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Cell Structures Flashcards

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1
Q

what are the features of a cytoskeleton?

A

highly ordered. dynamic network of filaments

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2
Q

what are the roles of the cytoskeleton?

A
  • responsible for maintaining cell shape
  • important in movement of cell and internal structures
  • cell morphology
  • cell migration
  • vesicle transport
  • cell division
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3
Q

why is the cytoskeleton dynamic?

A

has proteins that self-assembly in long polymers with repeating subunits

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4
Q

give a brief overview of micro-filaments (actin)

A
  • cellular movements
  • muscle contraction
  • cell division
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5
Q

give a brief overview of micro-tubules (tubulin)

A
  • scaffolds
  • cell shape
  • transport tracks
  • mitosis, pulls sister chromatids apart
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6
Q

give a brief overview of intermediate filaments

A

contain various proteins and provides tensile strength

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7
Q

how do these different components of the cytoskeleton interact?

A
  • network that extends through the cell
  • some overlap
  • reflects cooperation but each have unique functions
  • the monomers give the morphological features to set them apart
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8
Q

how is actin dynamic?

A

it is constantly polymerizing and depolymerization

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9
Q

how do cells move?

A

through change in cell shape which is driven by the actin cytoskeleton

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10
Q

when is cell movement important?

A

in healing wounds

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11
Q

what are the different features that help cells move?

A
  • tail
  • ruffle
  • filopodia
  • lamellipodium
  • actin bundles
  • stress fibres
  • leading edge
  • focal adhesions
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12
Q

what are ruffle?

A

assemblies that do not form tight adhesions with the substrate

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13
Q

what is filopodia?

A

finer cytoplasmic extensions. typical of slower moving cells.

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14
Q

what is lamellipodium?

A

broad membrane extension that move forward, typical of migrating cells

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15
Q

what are focal adhesions?

A

structures that form mechanical links between intracellular actin and extracellular substrate

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16
Q

what is needed for a cell to move along a surface?

A

needs to be able to hold onto something to pull it along

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17
Q

how does the cell move?

A
  • uses focal adhesions to pull it along
  • for instance holds onto the extracellular matrix
  • link the intracellular actin filaments to the extracellular matrix through integral membrane proteins (integrins)
  • as the cell moves focal adhesions assemble and disassemble (push and pull)
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18
Q

how do microfilaments generate force?

A

they generate force through the assembly of globular monomers (G-actin) into filamentous polymer (F-actin)

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19
Q

why does G-actin form a helical structure?

A

provides strength

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20
Q

describe F actin

A

a tight helix

repeating unit = 14 subunits

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21
Q

what are the features of G-actin?

A
  • found in all eukaryotes
  • highly conserved
  • multiple isoforms
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22
Q

what is the structure of G-actin?

A
  • two lobes separated by a large cleft
  • four domains with a hinge between domains I and II
  • the hinge allows lobes to move relative to each other forming a nucleotide binding cleft
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23
Q

why is nucleotide binding important in G-actin?

A

it stabilizes structure

G-actin stability (G-actin is unfolded in the absence of nucleotide)

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24
Q

what are the functions of G-actin?

A
  • nucleotide binding
  • can bind ATP and ADP - bound forms are identical apart from domain II
  • doesn’t have structure unless ATP/ADP bound
  • they have polarity as +ve end (barbed) and a -ve end (pointed)
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25
Q

what happens in a filament?

A

the -ve ends line up and point the same way. self assembles into F actin

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26
Q

what are the 3 stages of actin assembly?

A
  • nucleation
  • elongation
  • tread milling
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27
Q

what is nucleation?

A
  • formation of a trimer, 3 actin monomers

- occurs when concentration of action is higher than the critical concentration

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28
Q

why is the critical concentration important in nucleation?

A
  • More likely to interact with each other

- Rate of assembly will increase as concentration increases

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29
Q

what is elongation?

