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BI 338 Cells > Protein Transport (Lecture 12) > Flashcards

Protein Transport (Lecture 12) Flashcards

1
Q

______ is where most phospholipids are synthesized.

A

SER (smooth endoplasmic reticulum)

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2
Q

______ is where most membrane proteins are assemble.

A

RER (rough endoplasmic reticulum)

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3
Q

Translocation through the translocon is driven by ______.

A

Translation elongation

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4
Q

How does cotranslational translocation work?

A

Signal sequence of protein (NH3+) recruits a signal recognition particle (SRP)

SRP binds to the signal sequence arresting translation

SRP binds to a SRP receptor in the ER membrane

SRP is released and translocon opens, translation resumes

Signal peptidase cleaves signal sequence

Translation then continues until protein complete inside ER

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5
Q

Mammalian translocon consists of three proteins, collectively called
the ______ and n bacteria & archaea this is called ______

A

Sec61 complex, SecYEG

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6
Q

Sec61 i made up of ______, an integral membrane protein with ______ membrane-spanning α helices and two smaller proteins, termed ______ and ______.

A

Sec61α, 10

Sec61β, Sec61γ

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7
Q

What are the two reasons that other molecules don’t leak through the translocons?

A

—Hydrophobic hour glass-shaped channel has a helical plug that seals the translocon in absence of protein cargo

—Ring of hydrophobic Ile
residues at constriction of pore form a gasket, preventing passage of polar molecules

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8
Q

What is the purpose of the lateral exit in the translocon?

A

Allows a nascent protein hydrophobic transmembrane domain out of translocon and into the ER membrane lipid bilayer

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9
Q

The ______ recognizes a sequence on the C-terminal end of the hydrophobic core of the signal peptide and cleaves the chain specifically at this sequence once it has emerged into the luminal space of the ER.

A

Signal peptidase

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10
Q

True or False: In most eukaryotes, most secretory proteins enter the ER by cotranslational translocation

A

True

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11
Q

In yeast, some secretory proteins enter ER only ______ translation is complete.

A

After

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12
Q

______ tetrameric complex in ER membrane near translocon in post translational translocation.

A

Sec63

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13
Q

True or False: SRP and SRP receptor not involved in post-translational translocation.

A

True

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14
Q

Molecular chaperone ______ is in ER lumen and has a peptide-binding domain and a ATPase domain

A

BiP

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15
Q

______ is required for BiP to function.

A

ATP hydrolysis

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16
Q

True or False: Successive BiP binding acts as ‘ratchet’ to pull protein into ER.

A

True

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17
Q

Post-translocation is common for translocation into organelles like ______ & ______.

A

Mitochondria and chloroplasts

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18
Q

______ membrane proteins contain one or more hydrophobic membrane-spanning domains

A

Intergral

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19
Q

______ proteins have a polypeptide covalently to one or more lipid molecules.

A

Lipid-anchored

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20
Q

______ proteins are bound to the side of the membrane by by interactions with integral or lipid- anchored membrane proteins, or with lipid head groups

A

Peripheral

21
Q

Orientation of membrane proteins is known as?

A

Topology

22
Q

True or False: Membrane proteins do NOT flip-flop across the membrane.

A

True

23
Q

Topogenic sequences can be known as?

A

N- terminal signal sequences,
internal stop-transfer anchor sequences,
internal signal- anchor sequences

24
Q

______ direct the insertion of nascent proteins into the ER membrane.

A

Topogenic sequences

25
Q

What are the five types of transmembrane proteins?

A

Type I, II, III, IV and tail-anchored proteins

26
Q

What are the key features of Type I single-pass transmembrane proteins?

A

N-terminus on exoplasmic face, C-terminus on cytoplasmic face

27
Q

How are Type I transmembrane proteins made?

A

Signal peptidase cleaves signal sequence (lateral exit)

N-terminal grows into exoplasmic face

Stop-transfer anchor sequence halts translocon (lateral exit)

Polypeptide chain grows with C-terminus on cytosol face

28
Q

How are Type II transmembrane proteins made?

