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BIOL 2321 - Cell Biology > Signalling > Flashcards

Signalling Flashcards

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1
Q

What is signalling?

A

How a single cell responds to environmental changes

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2
Q

How does an extracellular signal affect cells?

A

the signal will not enter the cell, but it will induce changes within the cell

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3
Q

T or F: different signals will cause different cellular responses

A

true

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4
Q

What happens to a cell if there’s no incoming signals?

A

usually apoptosis

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5
Q

T or F: the same signal can cause different effects in different cells

A

true

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6
Q

What is an example of a signal?

A

neurotransmitters

hormones

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7
Q

What are the 2 basic ideas of how a cell can be changed?

A
  1. fast

2. slow

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8
Q

Describe the fast way to induce intracellular change

A

This occurs when extracellular signals change proteins that already exist

this results in a fast response

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9
Q

Describe the slow way to induce intracellular change

A

This occurs when extracellular signals alter gene expression

results in a slow response

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10
Q

What are the 3 basic steps of signalling pathways?

A
  1. a signalling cell sends the first messenger (signal ligand) to bind to a receptor on the plasma membrane of a second cell
  2. on the 2nd cell, a conformational change
    sends the signal across the membrane to the cytoplasmic domain
  3. one of two possible routes:
    a. cytoplasmic signal cascade
    b. a second messenger
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11
Q

What is the first messenger?

A

a signal ligand

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12
Q

What are the two possible routes that follow a conformational change causing a signal to be transduced across the membrane?

A

a cytoplasmic signal cascade

a second messenger

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13
Q

What generates the second messenger?

A

nearby effector enzymes in the cytoplasm

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14
Q

Where can the second messengers be located?

A

they can stay in the membrane or they can diffuse through the cytoplasm

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15
Q

What does the second messenger usually do?

A

binds and triggers cytoplasmic signalling (ex. a signal cascade)

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16
Q

What will the cytoplasmic domain of the receptor cause?

A

a change in a target protein (ex. phosphorylation)

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17
Q

What happens when a target protein changes conformation?

A

it will change the conformation of another downstream protein (either activating or inactivating it) - this is the signal cascade

the signal cascade will continue until the final protein influences cellular processes

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18
Q

When will the cytoplasmic signal cascade stop?

A

When the final protein influences cellular processes

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19
Q

What alters the conformations of signalling proteins?

A

kinases and phosphatases

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20
Q

What is the result of kinase and phosphatase altering the conformation of signalling proteins?

A

either the inactivation or activation within the cytoplasmic signal cascade

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21
Q

Approximately how many cellular proteins are subject to phosphorylation-dephosphorylation?

A

~1/3

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22
Q

What does kinase do?

A

phosphorylates proteins

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23
Q

What does phosphatase do?

A

dephosphorylates proteins

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24
Q

What are the 2 common types of receptors?

A

G-Protein-Coupled Receptors (GPCR)

Enzyme-coupled receptors (specifically RTKs)

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25
Q

What are GPCRs coupled to?

A

G-proteins

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26
Q

What is the largest superfamily of proteins encoded by animal genomes?

A

GPCRs

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27
Q

What is a GPCR?

A

a 7-pass transmembrane proteins

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28
Q

Where on a GPCR does a ligand bind? What does this cause?

A

binds to the extracellular side causing conformational changes on the cytosolic side

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29
Q

What side of the membrane will conformational changes occur after the ligand binds to a GPCR?

A

cytosolic side

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30
Q

How many times does GPCR pass through the membrane?

A

7

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31
Q

Describe the structure of GPCR

A

7-pass transmembrane protein

extracellular: 3 loops and the N-terminus bind the ligand
intracellular: 3 loops and the C-terminus bind to an associated G-protein

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32
Q

What part of a GPCR binds the ligand? What side of the membrane does this occur on?

A

the 3 loops and the N-terminal on the extracellular side

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33
Q

What part of a GPCR binds the G-protein? What side of the membrane does this occur on?

