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ESA 2- Membranes and Receptors > Membrane Proteins > Flashcards

Membrane Proteins Flashcards

1
Q

What kinds of evidence is there for membrane proteins?

A

Functional, biochemical

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2
Q

What functional evidence is there for membrane proteins?

A

Membranes have specific function, and specific function is conferred by proteins

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3
Q

What specific functions do membranes have?

A

Facilitated diffusion
Ion gradients
Specificity of cell responses

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4
Q

What biochemical evidence is there for membrane proteins?

A

Membrane fractionation and gel electrophoresis

Freeze fracture

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5
Q

How is SDS-PAGE performed on the erythrocyte membrane?

A

Take RBC and spin to remove plasma
Put in hypertonic solution
Spin in centrifuge to give membrane pellet
Put membrane into SDS-PAGE

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6
Q

What happens when you put a membrane in a hypertonic solution?

A

The cell will burst, releasing haemoglobin, leaving only the membrane

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7
Q

Why does the bursting of the RBC leave only the membrane?

A

Because there are no organelles in a RBC

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8
Q

What colour is a RBC membrane pellet?

A

White

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9
Q

What is the white membrane pellet called?

A

Erythrocyte ghost

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10
Q

What will be seen when the erythrocyte ghost is put into SDS-PAGE?

A

Seperation will give a number of bands, each corresponding to proteins found in the erythrocyte membrane

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11
Q

How many major proteins are detected in the erythrocyte membrane?

A

10

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12
Q

What remains in the RBC membrane after a salt wash?

A

Only band 3 and 7

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13
Q

What does only bands 3 and 7 remaining after a salt wash mean?

A

That all but the proteins that produce band 3 and 7 are peripheral membrane proteins, and must be on cytoplasmic face

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14
Q

Why must the proteins corresponding to bands removed by the cytoplasmic face of the membrane?

A

Since they are susceptible to proteolysis only when the cytoplasmic face of the membrane is accessible

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15
Q

What do the proteins removed by the salt wash comprise?

A

The cytoplasmic skeleton

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16
Q

What are proteins 3 and 7?

A

Covalently attached carbohydrate units, therefore glycoproteins

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17
Q

What does the highly hydrophilic nature of the extracellular carbohydrate groups act to do?

A

Lock the orientation of the protein in the membrane by preventing flip-flop rotation

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18
Q

How is the freeze fracture technique carried out?

A

Freeze cell in ice

Fracture with knife

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19
Q

What happens when you fracture the frozen cell with a knife?

A

The ice crystal will break around the weakest point, which is between the two lamallae of the bilayer

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20
Q

What is the result of the ice crystal breaking between the two lamallae of the bilayer?

A

The fracture pulls the two lamellae apart, taking the proteins with one of the lamellae, producing the P and C face

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21
Q

What is the P face?

A

The lamellae next to cytosol

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22
Q

What is the C face?

A

The lamallae next to extracellular water

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23
Q

How is the freeze fracture used to visualise proteins?

A

You take the crystal, shadow at a long angle with osmium or some other electron dense metal, and so build up a ‘snow drift’ against anything sticking up, or in holes, that can then be visualised with an electron microscope

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24
Q

What does the fluid mosaic theory of membrane structure say?

A

That biological membranes are composed of lipid bilayer associated with membrane proteins

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25
Q

How can the lipid bilayer be associated with membrane proteins?

A

May be deeply embedded in bilayer

May be associated with surface

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26
Q

What is it called when proteins are deeply embedded in the bilayer?

A

Integral

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27
Q

What is called when proteins are associated with the surface?

A

Peripheral

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28
Q

How can proteins move in the bilayer?

A

Conformational change
Rotational
Lateral

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29
Q

What allows proteins to change conformation?

A

The fluid membrane

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30
Q

What does lateral diffusion allow?

A

Recruitment of a partner to perform a function

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31
Q

Can membrane proteins perform flip-flop?

A

No

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32
Q

Why can’t membrane proteins perform flip-flop?

