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HELENP CBG: Cell Architecture, Communication and Regulation > CBG Lecture 35: Secretory Pathway:Protein Translocation > Flashcards

CBG Lecture 35: Secretory Pathway:Protein Translocation Flashcards

1
Q

how does epstein barr virus replicate

A

in an episome

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2
Q

how does EBV survive in a host cell

A

Survives in an infected cell by interfering with the T-cell adhesion response

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3
Q

in what can EBV be reactivated in

A

Can by reactivated in cancers

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4
Q

what do TRIM5 proteins do

A

prevent uncoating of viral capsid proteins

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5
Q

WHAT IS the signal hypothesis

A

signal sequences are typically 15-60aa, oftern @N-terminus, often cleaved by signal peptidases
GUNTER BLOBEL

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6
Q

how long are signals

A

15-60 aa

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7
Q

where does synthesis of all proteins begin

A

in cytoplasm

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8
Q

whatis the signal for ER translocation

A

N-terminal positive…hydrophobic….polar

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9
Q

what is the -3, -1 rule

A

residues @ position -3,-1 relative to cleavage site ,ust be small and neutral (lysine,arg) for cleavage to occur correctly

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10
Q

what is the SRP

A

signal recognition particle

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11
Q

what is Sec

A

a translocon that is an evolnary conserved passive pore

found on ER membrane

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12
Q

name an evolnary conserved passive pore

A

Sec translocon

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13
Q

which Sec in mammals and euks

A

Sec61

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14
Q

which Sec in proks and archaea

A

SecY

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15
Q

discuss structure of Sec translocon

A

heteroetrimeric complex

Sec61alpha, beta, gamma

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16
Q

which monomers of Sec translocon are essential

A

Sec61 alpha and Sec61beta

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17
Q

how is Sec61 a passive pore

A

associating partners provide the driving force
no ATP
depending on partner there are different ways channel could work

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18
Q

what are the Sec modes of action

A

Cotranslational -SecY
Posttranslational eukaryotic - ratcheting
Postranslational bacterial - pushing

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19
Q

what Sec is found exclusively in bacteria

A

SecA

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20
Q

which type of Sec transport requires addditional enerygy

A

brownian ratchet - eukaryotes Sec61

bacterial pushing - SecA

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21
Q

where does energy for cotranslational secretion in Sec come from

A

GTP -> GDP

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22
Q

describe cotranslational Sec61 mode of action

A

protein still being synthesized so pp only has one way to go - energy from GTP hydrolysis

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23
Q

when does postranslational translation happen

A

after protein synthesized - additional energy is necessary - ATP->ADP

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24
Q

discuss cotranslational translocatoin

A

most general mode, most membrane protein translocation

partners include SRP and SRP receptors

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25
Q

what are SRP partners of

A

cotranslational translocation

26
Q

where does the energy from cotranslational translocation come from

A

GTP hydrolysis in translation

27
Q

discuss protranslational translation in bacteria

A

uses SecA
the comleted pp chain is fed from the cytosolic side into translocator in the plasma membrane by the Sec A ATPase
ATP hydrolysis driven conformational change drives a piston like motion in SecA

28
Q

how are proteins translocated across the ER

A

by the Sec translocon which is a passive pore

29
Q

what does the mechanism of translocation depend on

A

interaction partners

30
Q

discuss cotranslational translocation in detail

A
  1. after protein starts to emerge from rib - it may be directed to ER (determined by signal sequence)
  2. signal sequence emerges from rib and recognised and bound by an SRP
  3. SRP binds rib and signal sequence and stalls translation
  4. SRP targets entire complex to RER by binding to an SRP receptor on ER membrane
  5. GTP molecules bind to SRP and SRPreceptor - trigers transfer of signal sequence from SRP to Sec61
  6. hydrolysis GTP->GDP = dissociation SRP from receptor and the ribosome-mRNA complex
  7. transfer of ribosome-mRNA complex to translocon allows the signal sequence to interact with short hydrophobic side chains inside the Sec which oipens the channel by moving the plug away
  8. as translocation proceeds - signal is cleaved by signal peptidase, pp freleased into ER lumen
31
Q