A
  • after nucleation filament extends through addition of actin monomers to each end of the trimer
  • creates f actin
  • all monomers have the same orientation
  • f actin is created in a polarisaed manner
  • f actin is polarised
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30
Q

which ends can G actin monomers add to?

A

both ends, but only when the concentration is high enough

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31
Q

what happens if you put in a solution below the critical concentration?

A

disassemble

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32
Q

where is there faster assembly?

A

the positive end

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33
Q

what is treadmilling?

A

monomers add to the +ve end and dissociate from -ve end

- filaments stay the same length but theres movement

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34
Q

what does treadmilling rely on?

A

actins atpase activity

  • hydorlysis is not required for polymerisation but important in treadmilling
  • critical concentration
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35
Q

what has higher ATPase activity?

A

Factin

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36
Q

describe the process of actin and ATP

A
  • actin with ATP joins at the +ve end of the filament
  • when bound it hydrolyses its ATP
  • longer its bound the more likely it is to have ADP
  • actin at -ve end more likely to have ADP
  • actin with ADP has a lower affinity to bind with F-actin
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37
Q

what are the differences between ATP actin and ADP actin?

A
  • conformational differences
  • ADP actin has a lower affinity than ATP actin
  • Dissociates more rapidly
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38
Q

what is the source of the critical concentration of actin polymerisation?

A
  • more recently polymerised ATP bounds ubunits at the +ve end but more ADp containing subunits at the -ve end
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39
Q

how is the ATP-ADP cycle completed?

A
  • with nucleotide exchange
  • at -ve end release and exchanges ADP to ATP
  • can rejoin at the positive end
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40
Q

what are the features filopodium?

A
  • parallel bundles
  • thin extensions from the leading edge
  • positive ends at one end
  • negative ends at the other
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41
Q

what are the features of lamellipodium?

A
  • branched and crosslinked netowrks
  • plate like projections that go out at the leading edge
  • extremely well ordered
  • ordered by proteins
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42
Q

what are the features of stress fibres?

A
  • anti parallel contractile structures
  • opposite direction from the leading edge
  • mixture of -ve and +ve ends
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43
Q

what are the features of the cortex?

A

branched and cross linked networks.

  • underlies the cell surface
  • tensile strength
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44
Q

why is actin remodelled?

A

in response to environmental cues, stimulate cell division, differentiation or locomotion

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45
Q

what are the roles of the cells regulatory mechanisms for actin?

A
  • assembly and disassemble actin
    1. inhibition of polymerisation of G to F
    2. nucleation of new actin filaments
    3. control actin filament length
    4. elongation/shortening of pre existing actin filaments
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46
Q

what are the regulatory mechanims for actin carried out by?

A
  • actin binding proteins
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47
Q

what are the roles of actin binding proteins?

A

regulate actin polymerisation and organisation

associate with monomers or filaments

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48
Q

what are some actin binding proteins involved in polymerisation?

A

thymosin beta 4

profilin

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49
Q

what is thymosin beta 4

A
  • Found in metazoic cells
  • Smaller peptide
  • Sequesters monomeric actin
  • Binds to the actin to prevent G actin from polymerising
  • Get a build-up of actin-ADP
  • Unable to add onto actin microfilaments
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50
Q

what is profilin?

A
  • Binds ADP – G-actin
  • Binds to monomeric G-actin
  • Increases rate of nucleotide exchange
  • Binds to opposite ends of nucleotide clefts
  • Prevents F-actin binding to the –ve end
  • F actin from the +ve end
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51
Q

how is nucleation regulation?

A

provides a template for the trimer to form

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52
Q

what are the proteins involved in regulating nucleation?

A

formins and Arp2/3

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53
Q

what is formin?

A

bind individual actin together

54
Q

what is Arp2/3

A

performs a similar function to formins, also initiates branching of microfilaments by binding to an actin filament and initating polymerisation of a new branch

55
Q

how is actin filament length regulated?

A

severing and capping proteins

56
Q

what is severing?

A

breaks actin filaments into shorter fragments

57
Q

what is capping?

