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HDSO Cell Biology > Chapter 11 > Flashcards

Chapter 11 Flashcards

1
Q

sorting and targeting of proteins to appropriate destinations are important tasks for eukaryotic cells

__________,_____________,___________,___________: involved in protein processing and connected by vesicular transport.

_________________: network of membrane-enclosed tubules and sacs (cisternae)- extend from nuclear membrane through the cytoplasm

the ER has two domains that perform different functions
rough ER is covered by ribosomes on the outer surface
smooth ER has no ribosomes and is involved in lipid metabolsim

A

endoplasmic reticulum, golgi apparatus, endosomes, lysosomes

endoplasmic reticulum (ER)

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2
Q

newly synthesized proteins were labeled with radioisotopes.

location of the radiolabeled proteins was then determined by autoradiography

after labeling, incubation with non-labeled amino acids for different lengths of time (called a “chase”) allowed them to track the labeled proteins through the _____________,____________,________ and then __________

A

through the ER, GOLGI APPARATUS, SECRETORY VESICLES, and then OUTSIDE THE CELL

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3
Q

these experiments define the __________
rough ER–> Golgi–>secretory vesicles –> cell exterior

further studies showed a similar pathway for non-secreted proteins

A

secretory pathway

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4
Q

in eukaryotic cells, initial sorting takes place while translation is in process

proteins synthesized on ________ stay in the cytosol or are transported to the nucleus and other organelles

proteins synthesized on ________ are translocated directly into the ER

A

free ribosomes

membrane bound ribosomes

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5
Q

____________________:

proteins move into the ER during their synthesis on _________:____________

A

cotranslational translocation

proteins move into the ER during their synthesis on MEMBRANE BOUND RIBOSOMES

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6
Q

_____________________:

proteins move into the ER after translation has been completed on ________________

A

postranslational translocation

proteins move into the ER after translation has been completed on FREE RIBOSOME

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7
Q

protein synthesis starts on ________________.

A

ribosomes that are free in the cytosol

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8
Q

cotranslational pathway:

ribosomes are targeted to the ER by a ____________ at the amino terminus which is removed when the growing polypeptide chain enter the ER

the role of signal sequence in targeting proteins to correct locations was determined by in vitro preparation of rough ER

A

SIGNAL SEQUENCE

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9
Q

when cells are distrupted and the nuclei centrifuged out, the Er breaks up into small vesicles called ___________

A

microsomes

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10
Q

___________and _______ found that translation of secretory proteins on free ribosomes retained the signal sequences and were slightly larger

when microsomes were added, the signal sequences were removed by ________________-

A

Blobel and Sabatini

proteolytic cleavage

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11
Q

signal sequences (about 20 amino acids) include a stretch of hydrophobic residues, and are usually located at the ______________ of the polypeptidechain

A

amino terminus

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12
Q

COTRANSLATIONAL TARGETING

  1. as they emerge on the ribosome, signal sequences are bound by a _________________
  2. SRPs consists of six polypeptides and small cytoplasmic RNA (________)
  3. the SRP binds the ribosome and the signal sequence, inhibing further translation
  4. the entire complex binds to a ________________ on the rough ER membrane
  5. the SRP is then released, and the ribosome binds to a membrane channel or _________________
  6. the signal sequence is inserted into the translocon, and ______________
  7. the signal sequence is cleaved by __________ and released into the ER lumen
A

COTRANSLATIONAL TARGETING:

  1. as they emerge from the ribosome, signal sequences are bound by a SIGNAL RECOGNITION PARTICLE (SRP)
  2. SRPs consist of six polypeptides and a small cytoplasmic RNA (SRP RNA)
  3. the SRP binds the ribosome and the signal sequence, inhibiting further translation
  4. the entire complex binds to a SRP RECEPTOR on the rough ER membrane
  5. the SRP is then released, and the ribosome binds to a membrane channel or TRANSLOCON
  6. the signal sequence is inserted into the translocon, and TRANSLATION RESUME
  7. the signal sequence is cleaved by SIGNAL PEPTIDASE and released into the ER lumen
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13
Q
  1. in posttranslation translocation (more common in yeast), polypeptides synthesized on free ribosomes have ________________ recognized by receptor proteins on the translocon. Polypeptides are targeted to the ER when translation is complete
  2. Proteins are synthesized on ___________________
  3. signal sequence are recognized by receptor protein *the Sec62/63 complex) associated with the translocon in the ER membrane
  4. _____ and _______ chaperones keep the polypeptides chains unfolded so hey can enter the translocon
  5. another Hsp70 chaperone in the Er _______ acts as a ratchet to pull the polypeptide chain through the channel and into the ER
A

