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lecture 8 Flashcards

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1
Q

why are there differences in actin in a cell vs. test tube

A

due to presence of actin binding proteins that regulate cytoskeletal form and function

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2
Q

what is first type of actin binding proteins

A

control filament assembly

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3
Q

what is second class of actin binding proteins

A

responsible for nucleating actin filaments/cytoskeleton

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4
Q

third class of actin proteins

A

shape structure of those filaments [arranges actin filaments]

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5
Q

what proteins in first class

A

thymosin, profilin, cofilin

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6
Q

what proteins in second class

A

formin, arp 2/3

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7
Q

what proteins in third class

A

fimbrin, alpha actinin, filamin

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8
Q

what do proteins that control filament assembly do

A

control the rxn of where monomers are being added or removed from actin filaments

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9
Q

what does thymosin do

A

binds to actin monomers and prevents them from being added to actin filaments

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10
Q

what does profilin do

A

opposite of thymosin; binds actin monomer and puts it on actin filaments hella fast (so it happens much faster than in test tube)

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11
Q

what does cofilin do

A

binds to existing actin filaments & breaks them apart into component pieces

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12
Q

why is cofilin important

A

limited # of monomers in cell, so it recycles old filaments into monomers so they can be used in new actin structures

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13
Q

what do actin binding proteins responsible for nucleating actin cytoskeleton do

A

kinda like cofactors/enzymes that catalyze nucleation step

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14
Q

why is nucleation faster in cell vs test tube

A

b/c of proteins that provide as scaffold to bring oligomer together in right place & right time –> overcome penalty of time it takes to seed a new filament

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15
Q

what do these 2nd class of proteins generate

A

2 distinct actin networks

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16
Q

what is formin

A

nucleates & facilitates polymerization of long, straight actin filaments

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17
Q

what is arp 2/3

A

branched, straight actin filaments

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18
Q

what does arp 2/3 do

A

does a branch coming off of a pre-existing filament –> enormous branched actin netowrk which forms lamellipodia

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19
Q

what is lamellipodia

A

leading edge of migrating cells

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20
Q

what kinds of filaments can the third class of protein make

A

filaments that are branched, tightly, or loosely bundled

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21
Q

what do different arrangements allow actin to do

A

function in specific ways in diff parts of cell

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22
Q

what are proteins that arrange actin filaments

A

fimbrin, alpha actinin, filamin

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23
Q

what is fimbrin

A

short cross linker; takes those long straight actin filaments and bundles them together into a very tightly woven bundled actin fiber

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24
Q

what is alpha actinin

A

loosely bundled filament, essential for helping myosin to fit in actin fibers to create contractile machinery (actomyosin stress fibers)

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25
Q

what is force generating aspect of cytoskeleton

A

alpha actinin

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26
Q

what is filamin

A

holds actin filaments together; flexible, dimer

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27
Q

describe filamin

A

can bend, stretch, adopt angles

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28
Q

where is filamin found

A

underneath plasma memrbane of cells in a cortex

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29
Q

what does filamin provide for cells

A

little more resilience so they don’t burst upon mechanical stress

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30
Q

what powers cell movement

A

myosin II contractility

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31
Q

what does profilin do

A

accelerates actin monomer addition to plus end

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32
Q

what does thymosin do

A

blocks actin monomer addition to plus end

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33
Q

what does monomer availability control

A

actin filament assembly

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34
Q

how does thymosin work

A

floating around in cytoplasm, bumps into actin monomer & binds it –> blocks addition to plus end

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35
Q

what happens when actin is bound to thymosin

A

cannot be added to growing actin filament

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36
Q

when is thymosin active

A

all the time; breaks are always on

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37
Q

what is the “accelerator” that overcomes this tendency

A

profilin

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38
Q

what does profilin do basically

A

opposite of thymosin; when it binds on actin monomer it’s attaching it to an actin filament immediately

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39
Q

what is profilin like

A

an enzyme that catalyzes rxn

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40
Q

is profilin regulated

A

yes; once signal activates it, it overcomes thymosin block allowing actin filaments to polymerize

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41
Q

how does thymosin-profilin work

A

dynamic EQ; even tho thymosin is always present and binds actin monomers, sometimes it releases it

