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Hilary Term 1 > Cytoskeleton I – Microtubules > Flashcards

Cytoskeleton I – Microtubules Flashcards

1
Q

Describe the eukaryotic cytoplasm

A
  • densely packed
  • filled with organelles
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2
Q

Describe the diffusion of the endocytic and secretory vesicles in the endomembrane system between the cellular interior and periphery

A

inefficient

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3
Q

Describe the effect of the inefficient diffusion of the endocytic and secretory vesicles in the endomembrane system between the cellular interior and periphery

A

bad for nutrients and gaseous exchange, and to prevent central mitochondrion oxygen starvation.

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4
Q

How are diffusion constraints solved in eukaryotic cells?

A

3D cytoskeletal transport network fills the cytoplasm, driving motile intracellular organelles

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5
Q

Describe the secondary functions of the cytoskeleton

A
  • control of cell shape and its structural support
  • muscle contraction
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6
Q

Describe cytoskeletal elements

A
  • long, unbranched, one-dimensional protein polymers
  • filaments assembly spontaneously
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7
Q

Describe actin microfilaments

A
  • 7nm
  • polar
  • dynamic
  • ATP-powered
  • found in all eukaryotes
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8
Q

Describe tubulin microtubule polymers

A
  • 25m
  • polar
    -dynamic
  • GTP-powered
  • hollow
  • thirteen protofilaments
  • alpha-beta heterodimer
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9
Q

Describe protomicrotubule polymerisation

A
  • 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
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10
Q

GTP

A

guanosine triphosphate

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11
Q

Describe the effects of kinetic inefficiency of GTP hydrolysis

A
  • 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
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12
Q

What is a GTP cap?

A

a GTP-bound filament

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13
Q

What happens if the GTP cap at the plus end is maintained?

A

GDP-bound monomers don’t dissociate

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14
Q

What happens if the GTP cap at the plus end is accidentally lost?

A
  • catstrophe event causes rapid shrinkage as the GDP-tubulin dimers depolymerise
  • can be rescued if the GTP cap is regained
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15
Q

Describe dynamic instability

A
  • 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
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16
Q

In vitro, the rate-limiting step of microtubule polymerisation is…

A

the initiation.

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17
Q

Describe the importance of gamma-tubulin in microtubule formation in Animalia

A
  • forms ring complexes (g-TuRC) with accessory proteins - bind to alpha-tubulins, determining the flament’s orientation.
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18
Q

Describe ring complexes

A
  • serve as structural templates for microtubule nucleations
  • anchored to intracellular organelles
  • nucleate within the centrosome
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19
Q

Describe the importance of gTuSC in microtubule formation in planta

A
  • exists in gTuSC
  • bind proteins of the nuclear envelope and cell edges
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20
Q

gTuSC

A

gamma tubulin small complices

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21
Q

Describe the guidance of microtubules and associated protein CC1 of the direction of cellulose polymerisation and cellulose microfibril formation in the cell wall.

A

enzyme complices in the plasma membrane extrude cellulose polymers as they travel along the cortical microtubules

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22
Q

Describe differing microtubule organisation across cell phases in animalia

A

in interphase, there exist approximately 50 microtubules, originating from the centrosome, containing a pair of centrioles

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23
Q

Describe the effect of gTuRC

A
  • 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
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24
Q

Describe catastrophins

A

regulate dynamic instability at either end by increasing catastrophic frequency and promoting disassembly

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25
Q

Describe longer, less dynamic and long-lasting microtubules

A

regulated at the growing end by MAPs

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26
Q

MAPs

A
  • 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
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27
Q

Describe the function of the microtubules in Animalia

A
  • 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
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28
Q

Describe microtubules bidirectional transport

A
  • two motor types: dyenins an dimer kinesins
  • allows stretching and clustering of the ER and Golgi with mass movement of vesicles along the microtubules
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29
Q

Describe dyenins

A

move towards the minus end

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30
Q

Describe dimer kinesins

A

usually move towards the plus end

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31
Q

Describe the relevance of dyenins and kinesins to COPI and COPII vesicles

A
  • 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
32
Q

Describe kinesin mechanics

A
  • 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
33
Q

What is important about the 8nm step of a kinesin

A

it is the distance between one tubulin dimer and its adjacent

34
Q

How is kinesin attachment mediated?