A

It is energetically unacceptable to take a large hydrophilic protein molecule through the bilayer
Taking a big molecule such as protein through the bilayer would disrupt the structure of the bilayer, destroying ion gradients

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33
Q

How can movement of proteins in the bilayer be described?

A

As dynamic- happens all the time

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34
Q

What restricts protein mobility?

A

Aggregates
Tethering
Interactions with other cells

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35
Q

Why do aggregates restrict protein mobility?

A

It is more difficult to move if proteins are in aggregated structures

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36
Q

What can a protein be tethered to?

A

Something outside the cell

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37
Q

Can proteins move most of the time?

A

Yes

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38
Q

When is a membrane protein fixed?

A

When involved in a specific function, such as synapse or cell-cell or cell-basement membrane interactions

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39
Q

What restraints on mobility are there?

A

Lipid mediated effects
Membrane protein associations
Association with extra-membraneous proteins (peripheral proteins)

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40
Q

How do lipid mediated effects impact mobility?

A

Proteins tend to separate out into the fluid phase, or cholesterol poor regions

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41
Q

What does cholesterol determine?

A

How proteins may be segregating in the membrane

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42
Q

What kind of regions are more fluid?

A

Cholesterol poor

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43
Q

What often clusters in high cholesterol regions?

A

Signalling proteins

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44
Q

Why do signalling proteins often cluster in high cholesterol regions?

A

Because they need to stay in one specific place

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45
Q

What extra-membranous proteins can restrain mobility?

A

The cytoskeleton

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46
Q

What are the types of membrane proteins?

A

Peripheral

Integral

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47
Q

How are peripheral membranes related to the membrane?

A

They are associated with the membrane- bound to the surace- but not inside

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48
Q

How are peripheral proteins bound to the surface?

A

By electrostatic and hydrogen bond interactions

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49
Q

How are peripheral membrane proteins removed?

A

By changes in pH or in ionic strength- they can be washed off by a high salt solution

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50
Q

How do integral proteins interact with the membrane?

A

They interact extensively with hydrophilic domains of the lipid bilayer

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51
Q

Can integral proteins be removed by manipulation of pH or ionic strength?

A

No

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52
Q

How are integral proteins removed?

A

By agents that compete for non-polar interactions, e.g. detergents and organic solvents

53
Q

What is often true of the R groups of amino acid residues in transmembrane domains of proteins?

A

They are largely hydrophilic, small or polar, uncharged

54
Q

What structure do transmembrane domains of proteins often have?

A

α-helical

55
Q

How many amino acids often make up transmembrane domains?

A

18-22

56
Q

What do hydropathy plots tell you?

A

How hydrophilic an amino sequence is

57
Q

What can hydropathy plots be used to determine?

A

The transmembrane domains

58
Q

How do hydropathy plots work?

A

Takes the first 20 amino acids, and gives them a score as to how many of the amino acids are hydrophilic.
It then moves down one amino acid, and takes another score

59
Q

What would you expect the hydropathy score of a transmembrane domain to be?

A

Very hydrophilic

60
Q

How are proteins always orientated?

A

One way or another, so specific parts of the proteins are on the inside and outside

61
Q

Can proteins be reversed in the membrane?

A

No

62
Q

What is asymmetrical orientation of proteins in biological membranes important for?

A

Function

63
Q

Give an example of why asymmetrical orientation of proteins in biological membranes is important for function?

A

E.g. a receptor for hydrophilic extracellular messenger molecules must have a recognition site directed towards the extracellular space to function

64
Q

How can proteins be associated in bilayers?

A

Peripheral protein associations
Single or multiple transmembrane domains
Post-translational lipid modification
Dolichol phosphate-linked polypeptides

65
Q

How do peripheral protein assocations work?

A

Protein A is attached to protein B, which is peripherally associated with the bilayer. Protein A is therefore not directly attached to the bilayer, but protein B is

66
Q

What happens when proteins have multiple transmembrane domains?

A

They have more than one part of the protein incorporated into the bilayer

67
Q

Give two examples of posttranslational lipid modification?

A

Mystroylation

Palmitoylation

68
Q

How can post-translational lipid modification associate proteins with the bilayer?

