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Phys-Pro - section 2 > Protein Secretion > Flashcards

Protein Secretion Flashcards

1
Q

Translocation =

A

transport of proteins into or across a membrane.

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2
Q

Secretion =

A

transport of proteins to the extracellular medium, into another cell, or to the bacterial cell surface.

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3
Q

Export =

A

translocation of proteins into the periplasm in Gram-negative bacteria

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4
Q

What are 2 systems used to translocate proteins across the cytoplasmic membrane?

A
  1. Sec system

2. Twin-arginine translocation (TAT) system

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5
Q

What does the Sec system translocate?

A

unfolded proteins

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6
Q

What is the job of SecYEG?

A

is the protein channel through which proteins exit

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7
Q

What is the role of SecA?

A

a cytoplasmic ATPase

-hydrolyzes ATP to provide energy

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8
Q

What is the role of SecB? What is its structure?

A
  1. a chaperone protein.
  2. binds to nascent preproteins to prevent premature folding and aggregation of these proteins.
  3. brings and delivers the preproteins to SecA
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9
Q

What components make up Sec translocase?

A

SecYEG and SecA

  • SecA joins SecYEG during protein translocation
  • SecA provides the energy by hydrolyzing ATP
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10
Q

Where is the signal (leader) sequence found?

A

At the N terminal on the protein to be translocated

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11
Q

What removes the signal peptide? when?

A

removed by signal peptidase during or after protein translocation

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12
Q

What is a protein with a leader sequence called?

A

A preprotein

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13
Q

What is a protein without a leader sequence called?

A

mature protein

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14
Q

What are the three regions of the signal sequence and what do they accomplish?

A
  1. an N-terminal basic region (binds to phospholipids)
  2. a central hydrophobic region (inserts into the membrane)
  3. a C-terminal uncharged region (signal peptidase recognition and cleavage site)
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15
Q

Describe the 6 steps in the proposed Sec transport system

A
  1. SecA binds to SecYEG to form the Sec translocase.
  2. SecB delivers preprotein to SecA, SecB may be released at this point (job is done)
  3. After binding to preprotein, SecA undergoes conformational changes and binds an ATP.
  4. Leader peptide binds the membrane with its N-terminal sequence and the central region inserts into the membrane forming a loop leading the preprotein to enter the translocase.
  5. ATP is hydrolyzed resulting in the release of preprotein from SecA. SecA leaves the membrane.
  6. The rest of the protein is translocated across the membrane by ΔP. Leader sequence is cleaved by a leader peptidase (SPaseI).
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16
Q

What is co-translational translocation?

A

When proteins are translocated across the membrane DURING translation

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17
Q

Where was co-translational translocation first discovered? what was it used for?

A

In eukaryotes for proteins destined for cell membrane, secretory vesicles, and lysosomes

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18
Q

In prokaryotes, what kinds of proteins are translocated by the co-translational model?

A

inner membrane proteins that do not have extensive periplasmic loops

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19
Q

What is SRP?

A

Signal recognition peptide

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20
Q

What is the role of the SRP in the protein translocation ? (3 steps)

A
  1. SRP binds to the signal sequence (non-cleavable) of the protein as it emerges from the ribosome.
  2. The complex (SRP/nascent protein/ribosome) binds to a membrane receptor (FtsY) and then to the Sec translocase.
  3. Translation continues at the membrane and the protein threads through the translocase as it is being synthesized
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21
Q

Give 3 features of the Twin-Arginine Translocation (TAT) system

A
  1. Sec-independent
  2. Post-translational
  3. ∆P driven
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22
Q

What does the TAT system translocate?

A

Translocates folded proteins

-such as cofactor-bound redox proteins and inner membrane proteins.

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23
Q

What feature is contained within the N-terminal signal peptide?

A

a highly conserved twin-arginine motif (SRRXFLK).

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24
Q

When is the signal peptide cleaved?

A

during translocation

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25
Q

The TAT translocase is composed of at least ____ integral membrane proteins.

A

4

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26
Q

How many types of secretion systems are there?

A

3

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27
Q

What is the main feature of type I secretion systems

A

Secretion of proteins in a single step without stable periplasmic intermediates

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28
Q

What are the 3 binding proteins involved in the Type I system ?