A

3 loops and the C-terminus on the intracellular side

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34
Q

How many subunits do the G-proteins that bind to GPCRs have? are they the same or different subunits? What does this make G-proteins?

A

they have 3 different subunits

therefore they are heterotrimeric

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35
Q

What are the 3 different subunits of G-proteins?

A

alpha
beta
gamma

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36
Q

In the GPCR pathway, how does the trimer G-protein start?

A

bound to GDP and inactive

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37
Q

What activates the G-proteins involved in the GPCR pathway?

A

ligand binding (a signal molecule) to the GPCR causes a conformational change in the GPCR that exchanges the GDP on GDP-bound G-protein for a GTP –> activating the G-protein

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38
Q

What happens when the G-protein is activated?

A

G-protein bound to GTP dissociates

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39
Q

How does the G-protein dissociate when it becomes active?

A

the beta and gamma subunits separate from the alpha subunit

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40
Q

Which subunits of the trimeric G-protein are lipid bound?

A

alpha and gamma

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41
Q

What happens after the G-protein dissociates?

A

Depending on the pathway, either activated beta and gamma will activate an effector

or

activated GTP-bound alpha will activate an effector

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42
Q

Where is the effector molecule located?

A

it is embedded in the membrane

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43
Q

Which effector molecule will we focus on for the GPCR pathway?

A

adenylyl cyclase

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44
Q

What happens when adenylyl cyclase (effector) is activated by GTP-bound alpha subunit?

A

adenylyl cyclase converts ATP into cAMP

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45
Q

What does adenylyl cyclase convert ATP into? What causes this to happen?

A

Adenylyl cyclase converts ATP into cAMP

happens when adenylyl cyclase is activated by the GTP-bound alpha subunit

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46
Q

What does cAMP stand for?

A

Cyclic AMP (Adenosine MonoPhosphate)

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47
Q

BRIEFLY state what the reaction of converting ATP into cAMP involves

A

removing 2 phosphates from ATP

cyclisizing the remaining phosphate to the ribose sugar

48
Q

What happens when adenylyl cyclase is activated?

A

the GTP bound to the alpha subunit of the G-protein will be hydrolyzed to GDP, turning the G-protein off

49
Q

What happens when the G-protein is turned off?

A

the alpha subunit again has affinity for the beta and gamma subunits

50
Q

What does the alpha subunit of the G-protein regaining affinity for the beta and gamma subunits cause?

A

alpha subunit of the G-protein dissociates from the adenlyl cyclase and binds to the beta and gamma subunits to reform the G-protein complex and the cycle can begin again

51
Q

What does cAMP activate?

A

Protein Kinase A (PKA)

52
Q

How does cAMP activate PKA?

A

cAMP binds to the PKA regulatory subunits of the PKA tetramer which causes them to dissociate from the catalytic subunits

the free catalytic subunits can now go around and phosphorylate things

53
Q

Where on the PKA does cAMP bind to?

A

on the PKA regulatory subunits of the PKA tetramer

54
Q

What is released from PKA when cAMP binds?

A

the catalytic subunits of the PKA tetramer are released to go phosphorylate a bunch of stuff

55
Q

Give some examples of things that can be phosphorylated after cAMP activates PKA

A

triglyceride lipase –> fatty acid formation

glycogen synthase –> glycogen formation

Nucleus –> DNA synthesis, differentiation, RNA synthesis

ER –> protein synthesis

phosphorylase –> glycogen breakdown

Microtubules –> assembly/disassembly

56
Q

Describe an example of what happens when the active PKA travels into the nucleus

A

it will phosphorylate the transcription factor CREB, allowing it to bind to the promoter and help in transcription initiation

57
Q

Describe the effect of signal amplification in regards to the GPCR pathway

A

a SINGLE ligand bound to a GPCR can activate SEVERAL G-protein complexes

Each G-protein complex activates an adenylyl cyclase

Each cAMP molecule activates several kinases

Each kinase activates several targets in each step of the cascade

58
Q

T or F: a large number of ligand molecules is required to produce a large cellular response

A

FALSE!

a small number of ligands can create a large effect

59
Q

What is it called when the GPCR signal is stopped?

