vertebrate retina
- outer segment contains stacks of membrane discs with photoreceptors
- cilium (axoneme) connects
- inner segment contains centrioles
Compare and contrast bacterial and eukaryotic flagella
non-homologous
Describe eukaryotic flagella
- tubulin
- intracellular filament
- moved by co-ordinated dyenins
Describe prokaryotic flagella
- flagellin
- extracellular filament
- moved by rotary motor complex
Describe LECA
most likely had two motile cilia/flagella with sensory function
Summarise the function of the cytoskeleton
movement + structural support + cell shape
Summarise the microtubules
- form relatively rigid filaments
- GTP hydrolysis results in dynamic instability
- use kinesin and dynein motor proteins for transport
- used to position organelles and direct vesicle trafficking in animal cells
- used to guide cell wall deposition in plant cells
- roles in whole cell motility through cilia and flagella
Summarise the intermediate filaments
provide structural support to animal cells and nuclei