A
  1. ATP binding cassette protein
  2. Membrane fusion protein
  3. Outer membrane protein
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29
Q

What is the role of the ATP binding cassette protein?

A

recognizes substrate and secretion signal

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30
Q

What is the role of the membrane fusion protein ?

A

Forms links between inner (where it is anchored) and outer membrane assembly.

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31
Q

What is the role of the outer membrane protein (OMP)

A

form a channel in the outer membrane

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32
Q

Substrates have a ___terminal secretion signal sequence

A

C

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33
Q

Is the C-terminal secretion signal cleaved or not?

A

No

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34
Q

What is the composition of the secretion signal?

A

conserved glycine rich repeat

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35
Q

What are examples of substrates with a C-terminal secretion signal secreted by Type I system?

A

toxins and enzymes

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36
Q

Is type I Sec dependent or independent?

A

Independent

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37
Q

What enzyme is secreted by uropathogenic and enterohemorrhagic E. coli.

A

Hemolysin

38
Q

What conduit does is used for hemolysin secretion?

A

The HlyD-TolC conduit

39
Q

What are the 2 steps involved in TIISS?

A
  1. Use the Sec-dependent pathway

2. Enter the TIISS pathway

40
Q

What are the 3 integral parts of the T2SS system? What does it span?

A
  1. Inner membrane ATPase
  2. Pseudopilus
  3. Homo-oligomeric protein pore in OM

Thought to span the entire Gram neg cell envelope

41
Q

What is the role of the ATPase in T2SS?

A

provides ATP hydrolysis believed to drive the secretion process.

42
Q

What is the role of the pseudopilus in T2SS?

A

act as a ‘piston’ to push protein substrates out.

-Extension/retraction is dependent on energy from ATP hydrolysis

43
Q

How do the protein substrates reach the T2SS?

A

By using the Sec system to reach the periplasm

44
Q

What kinds of features do proteins being excreted using the T2SS system have?

A
  • they are folded (in periplasm)
  • rich ß pleated sheet
  • have a secretion signal (not well understood)
45
Q

What are examples of proteins secreted by the T2SS system?

A

Cholera toxin, phospholipases, proteases, chitinases

46
Q

Describe how the T2SS system works (4 steps)

A
  1. Protein transported to periplasm by Sec system
  2. Protein folded in periplasm
  3. Pseudopilus is formed via energy from ATP hydrolysis
  4. Substrate protein is “pushed out” by the pseudopilius through the outer membrane pore
47
Q

What is T3SS used for and by?

A

Used by Gram-negative bacteria to secret/inject virulence proteins into the cytosol of a host cell

48
Q

What is the multi-unit secretory system called in T3SS ? What does it act as?

A

The injectisome or needle complex

-acts as a translocon

49
Q

What is carried through the T3SS system?

A

Unfolded proteins

50
Q

Where are the proteins carried in the T3SS system?

A

across three biological membranes (IM, OM, HostM)

51
Q

Do the proteins secreted via T3SS have a leader sequence?

A

no

52
Q

are the proteins secreted via T3SS sec-dependent or independent?

A

independent

53
Q

How is secretion initiated through T3SS?

A

Upon receiving an activation signal that signals bacterium adherence to target cell

54
Q

T3SS can also be used to alter the host cell’s….

A

cytoskeletal structure (e.g. actin rearrangement) resulting in engulfment of the bacteria

55
Q

What group of proteins are involved in the T3SS system?

A

The Yop proteins

56
Q

What is Yop N?

A

the plug on the Ysc channel on the bacteria?

57
Q

What do YopB, YopD, and LcrV do?

A

injected through the needle where they form a translocon on the target cell membrane

58
Q

What is are the 2 roles of T3S chaperones (T3SC)

A

Small proteins that interact and stabilize (in some cases) type III effector proteins.

Involved in maintaining type III effectors in a partially unfolded state

59
Q

T3SC often form

A

dimers

60
Q

Where are the T3SC found?

A

Exclusively in the host cell, they are not translocated to the target cell

61
Q

What is the role of T4SS?

A

To deliver DNA and proteins to another cell

  • either eukaryotic, a bacterial cell
  • can also deliver it to the extracellular medium
62
Q

What are the three classes of T4SS?