A

attachment/detachment hydrolysis cycle

35
Q

Describe the “morphology” of a kinesin top-down

A
  • cargo-binding tail domain
  • stalk
  • motor head domain
36
Q

Describe the similarities between actin and tubulin

A
  • have a plus and minus end
  • polarity means they can polymerise and depolymerise rapidly
37
Q

Describe the intermediate filaments - the basics

A
  • 10n
  • apolar
  • less dynamic
  • extensible (to 3.5x its length)
  • high tensile
  • proteinous
  • protofilament-forming
  • found only in animals
38
Q

Describe the intermediate filaments - the specifics

A
  • 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
39
Q

ULFs

A
  • small subunit aggregates
  • unit length filaments
40
Q

Describe the primary functions of the intermediate filaments

A
  • determination and maintenance of the nuclear and cell shape
  • structural support of these entities
41
Q

Give examples of intermediate filaments

A
  • keratins
  • vimentin
  • desmin
  • lamin
42
Q

Describe the nuclear lamina - the basics

A
  • important for the determination of nucleus shape, and secondarily for chromatin organisation]
  • lattice-like
43
Q

Describe the nuclear lamina - the specifics

A

formed by lamins at the interface between chromatin and the nuclear envelope: provides chromatin with an anchorage surface

44
Q

Describe how kinesin walks along microtubules

A
  • ATP-binding causes large conformational change
  • advances by one step and throws second head forward
  • hydrolysis relaxes the change and causes release from MT
45
Q

Describe dynein transport of vesicles - the basics

A
  • uses ATP
  • binding of dynein proteins to transport vesicles involves accessory proteins
46
Q

Describe dynein transport of vesicles - the specifics

A
  • 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
47
Q

Compare microtubules to microfilaments

A
  • 150-fold more rigid than microfilaments
  • can transmit compressive as well as tensile forces (push as well as pull)
48
Q

Describe the mitotic spindles

A
  • microtubule filament radiate from the spindle pole bodies
  • pull sister chromatids to opposing poles
  • position and push spindle poles
49
Q

Describe the eukaryotic cilia and flagella

A
  • structurally identical cellular extensions
  • contain complex arrays of microtubules
50
Q

Give to flagellated entities

A
  • sperm (1)
  • unicellular chlorophyte green alga Chlamydomonas (2)
51
Q

Give an example of a ciliated organism

A
  • Chromalveolate Paramecium
  • covered in short cilia
52
Q

Describe the basal bodies

A
  • cilia and flagella grow from them
  • they originate from centrioles in interphase cells
  • found in cilliated/flagellated plants cells
53
Q

Describe the cilium structure

A
  • 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
54
Q

Describe the A microtubule

A

13 protofilaments

55
Q

Describe the B microtubule

A

11 protofilaments

56
Q

Describe the ciliary dyneins

A

when on A microtubles, they walk along adjacent B microtubules

57
Q

What do the crosslinks between A and B microtubules result in?

A
  • bending
58
Q

The A and B microtubules make the

A

outer doublet microtubule

59
Q

Describe ciliary dynein action

A
  • 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
60
Q

Describe ciliary dynein

A

large protein complex with three motor heads

61
Q

What happens in isolated doublet microtubules?

A

dyenin produces microtubule sliding using ATP

62
Q

What happens in a normal flagellum?

A

dyenin causes microtubule bending

63
Q

nexin

A
  • links protein filaments
64
Q

Describe the cilia role in motility

A
  • short
  • aymmetric beat
  • force perpendicular to long axis
65
Q

Describe the flagellum role in motility

A
  • longer
  • (usually) symmetric sinusoidal beat
  • force (usually) parallel to long axis: generated along whole length
66
Q

Describe the role of the cilia in signalling and sensing

A
  • evolved as a sensory structure to detect extracellular mechanical and chemical signals
  • receptors, ion channels and transporter proteins localise to the cilium
67
Q

Describe human sperm cells

A
  • motile, sensory
68
Q

Describe olfactory neurones

A
  • non-motile, sensory
  • lacks central MT pair (9+0)
69
Q

Describe the rod cells of vertebrate retina

A
  • outer segment contains stacks of membrane discs with photoreceptors
  • cilium (axoneme) connects
  • inner segment contains centrioles
70
Q

Compare and contrast bacterial and eukaryotic flagella

A

non-homologous

71
Q

Describe eukaryotic flagella

A
  • tubulin
  • intracellular filament
  • moved by co-ordinated dyenins
72
Q

Describe prokaryotic flagella

A
  • flagellin
  • extracellular filament
  • moved by rotary motor complex
73
Q

Describe LECA

A

most likely had two motile cilia/flagella with sensory function

74
Q

Summarise the function of the cytoskeleton

A

movement + structural support + cell shape

75
Q

Summarise the microtubules

A
  • 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
76
Q

Summarise the intermediate filaments

A

provide structural support to animal cells and nuclei

Hilary Term 1 (61 decks)

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