A

Desensitization

60
Q

What is one way the GPCR cycle is stopped?

A

When a GRK (kinase) phosphorylates the GPCR and an arrestin causes endocytosis of the receptor to stop the signal

61
Q

What does GRK do?

A

it phosphorylates the GPCR when the cycle needs to be stopped

62
Q

What does the phosphorylation of the GPCR by GRK cause?

A

A protein called arrestin comes in to cause endocytosis of the GPCR and arrest the signal

63
Q

Describe arrestin

A

A protein that is recruited when GPCR is phosphorylated to stop the signal and end the cycle by causing endocytosis of the GPCR

64
Q

What does arrestin cause?

A

endocytosis of the GPCR and the stop of the signal

65
Q

What most commonly happens to the GPCR after it is endocytosed?

A

it will dissociate from the ligand in the endosome and return to the plasma membrane after a few minutes

66
Q

How can a signal be stopped for a longer time?

A

the GPCR can be degraded in the lysosome

67
Q

What does RTK stand for?

A

Receptor Tyrosine Kinases

68
Q

What are RTKs?

A

enzyme-coupled receptors

69
Q

What structure do RTKs have when they are inactive?

A

Monomers

70
Q

What structure do RTKs have when they are active?

A

dimer

71
Q

What are the 2 ways an RTK can become active?

A

a joint ligand can bind to the EC side of two inactive monomers and the monomers bind to form a dimer

OR

a single ligand can bind to one inactive monomer and a separate single ligand can bind to another inactive monomer, then the 2 monomers can bind to form a dimer

72
Q

What side do the ligands bind to RTKs?

A

the extracellular side

73
Q

Describe RTK dimerization

A

A ligand binds to the EC side of an RTK monomer and 2 monomers dimerize

74
Q

What is the result of RTK dimerization?

A

The tyrosine kinase domains on the cytosolic sides of each monomer are brought close enough together that the kinase domains can trans-phosphorylate the tyrosine on the opposite monomer

75
Q

Which amino acid and which enzyme are involved with RTKs?

A

amino acid = tyrosine

enzyme = kinase

76
Q

What side of the monomeric RTKs are the tyrosine kinase domains?

A

cytosolic

77
Q

What is the function of the kinase domain on RTKs?

A

When close enough together, the kinase domains will ‘trans-auto-phosphorylate’ a tyrosine on the opposite monomeric RTK

78
Q

When does trans-auto-phosphorylation of RTK occur?

A

After dimerization when the tyrosine kinase domains are close together

79
Q

What is trans-auto-phosphorylated?

A

a tyrosine amino acid on the opposite monomeric RTK of a dimer

80
Q

What enzymatic activity does RTK have when it’s active?

A

kinase activity

81
Q

T or F: trans-auto-phosphorylation only occurs once per RTK dimer

A

False

many phosphates can be added on the cytosolic domains

82
Q

Roughly how many human RTKs exist?

A

60

83
Q

What are many RTK ligands involved in? What consequence does this have? Explain

A

growth

consequence: cancer

they can have mutations such as RTKs in cancer cells being fixed as a dimer and always active –> always promoting growth

84
Q

What are many RTK ligands referred to as? give examples

A

growth factors

ex. 
epidermal growth factor
nerve growth factor
fibroblast growth factor
insulin
85
Q

T or F: insulin receptor is an RTK

A

true

86
Q

What happens after an RTK has been trans-auto-phosphorylated in the RTK pathway?

A

the phosphorylated kinase domains continue to phosphorylate other cytoplasmic tyrosine until there are a lot of P sites

87
Q

What is the purpose of creating many P sites in the RTK pathway?