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42
Q

what happens when thymosin releases actin monomers

A

if no profilin, thymosin just grabs free actin monomer again

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43
Q

what happens if there is profilin (regulation)

A

whenever thymosin gives up free actin monomer, profilin grabs it + puts it on actin filament

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44
Q

what dictates whether filament grows or not

A

thymosin keeps doing its thing; profilin is what dictates it (if profilin is activated, free actin monomers are grabbed & directed to polymerize)

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45
Q

what one of the actin nucleating proteins that overcomes natural lag phase

A

arp 2/3

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46
Q

describe arp 2/3 and actin

A

diff gene, separate proteins but very similar structure

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47
Q

what are arp 2 and arp 3

A

2 of the 3 necessary monomers that nucleate an actin filament

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48
Q

what is arp 2/3 like

A

it’s like a pre-formed dimer, shortcuts lag of nucleation by holding them in a complex

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49
Q

what does arp 2/3 need

A

another monomer to come along and overcome lag phase

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50
Q

can arp 2/3 be active/inactive

A

yes; signaling pathways can activate it

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51
Q

describe inactive arp2/3

A

held askew, not near each other

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52
Q

describe active arp2/3

A

conformational change lines them up in precise orientation

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53
Q

what happens when actin monomer is w/ active arp 2/3

A

rapid growth; it forms the stable oligomer that leads to rapid growth of actin filaments

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54
Q

what’s the 3rd part of this arp 2 /3 complex

A

third actin monomer to come in to get stable seed for filament growth

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55
Q

what is NPF

A

signaling protein that has been activated by upstream signaling cascade that is now responsible for activating arp 2/3

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56
Q

what does arp 2/3 do

A

accelerate polymerization & forms branched actin filaments/networks

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57
Q

what does arp2/3 bind to most of the time

A

binds to a pre-existing filament

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58
Q

describe the formation of filament w/ arp 2/3

A

all of the previous steps happen (conformational change, actin monomer joins, fuels further growth, etc.), just anchored to a pre-existing filament when active

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59
Q

what angle is actin branch at relative to mother filament

A

70*

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60
Q

describe the iterative process of arp2/3

A

enormous branched (complex) network thru multiple copies of arp2/3 activated, forms an initial branch, then another, then another etc.

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61
Q

what does this highly branched network allow

A

forms a flat fan-shaped structure, allows it to push membranes forward as actin filaments polymerize

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62
Q

what is fundamental machinery that drives lamellipodia based protrusion

A

arp2/3

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63
Q

where are lamellipodia

A

formed at leading edge of cells

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64
Q

what does arp2/3 allow

A

instead of needing 3 monomers to come together you only need one

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65
Q

describe orientation of inactivated arp2/3

A

open (not near each other)

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66
Q

describe orientation of activated arp2/3

A

closed; conformational change –> near each other

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67
Q

where does arp2/3 complex dock onto

A

side of pre-existing mother filament

68
Q

where do new active monomers join

A

plus end

69
Q

what do lamellipodia drive

A

cell motility & junction formation

70
Q

what does polymerization of branched actin do

A

push leading edge forward to drive cell movement

71
Q

where do we see densely branched actin network

A

at leading edge of migrating cells in a structure called lamellipodia

72
Q

what is job of lamellipodia

A

to push leading edge forward

73
Q

what do filaments do as they grow

A

convert chemical E to physical force to push plasma membrane in drxn of cell movement

74
Q

what does arp2/3 do thru its branched network forming abilities

A

helps form cell-cell adhesions

75
Q

where is another place you see branched actin network created by arp 2/3

A

junction b/w two epithelial cells (no lamellipodia)

76
Q

what happens to heart development if embryos treated w/ CK-666 that prevents lamellipodia formation thru inhibiting arp2/3 activity

A

cardiac cells would differentiate, but can’t move or migrate to where heart is gonna form (cuz no lamellipodia) –> profound defect

77
Q

what does no arp2/3 activity mean

A

no lamellipodia formation –> no migration (which is essential for development)

78
Q

what is other actin nucleating protein

A

formin

79
Q

what does formin do

A

accelerate polymerization (overcomes RDS of nucleation)