A

Same as type 1, but signal-anchor sequence instead of signal sequence

signal anchor sequence moved laterally out of translocon

N-terminus on cytoplasmic face, C- terminus on exoplasmic face

No cleavage

29
Q

How does Type III transmembrane proteins work?

A

Same as type II except:

N-terminus on exoplasmic face, C- terminus on cytosolic face

30
Q

How does insertion of tail-anchored proteins work?

A

Hydrophobic C-terminal tail binds to Get3

Get3 bind to Get1 and Get2

ATP hydrolysis inserts the C-terminal tail into the ER membrane

ADP is released and protein is moved laterally

31
Q

______ lipid anchor protein is attached to N-terminal Gly

A

Acylation

32
Q

______ lipid anchor protein is attached to C-terminal Cys

A

Prenylation

33
Q

______ lipid anchor protein is attached to C-terminus

A

GPI anchor

34
Q

______ cleaves the precursor protein within the exoplasmic-facing domain, near the stop-transfer anchor sequence (red)

A

GPI transamidase

35
Q

True or False: The GPI transamidase covalently links new C- terminus to the terminal amino group of a preformed GPI anchor

A

True

36
Q

What are the four main modifications in the ER.

A
  1. Covalent addition and processing of carbohydrates
    (glycosylation) in the ER and Golgi complex
  2. Formation of disulfide bonds in the ER
  3. Proper folding of polypeptide chains and assembly of multisubunit proteins in the ER
  4. Specific proteolytic cleavages in the ER, Golgi complex, and secretory vesicles.
37
Q

What are the three different sugars coming from the oligosaccharide pre made in glycosylation?

A

—Mannose
Glucose
N-acetylglucosamine (GlcNAc)

38
Q

True or False: In glycosylation, Full 14-residue precursor is transferred to certain Asn-X- Ser/Thr of a polypeptide as it emerges into the ER lumen

A

True

39
Q

What are the 3 reasons for oligosaccharide attachment?

A

—Required in some cases for proper protein folding
—
Confer stability on many secreted glycoproteins
—
Play a role in cell-to-cell adhesion and immune response

40
Q

______ help stabilize the tertiary and quaternary structure of many proteins

A

Difulside bonds

41
Q

True or False: Disulfide bond formation formed by the oxidative linkage of sulfhydryl groups (–SH, thiol groups) on two cysteine residues in the same or different polypeptide chains

A

True

42
Q

True or False: The disulfide bond formation can proceed only when a suitable oxidant is present

A

True

43
Q

In eukaryotic cells, where are disulfide bonds only formed?

A

Lumen of rough ER

44
Q

What if proteins are not properly folded?

A

Presence of unfolded proteins in the rough ER increases transcription of genes that encode ER chaperones and other folding catalysts

45
Q

What happens in the unfolded protein response?

A

BiP attaches to unfolded proteins

Ire1 forms dimer, endonuclease cuts Hac1 in mRNA

46
Q

WHat are the 4 steps in misfolded protein degradation.

A

—Modifications of N-linked oligosaccharides are used to monitor folding and for quality control
—
Misfolded proteins are targeted for transport from ER lumen back to the cytosol; called dislocation

ERAD (ER-associated degradation) complex enables dislocation of misfolded proteins through the membrane

Specific ubiquitin ligase enzymes that are components of the ERAD complex add ubiquitin residues to the dislocated peptides

47
Q

True or False: Proteins acquire oligosaccharides and disulfide bonds via ER enzymes

A

True

48
Q

______ and ______ work to ensure proper folding of new proteins or transport of unassembled/misfolded proteins back to the cytosol, where they are degraded in the ubiquitin-proteasome pathway

A

ER chaperones and lectins

49
Q

True or False: Only properly folded proteins and assembled subunits are transported from the rough ER to the Golgi complex in vesicles

A

True

BI 338 Cells (21 decks)

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