A

Lipid molecules attached to the protein can anchor it into the bilayer

69
Q

Are proteins attached by post-translational lipid modification attached solely using lipids?

A

They can be, or can be attached using both transmembrane domains and lipids

70
Q

What is the result when a protein is attached using transmembrane domains and lipids?

A

The protein is further restricted back into the membrane, as not only is the α-helical domain locking it into the membrane, but also a fatty acid

71
Q

What happens with dolichol phosphate-linked polypeptides?

A

A lipid is integrated into the bilayer. That lipid is attached to a carbohydrate via a phoshphate. The carbohydrate is attached to the polypeptide, via another phosphate on the other end

72
Q

What is the erythrocyte cytoskeleton made up of?

A

A network spectrin and actin molecules

73
Q

What kind of protein is spectrin?

A

Fibrous

74
Q

What are the subunits of spectrin?

A

α and ß

75
Q

Describe the structure of spectrin?

A

Long, floppy, rod-like molecule

76
Q

How is the final structure of spectrin formed?

A

α and ß subunits wind together to form an antiparallel heterodimer
Two heterodimers form head to head associations, to form a heterotetramer of α 2 ß 2
These rods are crosslinked into networks by short actin protofilaments and band 4.1 and adducin molecules
These form interactions towards the end of the spectrin rods

77
Q

What is the short actin protofilament made up of?

A

~14 actin monomers

78
Q

What does spectrin form?

A

A lattice, cage like structure

79
Q

What happens to the cage like spectrin structure?

A

It is grafted onto the inside of the RBC membrane

80
Q

How is the spectrin cage attached to the membrane?

A

Through adapter proteins

81
Q

How is spectrin linked to band 3 protein?

A

Via ankyrin

82
Q

How is spectrin linked to glycophorin A?

A

Via band 4.1

83
Q

What is the effect of attachment of integral membrane proteins to the cytoskeleton?

A

It restricts the lateral mobility of the membane protein

84
Q

What is the erythrocyte cytoskeleton important for?

A

Maintaining deformability

85
Q

Why is deformability necessary for erythrocytes?

A

So they can make their passage through capillary beds without lysis

86
Q

Give 3 types of haemolytic anaemias

A

Hereditary spherocytosis
Hereditary Elliptocytosis
Ankyrin defects

87
Q

Is hereditary spherocytosis a dominant or recessive disease?

A

Dominant

88
Q

What is the defect inhereditary spherocytosis?

A

Spectrin is depleted by 40-50%, and so there is a reduced cage-like structure

89
Q

What is the result ofhereditary spherocytosis?

A

Erythrocytes round up, losing their biconcave shape, and so are less resistant to lysis

90
Q

What doeshereditary spherocytosis result in?

A

Anaemia

91
Q

Why doeshereditary spherocytosis result in anaemia?

A

Because the rounded cells get stuck in the capillary, and the plasma rushing past rips the cells apart. The cell fragments are then removed by the spleen, and so the RBC’s have a shortened in vivo survival time.
The bone marrow is unable to compensate for the reduced life span

92
Q

What is the result of anaemia?

A

Reduced ability to carry oxygen, so patients are constantly tired and weak

93
Q

What is the treatment forhereditary spherocytosis?

A

Blood transfusions ~every 100 days

94
Q

What is Hereditary Elliptocytosis a defect in?

A

Spectrin molecules- they are present, but don’t assemble the end-to-end structure as well

95
Q

What is a result of spectrin being less able to form their end-to-end structure inHereditary Elliptocytosis?

A

They are unable to form heterotetramers, and so they form fragile, rugby ball (elliptoid) shaped RBCs

96
Q

How are haemolytic anaemias treated?

A

Cytochalasin drugs

97
Q

What do cytochalasin drugs do?

A

Cap the growing end of polymerising actin filaments, and so can alter the deformity of the erythrocyte

98
Q

How are membrane proteins and those to be secreted or targeted to lysosomes synthesised?

A

Against the mRNA template by the ribosomes

99
Q

What are proteins sometimes produced with?

A

A leader sequence of hydrophobic amino acids

100
Q

What is the leader sequence of a protein?