A

stop the filament from growing or shrinking, acts to stabilise the filament

58
Q

what happens if an actin filament is capped at both ends?

A

it will not lose or gain monomers

59
Q

what is Gelsolin?

A
  • dissolves actin meshes
60
Q

what is the function of Gelsolin?

A
  • disrupts subunit organisation by binding to Factin
  • severing filament
  • capping +ve end
  • uncapped -ve end can disassemble
  • has 2 different properties
61
Q

how is Gelsolin stimulated?

A
  • by elavated Ca2+ concentration

- trnaslates extracellular signals

62
Q

how is Gelsolin inhibited?

A
  • inhibited by P1P2 - different cell signalling molecules
63
Q

what is Cofilin (ADF-cofilin)?

A

binds ADP actin in bothe monomeric G actin and in F actin

64
Q

what is the function of Cofilin?

A

increases the rate of depolymerisation of ADP actin

  • found mainly at the -ve end
  • dissociates subunits
  • prevents nucleotide exchange (stopping polymerization on the +ve end)
65
Q

what are some examples of cross linker proteins?

A
  • alpha actinin
  • filamin
  • fimbrin
66
Q

what is alpha actinin?

A
  • Contractile bundle
  • Stress fibre
  • Contains 1 actin binding domain as its part of a bigger protein (dimeric)
  • Anti-parallel
  • Head to tail structure
67
Q

what is filamin?

A
  • Gel like network
  • Cell cortex
  • Contains 1 actin binding domain as its part of a bigger protein (dimeric)
  • Coordinating the network in the cortex
  • V shape structure
  • Binds to F-actin  organises the network
68
Q

what is fimbrin?

A
  • Tight and parallel bundle
  • Filopodium
  • Contains 2 actin binding domains (monomeric)
69
Q

what do all cross linker proteins contain?

A

an actin binding domain (tight parallel)

70
Q

what is spectrin?

A
  • tetrameric binding
  • Organises filaments into a specific network
  • Almost exclusively in RBC  gives them their shape
71
Q

what happens if there are mutations in spectrin?

A
  • underlie severe anemia - RBC lose their shape and are not as efficient at carrying oxygen
72
Q

what is dystrophin?

A
  • monomeric binding
  • Carries a single binding domain
  • Binds and links a multi-protein complex found in plasma membrane of muscle fibres
  • Connecting muscle fibre cytoskeleton to extracellular matrix
  • Role in the force generation in muscle
73
Q

what do mutations in dystrophin cause?

A
  • underlie muscular dystrophies
74
Q

what role do actin filaments have in the cell?

A
  • serve as tracks for transport of components
  • myosins walk vesicle along actin filaments
  • sometimes very long distances
75
Q

what are the key features of myosin?

A
  • multigene family
  • 1 or 2 heavy chains and light chains
  • heavy chains have a globular head
76
Q

why is the globular head important in myosin?

A
  • binds to actin (which has ATPase activity)

- allows it to walk a vesicle along the cytoskeleton

77
Q

what are the different types of myosin?

A

myosin I, II and V

78
Q

what is myosin I?

A
  • tail binds to membranes
  • cytoskeletal - membrane interaction
  • filopodia, microvilli
  • link cytoskeleton to plasma membrane
  • (monomer)
79
Q

what is myosin V?

A
  • cytoskeletal - membrane interaction
  • vesicle transport
  • can link to transport vesicles to power transport
  • leave the globular heads to walk along the cytoskeleton
  • (dimer)
80
Q

what is myosin II?

A

 Doesn’t link actin to a membrane but generates contractile force
 Muscle contraction, cytokinesis
 Tightens contractile ring  cell division
 Found in muscle fibres  instriated muscle they are assembled into higher ordered structures, with tails packed together to form a thick filament where actin-binding globular heads protrude
 Organised into thick filaments
 All the tails together with the heads sticking out
 Regulated by phosphorylation
- (dimer)

81
Q

what is the sliding filament model in striated muscle contraction?

