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BIOC 303 > 2.1 - Transmembrane Transport (Insertion into ER Membrane) > Flashcards

2.1 - Transmembrane Transport (Insertion into ER Membrane) Flashcards

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1
Q

how is a protein inserted into the ER lumen/membrane?

A
  • ribosome docks onto ER membrane surface - injects polypeptide in the ER lumen as it is being synthesized - ER signal sequence cleaved off by signal peptidase
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2
Q

Type 1 ER protein

A

ER protein with one TM segment and an ER signal sequence (amino end in ER lumen)

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3
Q

Type 1 ER protein - topology

A

s18

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4
Q

Type 1 ER protein - hydropathy plot

A

s19

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5
Q

Type 2 ER protein

A

ER protein with one TM segment start-transfer sequence (amino end in cytosol)

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6
Q

Type 2 ER protein - topology

A

s20

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7
Q

Type 2 ER protein - hydropathy plot

A

s21

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8
Q

signal-anchor sequence

A

found in type 2 and 3 ER proteins

  • start-transfer sequence near N-term of protein that is a TMS recognized by SRP
  • positive is always facing the cytosol
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9
Q

why is the positive charge of the start-transfer sequence of type 2/3 ER proteins always towards the cytosol?

A

negatively charged lipids are present around the translocation channel

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10
Q

Type 3 ER

A

ER protein with one TM segment start-transfer sequence (amino end in ER lumen)

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11
Q

Type 3 ER protein - topology

A

s23

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12
Q

Type 3 ER protein - hydropathy plot -

A

probably same as type 2

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13
Q

Type 4 ER protein

A

multi-pass ER protein with a start transfer-sequence and a start-transfer sequence (can have multiple

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14
Q

positive-inside rule with alkaline phosphatase

A

1) hairpin loop inside = blue

2) hairpin loop outside = white

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15
Q

explain the charge gradient across the ER membrane at the translocon

A
  • cytoplasmic side of ER membrane (-)/basic
  • ER lumen side of ER membrane (+)/acidic
  • makes a charge gradient across the membrane, so harder to insert + residues across the membrane
  • expect that cytosolic loops will have more + charged residues
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16
Q

SecYEG

A

translocation channel/translocon of prokaryotes

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17
Q

Sec61aBg

A

translocation channel/translocon of eukaryotes

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18
Q

SecYEg

A

translocation channel/translocon of archaea

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19
Q

translocon

A

membrane channel in the ER membrane through which the polypeptide chain is transferred through

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20
Q

translocon structure

A

membrane channel protein made of 3 subunits

  • pore ring made of 6 isoleucines gated by a short helix when closed
  • SecY/Sec61 alpha
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21
Q

how can protein structure be determined experimentally?

A

X-ray diffraction of protein crystals

  • beam of X-ray (short wavelengths) directed across crystal
  • some X-rays are scattered in waves based on the structure and the diffraction pattern can be used to make an electron density map
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22
Q

SecYEG

Sec61aBg

A

???

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23
Q

what happens if you remove the plug from translocon

A

cell dies

- membrane loses impermeability to ions

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24
Q

what is the translocon pore ring made of

A

6 isoleucines (hydrophobic amino acid resides)

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25
what is the function of the pore ring
forms a gasket around the polypeptide chain in transit so that it is ion tight
26
what is the equivalent of the Sec61B subunit in bacterial translocon?
SecG
27
what is the equivalent of the Sec61g subunit in bacterial translocon?
SecE
28
what is the purpose of the seam opening of Sec complex
lateral opening along seam allows the translocating peptide chain to access the lipid bilayer
29
how is the TMS released from the Sec complex
two halves of SecY/Sec61a opening
30
describe secondary protein structure of porins
beta-barrel proteins
31
why are porins not found in the IM of bacteria
???
32
two modes of protein translocation (2)
1) co-translational translocation | 2) post-translational translocation
33
co-translational translocation
translocation during translation
34
post-translational translocation
translation before translocation. Chaperone proteins maintains the proteins in a loosely folded state
35
how does post-translational translocation work in bacteria?
SecA ATPase uses ATP to push the protein across the SecY complex (ratchet mech)
36
how does post-translational translocation work in yeast?
BiP ATPase binds polypeptide chains as it emerges from the pore (pulling mechanism)
37
what kind of translocation occurs across the ER membrane
co-translational translocation
38
what does the translocator need to feed the polypeptide through the pore
accessory proteins (SecA ATPase in bact, BiP ATPase in euk)
39
is yeast prokaryotic or eukaryotic
eukaryotic (has a nucleus)
40
BiP ATPase function (2)
- protein that pull proteins in post-translational translocation (yeast) across ER membrane - also acts as a chaperon
41
OST complex location
oligosaccharide transferase complex | - located in ER lumen
42
OST complex function
glycosylate proteins that are in the ER lumen
43
what proportion of eukaryotic proteins are glycosylated
50%
44
why are so few proteins in the cytosol are glycosylated
OST (oligosaccharide transferase) are only located in the ER lumen
45
how does the carbohydrate layer of the ER useful for the cell
protects cells against mechanical and chemical damage
46
how does glycosylation in ER lumen work
N-linked glycosylation - oligosaccharide transferred to Asn on polypeptide via oligosaccharide transferase
47
what holds the preformed oligosaccharide in the ER lumen prior to it being used for glycosylation
dolichol (special lipid) holds preformed oligosaccharide (14 sugar units) in ER lumen via a high energy pyrophosphate bond
48
where does O-linked glycosylation occur and what proportion is it performed in
In the Golgi
49
what is O-linked glycosylation
preformed oligosaccharide linked to OH group of serine (S) or threonine (T)
50
what are oligosaccharides used for
used as tags to mark the state of protein folding
51
calnexin
ER chaperone protein | - binds to oligosaccharide on incompletely folded proteins and retains them in the ER until the protein is folded
52
calreticulin
ER chaperone protein - binds to oligosaccharide on incompletely folded proteins and retains them in the ER until the protein is folded
53
lectin
carbohydrate-binding proteins
54
PDI
protein disulfide isomerase | - protein in the ER that helps the formation of S-S bonds
55
where do disulfide bonds form
in the ER usually, the reducing environment makes it hard for S-S bonds to form in the cytosol
56
GPI
glycophosphatidylinositol anchor
57
GPI function
1) protein transportation 2) cell adhesion 3) cell wall synthesis 4) cell surface protection
58
how is GPI added onto proteins?
ER protein, GPI-transamidase, cleaves off C-terminal transmembrane segment and attaches GPI anchor to C-terminus of protein
59
what kind of proteins is GPI found on
plasma membrane proteins
60
HA
influenza hemagglutinin - antigenic glycoprotein responsible for binding of virus to cell (target is sialic acid)
61
why is trypanosoma brucei so potent
parasite that is able to cross blood brain barrier - VSG (variable surface glycoprotein) coat prevents the immune system from accessing the PM epitopes of the parasite - allows it to evade the immune system - antigenic variation allows it to avoid specific immune responses

BIOC 303 (14 decks)

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