A

18-30 amino acids with a number of basic residues at the N-terminus

101
Q

What happens to the leader sequence as it emerges from the ribosome?

A

It is recognised by a signal recognition particle

102
Q

What is a SRP?

A

A large protein/RNA complex

103
Q

What does the SRP do?

A

Grabs the leader sequence and the ribosome and locks them together

104
Q

What is the result of the SRP locking of the leader sequence and the ribosome?

A

Prevents anything from happening until it reaches the ER

105
Q

What recognises the SRP?

A

A docking protein/SRP receptor

106
Q

What happens when the SRP is recognised by a docking protein?

A

It brings the ribosome down onto the ER
In making the interaction with the docking protein, the SRP is released from the signal sequence of the nascent polypeptide

107
Q

What is the result of the release of the SRP from the polypeptide?

A

Removes the inhibitory constraint on further translation

108
Q

What is the leader sequence recognised by once in ER?

A

The signal sequence receptor (SSR) within a translocator complex (Sec61) in the ER

109
Q

What does the SSR do?

A

Takes the signal and starts feeding through into lumen of ER

110
Q

Where is the ribosome as the protein is translated?

A

Sitting on the ER, making the protein into the lumen, anchored to a pore complex through which the glowing polypeptide chain is extruded

111
Q

When does protein synthesis into the ER cease?

A

When the C-terminal is released from the ribosome

112
Q

What happens in the case of a secreted or lysosomal protein?

A

When synthesis is completed, the protein is translocated into the lumen of the ER

113
Q

How is a protein synthesised when it needs to remain in the membrane?

A

As the protein is synthesised, if a stop-transfer signal is found, it sticks in the membrane

114
Q

What is a stop-transfer signal?

A

A highly hydrophobic primary sequence followed directly by charged amino acids, which, in alpha-helical form, is long enough to span the hydrophobic core of the bilayer

115
Q

What does the stop-transfer sequence from?

A

The transmembranous region of the protein

116
Q

Why does the stop-transfer sequence form the transmembranous region of the protein?

A

Because its thermodynamically happier to be inside the hydrophobic domain of the bilayer

117
Q

How is a membrane protein released from the protein translocator into the lipid bilayer?

A

A lateral gating mechanism

118
Q

What happens as the ribosomes continues to synthesise a membrane protein after the stop-transfer signal?

A

It gets pushed away from the membrane, and continues to synthesise the protein in the cytoplasm

119
Q

What is the result of the synthesis of a protein with a stop-transfer signal?

A

A transmembrane protein with it’s N-terminal directly into the lumen and it’s C-terminal to the cytoplasm

120
Q

What happens to both secretory and membrane incorporated proteins?

A

The signal sequence if cleaved from the new protein by signal peptidases

121
Q

When is the signal sequence cleaved from the new protein?

A

Even before protein synthesis is completed

122
Q

What is likely when a protein has multiple transmembrane domains?

A

That the folding of the nascent protein against the constraint of the first transmembrane segment is the driving force for insertion of other domains

123
Q

What possibly controls membrane insertion when there are multiple spanning transmembrane domains?

A

A series of start- and stop-transfer sequences within the primary structure

124
Q

What assists in stabilising the partially folded growing polypeptide when there are multiple transmembrane?

A

The association of luminal binding proteins (e.g. BiP) related to the family of heat-shock (chaperone) proteins

125
Q

In what manner are transmembrane domains added?

A

In pairs

126
Q

How are nascent chains further processed?

A

As they pass from the ER and through the cis to trans Golgi

127
Q

When does a new protein continue along the secretory pathway until?

A

Secretory vesicles fuse with the plasma membrane

128
Q

What happens when secretory vesicles fuse with the plasma membrane?

A

Secreted proteins are released from the cell, and membrane proteins are delivered such that regions of the protein that were located in the cytoplasm during synthesis remain with this orientation

129
Q

Why are specific carbohydrate groups on membrane proteins important?

A

May be important for cellular recognition to allow tissues to form an immune recognition

ESA 2- Membranes and Receptors (13 decks)

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