A
  • sarcomere is the function unit - thick and thin filaments
  • actin filaments anchored to the z disk
  • controlled by the interaction of myosin heads sticking out from the fick filaments with actin in the thin filaments
82
Q

what is the first stage of the sliding filament model?

A
  • myosin movement energy

- ATP hydrolysis channeled through changes in myosin heavy chain

83
Q

what is the second stage of the sliding filament model?

A
  • ATP binding causes conformation change in myosin

- disruption in actin binding site (releases)

84
Q

what is the third stage of the sliding filament model?

A

• ATP hydrolysis causes conformational change in myosin, hydrolysis products trapped
- Actin binding site restored (head pivots and binds)

85
Q

what is the foruth stage of the sliding filament model?

A
  • Conformational changes in head and neck are transmitted and amplified to other parts of the molecule through the light chains bound to the neck (power stroke)
  • Structures slide past each other and shorten the sarcomere
86
Q

what are the key features of intermediate filaments?

A
  • lots of different types - large diveristy in size and sequence
  • there are 5 major classes
87
Q

what are keratins?

A
  • make up cystole intermediate filaments
  • extend to the cell membrane in skin epithelial cells
  • type 1 and type 2
88
Q

what are the main features of keratins?

A
  • fibrous proteins
  • outer epithelial = skin cells
  • prominent in skin, hair and nails
  • heterodimers of basic and acidic subunits
89
Q

what are mutations of keratins involved in?

A

mutations involved in skin disease

90
Q

what are type 1 keratins?

A

acidic keratins

91
Q

what are type 2 keratins?

A

basic keratins

92
Q

what are vimentins?

A
  • type III intermediate filaments
  • widely distributed (stromal tissues, lymphocytes, endothelial cells, fibroblasts)
  • supports cell membranes and keeps nucleus and organelles in position
93
Q

what are neuronal IF proteins?

A
  • type IV intermediate filaments
  • neurofilaments
  • structural role in axons
  • determine axon diameter and hence their speed of conduction
94
Q

what are lamins?

A
  • type V intermediate filaments
  • fibrous network supporting the inner nuclear membrane
  • may organise different types of chromatin (gene regulation)
95
Q

what is the process of assembly of intermediate filaments?

A
  • 2 monomers come together and wrap around to form a parallel dimer
  • two heads (N terminal) and two tails C terminal)
  • 2 dimers go head to tail to form an antiparallel tetramer
  • tetramers stack end on ened to form a profilament
  • they double up to form a protofibril
  • 4 protofibirls wrap around each other to form an intermediate filament
96
Q

where are the differences mainly found in intermediate filaments?

A
  • the globular heads and tails

- the alpha helical region is highly conserved

97
Q

what are the different forms of intermediate filaments?

A
  • heterpolymers (keratin type I and type II)
  • homopolymers
    (vimentins can be either)
98
Q

what helps form heteropolymers in intermediate filaments?

A

spacer sequence

99
Q

what does intermediate filament assembly not require?

A

doesn’t need ATP or GTP - a spontaneous process

100
Q

what is the physiological role of intermediate filaments?

A
  • anchor to cell junctions (cell adhesion)
  • cell integrity
  • position structures within the cell
101
Q

describe stem cells in the basal epidermal layer

A
  • different and change keratin expression profile as they progress to the outer layer
  • the differentiation causes different characteristics of the skin
102
Q

what are keratin 5/14 heterodimer?

A
  • assemble into IFs in basal keratinocytes

- critical for cell structure

103
Q

what happens if there are mutations in keratin5/14?

A

 N – or C – terminal mutations  unable to form (end to end) protofilaments
 Cells at the base of the epidermis are weakened
 Epidermis and dermis easily separate
 Blistering disease  epidermolysis bullosa simples: slight abrasions cause serious wounds. Demonstrates the importance if keratins.

104
Q

what is hutchinson - gilford progeria syndrome?