A

they serve as docking sites for other proteins like kinases, transcription factors, 2nd messengers, etc.

88
Q

What is the P site?

A

Phosphorylated tyrosine

89
Q

What kind of proteins can use the P sites as docking sites?

A

kinases
transcription factors
2nd messengers

90
Q

What is the purpose of other proteins (like transcription factors or 2nd messengers) docking to the phosphorylated and dimerized RTK?

A

they will carry the signal forward

91
Q

What are two examples of things that the RTK pathway regulates?

A

transcription

calcium release

92
Q

Why is it strictly called a Receptor Tyrosine Kinase and not a Tyrosine Kinase Receptor?

A

tyrosine kinase receptor suggests that it’s a receptor that binds tyrosine kinases (recall that an insulin receptor would bind insulin)

but this is NOT what happens here

a LIGAND binds to the enzyme-coupled receptor (RTK) and the RTK has tyrosine and kinase domains already

93
Q

Describe phospholipase C

A

A membrane-bound enzyme that cleaves phospholipids at a specific site when it is activated

94
Q

How is PLC activated?

A

by specific GPCRs or RTKs

95
Q

In a GPCR pathway, how can a PLC function?

A

as the effector instead of adenylyl cyclase

96
Q

In an RTK pathway, how can PLC function?

A

it can dock to one of the phosphorylated tyrosines

97
Q

What kind of phospholipids will PLC cleave once activated?

A

specific membrane phospholipids with inositol headgroups called PIP2

98
Q

What is inositol?

A

alcohol-sugar

99
Q

What is the key inner leaflet signalling lipid?

A

phosphatidylinositol-4,5-biphosphate (aka PIP2)

100
Q

What does PIP2 stand for?

A

Phosphatidylinositol-4,5-biphosphate

101
Q

How many molecules does PLC cleave PIP2 into? what are they?

A

2 molecules

inositol triphosphate (IP3)

diacylglycerol (DAG)

102
Q

What does IP3 stand for? How is it formed?

A

Inositol triphosphate

one of the products of PIP2 being cleaved by PLC

103
Q

What does DAG stand for? how is it formed?

A

diacylglycerol

one of the products formed when PIP2 is cleaved by PLC

104
Q

What are IP3 and DAG?

A

they are second messengers used in signalling

105
Q

Where is DAG located when formed?

A

it is embedded in the membrane

106
Q

What is the function of DAG?

A

activates a kinase cascade (Protein Kinase C)

107
Q

Where is IP3 located once formed?

A

IP3 diffuses through the cytoplasm

108
Q

What is the function of IP3?

A

when it diffuses through the cytoplasm it binds to ligand-gated Ca2+ channels in the sER causing the release of Ca2+ into the cytosol

109
Q

T or F: PIP2 is made in advance

A

true

110
Q

Where is PIP2 located on the membrane?

A

on the inner leaflet of the membrane

111
Q

In the GPCR pathway, how is PLC activated?

A

GPCR is first activated when a ligand binds

then PLC is activated by the G-protein

112
Q

What activates PLC in the GPCR pathway?

A

a G-protein

113
Q

What happens when the PLC is activated?

A

PIP2 is cleaved into DAG and IP3

DAG activates a kinase cascade

IP3 diffuses through the cytoplasm and binds to ligand-gated Ca2+ channels to release Ca2+

114
Q

What does IP3 bind to in the cytoplasm?

A

ligand-gated Ca2+ channels in the sER

115
Q

What kind of processes occur because of PLC?

A

Ca2+ regulated processes (ex. exocytosis, cell cycle changes)

116
Q

T or F: Ca2+ is a second messenger

A

true

117
Q

What happens when intracellular Ca2+ is released as a result of PLC?

A

the activation of Protein Kinase C that is bound to DAG

BIOL 2321 - Cell Biology (26 decks)

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