80
Q

what does formin generate

A

straight filaments

81
Q

describe molecular mechanism of formins

A

surfs growing plus end /(instead of sitting at minus end of filament and allowing it to grow a firm foundation)

82
Q

what do formins continuously do

A

add actin monomers to plus end of growing actin filaments to generate long filaments that can be bundled into stress fibers

83
Q

describe formin structure

A

dimer

84
Q

what does dimer do w/ addition of new monomer

A

shifts; walks up plus end as new monomers are added

85
Q

is formin straight or branching

A

not branching; it’s straight

86
Q

what happens if formin is the only thing going on

A

just get long, straight actin filaments that form rapidly

87
Q

what are long filaments from forming the building blocks for

A

loosely bundled filaments, tightly bundled filaments

88
Q

what do formins build

A

actin stress fibers

89
Q

what do formins nucleate

A

long actin filaments that are bundled into stress fibers

90
Q

what do formins make

A

precursor straight filaments that go into long stress fibers

91
Q

where are these long stress fibers

A

run underneath cell, responsible for generating majority of contractile forces

92
Q

how do formin and profilin work together

A

form surfs growing plus end, elongating filament; gets hand off of actin monomers from activated profilin

93
Q

what is anchored at minus end, arp2/3 or formin

A

ARP (not formin)

94
Q

what does arp 2/3 form

A

branched filaments

95
Q

what does formin form

A

straight filaments (surfs plus end as new monomers are added)

96
Q

what else can profilin help besides formin

A

also feeds actin monomres to arp2/3

97
Q

what is common denominator in creation of actin filaments

A

profilin

98
Q

what is a way to regulate whether it’s gonna grow or shrink

A

profilin

99
Q

what is cofilin

A

disassembles old actin structure, gets monomers recycled back into cytoplasm to build new actin structures

100
Q

where does cofilin bind

A

binds along actin filament like a copolymer

101
Q

what does cofilin induce

A

binds, induces conformational change that puts filament under mechanical strain

102
Q

what does putting filament under mechanical strain do

A

primes it to fall apart

103
Q

basically what does cofilin do to shape

A

binds, gives it a twist, breaks it into pieces

104
Q

describe appearance of filament when cofilin is there

A

thicker (cuz there’s another protein there)

105
Q

what does cofilin do to twists in the helix

A

shortens the twist in the helix, cuz its getting ready to fall apart into component pieces

106
Q

what is lamellipodia generated by

A

arp 2/3

107
Q

what are filopodia and stress fibers generated by

A

formin

108
Q

what is cell cortex (forms mix of branched and unbranched filaments underneath plasma membrane) generated by

A

filamin

109
Q

once filaments (long & straight) are generated by formin, what happens

A

fimbrin binds actin filaments into tight bundle

110
Q

what does this tight bundle of actin filaments prevent

A

prevents addition of other proteins cuz there’s not enough space

111
Q

what is actin filaments and fimbrin

A

parallel bundle; tight packing prevents myosin II fron entering bundle

112
Q

what type of structural actin bundle do we see in filopodia

A

parallel bundle; hella tight

113
Q

what does alpha actinin do

A

longer cross linker that binds them with more space b/w adjacent filaments –> loose packing

114
Q

what does the loose packing of a-actinin allow

A

allows myosin 2 to enter bundle

115
Q

why is loose packing important

A

we wanna build an actomyosin stress fiber, so we need space for myosin motor proteins to enter network, bind actin, and act on them

116
Q

describe actin filaments in filopodia/fimbrin mediated bundles

A

parallel

117
Q

what does parallel mean for plus and minus ends

A

plus and minus ends are in same orientation

118
Q

describe actin in alpha actinin

A

plus and minus ends alternate

119
Q

what does filamin dimer do

A

cross links proteins but doesn’t bundle them

120
Q

what dooes filamin dimer do

A

hold them in a loose, flexible network

121
Q

describe myosin II

A

dimer of 2 heavy chains, 4 light chains

122
Q

what are heavy chains

A

green structures w/ globular head domain; this part of protein binds to actin

123
Q

what do heavy chains have

A

long, flexible tails

124
Q

what do the long tails form

A

coiled-coil alpha helix

125
Q

how many heavy chains in every myosin II protein complex

A

2

126
Q

how many light chains,

A

2 diff copies –> 4

127
Q

where are light chains found

A

around neck/hinge region

128
Q

what happens when myosin II is active

A

globular head domain can change conformation forward & backward as it walks along filament

