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BS202 > 12.1 The Endoplasmic Reticulum > Flashcards

12.1 The Endoplasmic Reticulum Flashcards

1
Q
  • a network of membrane-enclosed tubules and sacs that extends from the nuclear membrane throughout the cytoplasm
  • largest organelle of most eukaryotic cells
A

endoplasmic reticulum

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2
Q

rough ER is covered with (1), and thus it mainly functions in (2)

A
  1. ribosomes
  2. protein processing
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3
Q

() is not associated with ribosomes and is involved in lipid metabolism

A

smooth ER

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4
Q

when cells are disrupted, the ER breaks up into small vesicles called ()

A

microsomes

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5
Q

what are the sacs in the ER called

A

cisternae

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6
Q

() is the first step in protein trafficking along the secretory pathway

A

Entrance into the ER

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7
Q

using the (), George Palade and his colleagues were able to show how proteins are secreted

A

pulse-chase experiment

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8
Q

how were newly synthesized proteins labelled in the pulse-chase experiment

A

using radioactive amino acids that the pancreatic cell was exposed to

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9
Q

state the secretory pathway

A

rough ER → Golgi → secretory vesicles → cell exterior

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10
Q

what is the “pulse” stage of the pulse-chase experiment

A

exposure of cell to radioactive AAs

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11
Q

what is the “chase” stage of the pulse-chase experiment?

A

incubating the cells in a media with nonradioactive AA

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12
Q

the site of secretory protein synthesis

A

rough ER

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13
Q

how do proteins travel to the cell surface from the Golgi

A

they are packaged in secretory vesicles

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14
Q

proteins whose destinations are the cytosol, mitochondria, chloroplasts, or the nuclear interior are synthesized by ()

A

free ribosomes

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15
Q

compare and contrast free ribosomes from membrane-bound ribosomes

A

free and membrane bound ribosomes are virtually indistinguishable; a ribosome will bind to the ER membrane only if the protein it is translating needs to be secreted

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16
Q

what are the 2 kinds of translocation

A
  1. cotranslational translocation
  2. posttranslational translocation
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17
Q

compare and contrast cotranslational from posttranslational translocation

A
  • in cotranslational translocation, proteins are translocated into the ER during their translation
  • in posttranslational translocation, proteins are translocated into the ER once translation has been completed
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18
Q

an amino acid sequence that the N-terminus of a polypeptide chain that signals the ribosome to be targeted to the ER membrane; it is cleaved during the polypeptide chain’s transfer into the ER lumen

A

signal sequence

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19
Q

the signal sequence is cleaved by () during translation

A

signal peptidase

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20
Q

this hypothesis posited that a short amino acid sequence was responsible for targeting ribosomes to the ER membrane when they are translating secretory proteins

A

signal hypothesis

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21
Q

how did experiments on the signal hypothesis confirm the role of signal sequences?

A
  • (secreted) proteins translated by free ribosomes were slightly larger because the signal sequences weren’t cleaved
  • additionally, the addition of a signal sequence to a normally non-secreted protein is sufficient to direct the recombinant protein into the ER
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22
Q

signal sequences are recognized and bound by the ()

A

signal recognition particle (SRP)

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23
Q

the SRP consists of (1) and (2)

A
  1. 6 polypeptides
  2. a small cytoplasmic RNA (SRP RNA)
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24
Q

the SRP binds to the ribosome and signal sequence, and then targets the entire complex to the rough ER by binding to the () on the ER membrane

A

SRP receptor

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25
Q

what do you call the protein translocation complex on the ER membrane that the ribosome binds to

A

translocon

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26
Q

() opens the translocon by moving a plug away from the translocon channel

A

insertion of the signal sequence

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27
Q

in both yeast and mammalian cells, translocons are complexes are complexes of 3 transmembrane proteins called ()