A
  • Rare premature ageing disease
  • Caused by mutations in the LMNA gene producing an abnormal form of the nuclear intermediate filament Lamin A
  • Nuclear envelope becomes unstable and prone to damage and mutations  cause phenotypes associated with ageing
  • Osteoporosis, hair loss, cardiovascular disorders, diabetes, muscle atrophy
105
Q

what are microtubules?

A
  • polymers of globular tubulins

- cellular tracks used by microtubule motor proteins to transport

106
Q

what are some example of motor proteins?

A

kinesin and dynein

107
Q

what heterodimers do tubulin form?

A

alpha tubulin and beta tubulin

108
Q

what is alpha tubulin?

A
  • binds GTP irreversibly
  • GTP is not hydorlysed
  • non exchangeable GTP
109
Q

what is beta tubulin?

A
  • binds GTP reversibly
  • GTP hydrolysed to GDP
  • exchangeable GTP
110
Q

how do heterodimers (tubulin) polymerise?

A
  • polymerise head to tail to form polar tubular structure

- they have polarity

111
Q

how is a tubulin protofilament formed?

A
  • assemble end to end to form a protofilament, this then forms a sheet which curls into a tub
112
Q

describe protofilament assembly? (tubulin)

A
  • Tubulin dimers exist in cytoplasm
  • As concentration increases to above critical concentration for assembly
  • Assembly longitudinally to form short profliaments
113
Q

describe sheet assembly (tubulin)

A
  • protofilaments associate laterally into curved sheets
  • increases stability
  • can carry on forming longitudinally
114
Q

how are the tubules formed?

A
  • sheets wrap around to form hollow stable structures
  • grows by adding more dimers
  • rate of assembly faster at the (+) end
  • orientated so that beta tubulin with hydrolysable GTP is at the (+) end
  • GTP on beta tubulin hydrolyses on binding
  • if rate of addition is greater that GTP hydrolysis, a GTP cap is produced- serves to stabilise the tubule
115
Q

when are microtubules formed?

A

when dimer concentration is higher than the critical concentration
- as tubules grow pool of dimers will decrease

116
Q

what happens to microtubules if the dimers go below Cc?

A

microtubules start to disassemble

- known as dynamic instability

117
Q

what are microtubules anchored to?

A

most are anchored at their -ve end to a microtubule organisation centre (MTOC)
-located near the nucleus which directs assembly and orientation

118
Q

what is the MTOC in animals?

A

the centrosome (pair of centrioles)

119
Q

what is the centrosome?

A

centrioles are associated with the pericentriolar matrix, through proteins to (-) end of microtubules leaving (+) end free to grow and shrink

120
Q

what is a microtubules function during mitosis?

A
  • centrioles replicate and go to opposite poles od diving cells
  • helps separate chromosome
  • use microtubules disassembly
121
Q

what are some other types of MTOC?

A

flagella

dendrites

122
Q

how is a flagella a MTOC?

A

basal body to organise their microtubules

123
Q

how is a dendrite a MTOC?

A

show mixed polarity

important for transport

124
Q

what is the microtubules role in transport?

A
  • cellular tracks
  • traffic vesicles, organelles and chromosomes
  • used by motor proteins
  • work in oppsitie directions
125
Q

what direction do kinesins work?

A

anterograde

126
Q

what direction do dyneins work?

A

retrograde

127
Q

how do vesicles cope when the cell is MT poor but MF rich?

A

a single vesicle may have both kinesin and myosin motors attached

128
Q

describe the structure of kinesins

A

similar to myosin, have globular heads which binds to MT

129
Q

describe the function of kinesins

A
  • direction of movement is towards to positive end

- ATP dependent, use energy to walk the cargo

130
Q

describle cysotolic kinesins in vesicle transport

A
  • Kinesin head domains (ATPase activity) dock with microtubules
  • MT remains stationary and vesicle transported towards (+) end (anterograde)
  • Kinesins ‘walk’ cargo along microtubule tracks

Cell Biology (10 decks)

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