129
Q

what does neck/hinge region allow

A

flexibility

130
Q

what is the ‘business’ part of protein

A

heavy chains w/ actin binding region

131
Q

why are heavy chains the business region

A

it binds actin and converts chemical E into force

132
Q

what are light chains

A

regulatory chains

133
Q

what happens to light chains

A

they get phosphorylated or de-phosphorylated to tell myosin 2 to be active or inactive

134
Q

what do heavy chains of the dimer do

A

mediate formation of bipolar filaments (many copies of bundled myosin II dimers)

135
Q

what does regulatory light chain control

A

whether myosin 2 is just floating around cytoplasm as a dimer or polymerizing into larger assembly of bipolar filaments

136
Q

what are bipolar filaments

A

many copies of myosin II heavy chains coming together in a specific orientation

137
Q

what does phosphorylation of regulatory light chains control

A

whether they’re single or form a filament

138
Q

what do bipolar filaments have

A

mirror symmetry –> face each other (Helps generate force)

139
Q

how do bipolar filaments lead to contraction & shorten network

A

motors walk on opposite directions on filaments; single bipolar filaments move actin filaments in opposite directions

140
Q

what would happen if motors were all in same direction

A

filament would just slide left or right; filament would be moving not shortened

141
Q

describe inactive state of myosin II

A

regulatory light chains aren’t phosphorylated, myosin motor doesn’t bind actin, coiled coil is held in conformation

142
Q

basically what is inactive state

A

can’t bind actin or other copies of myosin 2 motor

143
Q

what are two upstream kinase that become activated

A

MLCK and ROCK

144
Q

how are kinases activated

A

rhoA activates ROCK/MLCK, phosphorylates reg. light chains to activate myosin 2 activity & myosin bipolar filament assembly

145
Q

what is the mechanism for contractility

A

something activates rhoA, rhoA activates ROCK/MLCK, triggers ^^ process leading to actomyosin contractility

146
Q

what to both MLCK and ROCK have in common

A

both phosphorylate light chains leading to actin binding site being available; myosin tail released & can assemble w/ other copies to form bipolar filaments

147
Q

why is myosin tail being released important

A

allows it to assemble other copies and form bipolar filaments

148
Q

how does myosin 2 generate force

A

couples ATP hydrolysis to conformational changes

149
Q

what does myosin-ATP cycle do

A

generates contractility important in generating forces and processes like cell migration & tissue shit

150
Q

where does cycle start

A

myosin in its nucleotide free form –> attached to actin filaments

151
Q

what comes next

A

ATP binds onto myosin head, induces release from actin

152
Q

what happens to ATP

A

hydrolyzed, triggers conformational change that advances myosin head one monomer forward

153
Q

now what is ATP in position to bind

A

the next actin monomer in the filament

154
Q

what’s the next step

A

inorganic phosphate is released (weak binding of head domain to new monomer)

155
Q

what does weak binding trigger

A

ADP release, and power stroke

156
Q

what is power stroke

A

where conformation goes backward, resetting to original position

157
Q

basically sum up how myosin 2 works

A

starts bound to filament, releases filament, moves forward, binds filament, as its bound to filament it undergoes powerstroke to pull filament one monomer to the left

158
Q

how many copies of myosin motor domain does bipolar filament have

A

100 copies

159
Q

what generates contractile forces in cell

A

100 copies of myosin motor domain in bipolar filament all simultaneously doing this in opposite directions

160
Q

what happens once ADP is released

A

back to nucleotide free state that is firmly anchored to filament waiting for next ATP to join

161
Q

what does contractility apply to

A

stress fibers, cell migration, nuclear movement –> anytime force generated in a cell

162
Q

what make actin interesting (structurally & functionally)

A

actin binding proteins

163
Q

what does arp2/3 form

A

branched filaments (lamellipodia)

164
Q

what does formin generate

A

single actin filaments that can be bundled

165
Q

what does myosin II bipolar filaments contract

A

contract stress fibers

166
Q

what activates myosin II

A

phosphorylation of myosin light chain

bio 211 (18 decks)

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