A

Sec61 proteins

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28
Q

summary of cotranslational targeting of secretory proteins into the ER

A
  1. once signal sequence emerges from (i.e. is translated by) the ribosome, it is recognized and bound by the SRP
  2. SRP escorts the ribosome-polypeptide-SRP complex to the ER membrane where it binds to the SRP receptor
  3. SRP is released and ribosome binds to translocon; insertion of signal sequence opens translocon channel
  4. translation resumes and the signal sequence is cleaved by signal peptidase
  5. continued translation drives translocation of the growing polypeptide chain across the membrane
  6. the completed polypeptide chain is released within the ER lumen
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29
Q

proteins destined for incorporation into the plasma membrane or the membranes of other compartments are () instead of being released into the lumen

A

initially inserted into the ER membrane

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30
Q

Transmembrane proteins are inserted into the ER membrane by ()

A

⍺-helical membrane-spanning regions

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31
Q

transmembrane proteins are transported through the secretory pathway as () rather than soluble proteins

A

membrane components

32
Q

the transmembrane sequences are (), thus allowing them to integrate with the phospholipid bilayer

A

hydrophobic

33
Q

the lumen of the ER is topologically equivalent to the

A

exterior of the cell

34
Q

Transmembrane regions signal a change in the translocon and lead to the ()

A

insertion of the polypeptide chain into the ER membrane

35
Q

summarize how a transmembrane protein with a regular signal sequence is inserted into the ER membrane

A
  1. signal sequence allows SRP to target the translating ribosome to the ER membrane
  2. translation of a membrane-spanning ⍺-helix in the middle of a protein halts translocation
  3. helices exit the translocon laterally into the ER membrane and further translocation is blocked
  4. translation proceeds as normal from the cytosolic side of the ER
36
Q

The orientation of transmembrane proteins is () as they travel along the secretory pathway

A

established in the ER and maintained

37
Q

how are transmembrane proteins with no signal sequence targeted to the ER membrane

A
  • SRP recognizes the internal transmembrane sequence present in the polypeptide chain and targets the translating ribosome to the SRP receptor
  • there is no cleaving of a signal sequence
38
Q

orientation of the transmembrane proteins is determined by the ()

A

amino acids immediately flanking the transmembrane region

39
Q

polypeptide chains are inserted into the translocon with (+) charged residues on the

A

cytosolic side

40
Q

a series of transmembrane sequences with altering orientations results in proteins that

A

span the membrane multiple times

41
Q

how are transmembrane proteins with transmembrane sequences at the C-terminus brought to the ER membrane

A
  • transmembrane domains of these proteins are recognized by TRC40
    (distinct targeting factor) once translation is complete
  • TRC40 then escorts the proteins to the ER membrane, where it is inserted via the GET1-GET2 receptor
42
Q

for secretory proteins, most protein processing occurs either during (1) or (2)

A
  1. translocation across the ER membrane
  2. within the ER lumen
43
Q

the primary role of the lumenal ER proteins is to

A

assist in the folding and assembly of newly translocated polypeptides

44
Q

polypeptides fold into their 3D conformations within the ER, assisted by ()

A

molecular chaperones that facilitate folding

45
Q

the Hsp70 chaperone, (), is thought to bind to the unfolded polypeptide chain as it crosses the membrane and then mediate protein folding within the ER

A

BiP

46
Q

the formation of disulfide bonds in the oxidizing environment of the ER is facilitated by the enzyme () located in the ER lumen

A

protein disulfide isomerase (PDI)

47
Q

the process of glycosylating proteins on specific asparagine residues

A

N-linked glycosylation

48
Q

give an overview of N-linked glycosylation

A
  • an oligosaccharide (consisting of 14 sugars) is added to an acceptor asparagine residue on the growing polypeptide chain
  • 3 glucose residues are removed while the protein is still within the ER
  • the oligosaccharide is then transferred as a unit to acceptor asparagine
49
Q

() synthesizes the oligosaccharide that is added to the growing polypeptide chain in N-linked glycosylation

A

dolichol

50
Q

in N-linked glycosylation, the oligosaccharide is added to acceptor asparagine in the consensus sequence ()

A

Asn-(X)-Ser/Thr

51
Q

glycosylation helps:

A
  1. prevent protein aggregration
  2. promote protein folding and sorting
52
Q

glycolipids that anchor proteins to the plasma membrane and contain phosphatidylinositol