A
  1. conjugative machines (like F-pili)
  2. DNA uptake and release
  3. Effector secretion
63
Q

What is a T-DNA?

A

(Transfer DNA) is a plasmid that carries genes for opines and phytohormones

64
Q

What does the secretion system consist of in T4SS?

A

a channel and a pilus

65
Q

In effector secretion, what is the role of the effector proteins? Where do they go?

A

Effector proteins are also transported in addition to the DNA.

Effectors aid the T-DNA to move to the nucleus and integrate into the plant chromosome.

66
Q

What is the role of the T4SS in Bordetella pertussis? What is excreted and where? (4 points)

A
  1. pertussis toxin is exported to the periplasm by the Sec system
  2. assembled into holotoxin
  3. moved into the secretion apparatus
  4. secreted into the extracellular medium
67
Q

Has DNA been excreted by B. pertussis?

A

not been shown to do so

68
Q

How does Agrobacterium cause the tumourous growth of plant crown gall

A

The T-DNA is incorporated into the plant genome

  • Causes the plant to produce phytohormone
  • Results in unregulated growth
  • The synthesis of the opine is used as nutrient for the bacteria
69
Q

What is the T5SS also called? Why?

A

The autotransporter pathway because the proteins transport themselves across te outer membrane

70
Q

What 3 domains do the proteins excreted through the T5SS have?

A
  1. N-terminal signal sequence
  2. Integral passenger domain
  3. C-terminal helper domain
71
Q

How do the proteins reach the T5SS

A

exported to the periplasm by the Sec system using the N-terminal signal sequence

72
Q

How do the proteins pass through the T5SS?

A
  1. The “helper” domain inserts into the OM and forms a pore
  2. The passenger domain travels through the pore to the outside of the cell.
    - may or may not be cleaved from the helper domain
73
Q

What important B. pertussis virulence factor is secreted through a T5SS? What does it do?

A

Pertactin. Allows the B. pertussis to attach to ciliated tracheal cells

74
Q

Is T5SS sec dependent or independent

A

Sec dependent

75
Q

What is the T6SS? In what bacteria is it found?

A

T6SS apparatus is structurally similar to T4 phage tail

Widely distributed in gram negative bacteria

76
Q

What two proteins are integral to the functioning of T6SS?

A

Hcp and VgrG

77
Q

What is Hcp?

A

hemolysin co-regulated protein

78
Q

What is VgrG?

A

valine glycine repeat protein G

79
Q

In Pseudomonas, the T6SS is used to deliver effector proteins to the target cell ___?

A

Periplasm

80
Q

What are the 2 effector proteins called and what do they do?

A

Effectors, e1 and e3, are peptidoglycan hydrolyases.

-Recipient bacterial cells are lyzed by the action of e1 and e3.

81
Q

Why are donor bacteria immune to e1 and e3?

A

Because they produce immunity proteins (i1 and i3)

82
Q

When is T6SS delivery of bacteriolytic effector proteins important ?

A

niche competition in natural environments and in polymicrobial infections

83
Q

Improperly (mis)folded proteins are highly susceptible to degradation by:

A

Proteases

84
Q

Accumulation of misfolded proteins in the periplasm will lead to:

A

mounting a stress response

85
Q

E. coli produces many ___ which help with proper protein folding in the periplasm

A

extracytoplasmic chaperones

86
Q

What class of extracytoplasmic chaperones catalyze disulphide bond formation ?

A

thiol-disulfide oxidoreductase enzymes

87
Q

In E. coli, the thiol-disulfide oxidoreductase _____ forms disulfide bonds in periplasmic proteins

A

DbsA

88
Q

How does the DbsA form disulphide bonds?

A

DsbA has a disulfide bond in its active site CXXC

- accepts electrons from the protein and becomes reduced

89
Q

Reduced DsbA is reoxidized by donating the electrons to a membrane protein ___

A

DbsB

90
Q

where does DbsB pass the electrons to? `

A

Ubiquinone in the ETC

91
Q

other thiol-disulfide oxidoreductases, DsbC and DsbG, are also present in the periplasm; they function as:

A

isomerase-reductases to correct incorrectly bonded disulfide bonds.

Phys-Pro - section 2 (5 decks)

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