how is cotranslational translocation possible

A

because ribosomes are bound to the ER and not other organelles

32
Q

what is an SRP

A

Signal Receptor Particle consisting of 6pps and a scmall cytoplasmic RNA

33
Q

what does an SRP do

A

binds hydrophobic signal on protein

34
Q

discuss post-translational translocation by ratcheting mechanism

A

ratcheting mechanism
-protein first synthesized then transported to ER
chaperonin proteins called SecB bind to the pp during synthesis to prevent it from folding prematurely
-accessing proteins to fit pp into pore and drive translocation - composed of Sec62,Sec63,Sec71, and Sec72 is attched to the Sec61 and deposits BiP molecules onto the translocating chain as it emerges into the Er lumen
ATP driven cycles of BiP binding and release pull the protein into the lumen
-unidirectional translocation is driven by cycles of BiP binding and release - Brownian ratchet

35
Q

what is SecB

A

a chaperonin protein which nbinds to pp in posttranslaitonal translocation to ensrure it doesnt fold up prematuerely

36
Q

what is brownian ratchet

A

successive binding of BiP-ADO prevents pp backslide back into the cytosol

37
Q

name some cells that use brownian ratchet

A

yeast and higher eukaryotes

38
Q

how is unidrectional translocation enabled in brownian ratchet

A

by cycles of BiP binding and release - successive binding of BiP-ADP prevents backslide

39
Q

what is the universal mechanism of initiation of translocation in cotranslational translocation

A

when signal peptide emerges from ribosome and is recognised by the SRP

40
Q

how does initiation of post translocational translocation happen

A

after protein synthesis is complete - binding of BiP

41
Q

what is SecY in higher eukaryptes aka

A

Sec61

42
Q

discuss general architecture of SecY

A

3 SUs - alpha -helical sand made up of two halves like a clam
beta-nonessential as its peripheral
gamma
transmembrane protein

43
Q

discuss the translocation pore and plug

A

helical plug of alpha SU blocks the channel in closed state
Pore ring lined by hydrophobic residues - gasket like seal
crosslinking experiment confirms movement of the plug during translocation

44
Q

why can pp pass laterally through clamp

A

because it has helical signal sequence enabling ateral passing

45
Q

where is intercalation of signal sequence between

A

TM2b/TM7

46
Q

discuss the model SecY pore for translocation

A

confirmed by structures of translocating ribosome - has hourglass shape

  • helical plug fills middle of channel
  • pp chain grows from ribosome
  • signal sequence is alpha helical and intercalates between helices allowing it to displace from membrane so signal peptidase cuts it off
47
Q

how many classes of transmembrane protein are there

A

4

48
Q

what are the different classes of transpmembrane proteins

A
  1. single pass with cleaved ER signal sequence - N terminus internally
  2. single pass INTERNAL SIGNAL SEQUENCE - N-terminus@luminal side
  3. INTERNAL SIGNAL SEQUENCE - N terminus @ lumimnal side
  4. MULTIPASS TRANSMEMBRANE PROTEINS
49
Q

give some examples of type 1 membrane proteins

A 
cleavable signal sequence and downstream sequence
glycophorin
influenza HA protein
insulin receptor
growth hormone receptor
50
Q

give an example of type 2 membrane protein

A

transferrin receptor
golgi galactosyltransferase
influenza HN protein

51
Q

give an example of type 3 receptor

A

cytochrome P450

52
Q

what type receptor is cytochrome P450

A

type 3 receptor

53
Q

give an example of a type 4 receptor

A

GPCR
voltage gated Ca2+
Sec61

54
Q

what two types of membrane proteins are basically the same

A

type 1 and type 3

type 3 is same as type 1, just with NH3+ on ERgolgi side of membrane

55
Q

what type receptor is a GPCR,VOLTAGE gated Ca2+,Sec61

A

type 4 transmembrane receptor

56
Q

how is a single pass type 1 TM protein with a cleaved ER signal integrated into the ER membrane

A

cotranslational translocation and lateral gating

remains as a single alpha helical membrane spanning segment with N terminus on lumenal side and C terminus cytosolic

57
Q

what do topogenic sequences of peptides determine

A

the orientation in the membrane

58
Q

what part of pp is topogenic sequence

A

signal anchor sequence

59
Q

how can a topogenic sequence be predicted

A

by bioinformatics tools

60
Q

what type of TM is a signal-anchor

A

type 2

HELENP CBG: Cell Architecture, Communication and Regulation (5 decks)

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