A

glycosylphosphatidylinositol (GPI) anchors

53
Q

GPI anchors are assembled in the ()

A

ER membrane

54
Q

how are GPI anchors added to a protein

A
  • they are added immediately after completion of protein synthesis
  • the C-terminal transmembrane domain is cleaved and exchanged for the GPI anchor
  • these proteins are attached to the membrane only by their associated glycolipid
55
Q

in (), misfolded proteins are identified, returned from the ER to the cytosol

A

ER-associated degradation (ERAD)

56
Q

in ER-associated degradation (ERAD), misfolded proteins are degraded by the ()

A

ubiquitin-proteosome system

57
Q

the () is activated when there is an excess of unfolded proteins accumulated in the ER

A

unfolded protein response pathway (UPR pathway)

58
Q

in summary, activation of the UPR pathway leads to (1) and (2)

A
  1. expansion of the ER and production (transcription) of additional chaperones (and enzymes) to meet the need for increased protein folding
  2. transient reduction in the amount of newly synthesized proteins entering the ER
59
Q

sustained activity of the UPR pathway leads to ()

A

cell death

60
Q

specifically, the UPR results from the activation (by unfolded proteins) of the ff. signalling molecules (stress sensors) in the ER membrane

A
  1. IRE1
  2. ATF6
  3. PERK
61
Q

what does IRE1 do to stimulate UPR

A

results in cleavage and activation of the mRNA encoding a transcription factor (XBP1), which stimulates the transcription of UPR target genes

62
Q

what does ATF6 do to stimulate UPR

A

cleaved to release ATF6 transcription factor

63
Q

what does PERK do to stimulate UPR

A

a protein kinase that phosphorylates the translation factor elF2, which inhibits general translation (reduces amount of protein entering ER); it also results in the preferential transcription factor ATF4 (contributes to induction of UPR target genes)

64
Q

Membrane lipids are synthesized in the ()

A

smooth ER

65
Q

membrane lipids are synthesized in association with () rather than in the aqueous environment of the cytosol

A

already existing cellular membranes

66
Q

the membranes of eukaryotic cells are composed of 3 main types of lipids:

A
  1. phospholipids
  2. glycolipids
  3. cholesterol
67
Q

basic structural components of the membrane; mostly derived from glycerol

A

phospholipids

68
Q

overview of ER membrane lipid synthesis

A
  1. in the cytosolic side of the ER, membrane-bound enzymes transfer fatty acids from coenzyme A (fatty acyl CoA complexes) to glycerol-3-phosphate → results in phosphatidic acid
  2. the resulting phosphatidic acid is inserted into the membrane
  3. enzymes (phosphatase) in the cytosolic side of ER convert phosphatidic acid to diacylglycerol
  4. enzymes then catalyze the addition of different polar head groups to diacylglycerol; adding phosphocholine results in phosphatidylcholine
69
Q

to ensure even growth of both the cytosolic and lumenal sides of the membrane, () catalyze the rapid translocation fo phospholipids across the ER membrane

A

flippases

70
Q

The smooth ER is the site of (1) and (2)

A
  1. synthesis of steroid hormones
  2. degradation of many toxic compounds
71
Q

the smooth ER also serves as the major site of synthesis of (1) and (2)

A
  1. cholesterol
  2. ceramide
72
Q

overview of protein/lipid export from ER

A
  1. vesicles bud off from a specialized region of the ER called the ER exit side (ERES)
  2. vesicles then fuse to form the vesicles and tubules of the ER-Golgi intermediate compartment (ERGIC)
  3. from the ERGIC, the proteins are then carried to the Golgi
73
Q

lumenal proteins targeted for the Golgi are bound by () that are selectively packaged into vesicles

A

transmembrane proteins

74
Q

resident ER proteins destined to remain in ER lumen are marked by () located at their C-terminus; a recycling receptor in the ERGIC or Golgi recognizes these sequences and transports these proteins back to the ER

A

(Lys-Asp-Glu-Leu) KDEL retrieval sequences

75
Q

KDEL retrieval sequences are recognized by

A

a recycling receptor in the ERGIC or Golgi

BS202 (36 decks)

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