SIGNAL SEQUENCE

  1. in posttranslation translocation (more common in yeast), polypeptides synthesized on free ribosomes have SIGNAL SEQUENCES recognized by receptor proteins on the translocon. Polypeptides are targeted to the ER when translation is complete
  2. Proteins are synthesized on FREE CYTOSOLIC-BOUND RIBOSOMES
  3. signal sequence are recognized by receptor protein *the Sec62/63 complex) associated with the translocon in the ER membrane
  4. HSP 70 and HSP 40 chaperones keep the polypeptides chains unfolded so hey can enter the translocon
  5. another Hsp70 chaperone in the Er (BIP) acts as a ratchet to pull the polypeptide chain through the channel and into the ER
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14
Q

_______________________
1. initially inserted into ER membrane instead of being released into lumen

  1. they are transported along secretory pathway as membrane components rather than as soluble proteins
  2. the _________________ of internal membrane proteins: usually alpha _________ with hydrophobic amino acids
  3. orientations vary-the amino (N) or the carboxyl (C) terminus is on the cytosolic side
  4. some proteins have multiple membrane-spanning regions
A

PROTEINS DESTINED FOR INCORPORATION INTO MEMBRANES

the MEMBRANE-SPANNING REGIONS of integral membrane proteins: usually alpha HELICAL REGIONS with hydrophobic amino acids

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15
Q

the lumen of the ER is topologically equivalent to the _________________

domains of membrane proteins that are exposed on the cell surface correspond to regions of polypeptide chains that are translocated into the ER lumen

A

the lumen of the ER is topologically equivalent to the EXTERIOR OF THE CELL

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16
Q

many proteins are inserted directly into the ER membrane by ________________

these sequences are recognized by SRP, but not cleaved by signal peptidase

A

many proteins are inserted directly into the ER membrane by INTERNAL TRANSMEMBRANE SEQUENCES

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17
Q

the transmembrane alpha helices ________________________________

the proteins can often be oriented in either direction across the membrane

A

the transmembrane alpha helices EXIT THE TRANSLOCON LATERALLY AND ANCHOR PROTEINS IN THE ER MEMBRANE

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18
Q

some proteins have an _________________, and a ___________________ in the middle of the protein that halts translocation and anchors the polypeptide in the membrane

the carboxy terminal portion of the growing polypeptide remains in the cytosol

A

some protein have an AMINO TERMINAL SIGNAL SEQUENCE, and a TRANSMEMBRANE alpha HELIX in the middle of the protein that halts translocation and anchors the polypeptide in the membrane

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19
Q

protein folding and processing can occur

1.?
2?

A
  1. during translocation across the ER membrane

2. within the ER lumen

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20
Q

the primary role of lumenal ER proteins is to ?

1?
2?

A
  1. assist in folding

2. assembly of newly translocated polypeptides

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21
Q

the __________________- is thought to bind to an unfolded polypeptide chain as it crosses the membrane, then mediate folding and assembly of multisubunit protiens

A

Hsp 70 chaperone BiP

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22
Q
  1. formation of disulfide bonds is important in protein folding
    - in the cytosol, which is a _____________, most cysteine residues are in their reduced (—SH) state.
    - in the ER, an ___________ promotes disulfide (S—-S) bond formation facilitated by _____________–
A

in the cytosol, which is a REDUCING ENVIRONMENT, most cysteine residues are in their reduced (-SH) state

in the ER, and OXIDIZING ENVIRONMENT promotes disulfide (S-S) bond formation, facilitated by PROTEIN DISULFIDE ISOMERASE

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23
Q
  1. proteins are glycosylated on specific ___________ (N-linked glycosylation) as they are translocated into the ER

the oligosaccharide is synthesized on a lipid (dolichol)carrier

glycosylation helps prevent protein aggregation in the ER and provides signals for subsequent sorting

A

proteins are glycosylated on specific ASPARAGINE RESIDUES as they are translocated into the ER

24
Q

protein folding in the ER is slow and inefficient, and man are misfolded. misfolded proteins _______ by __________. misfolded proteins are identified, ____________ and degraded by the ___________

chaperones and protein processing enzymes in the ER lumens can act as sensors of misfolded proteins

A

protein folding in the ER is slow and inefficient, and man are misfolded. misfolded proteins REMOVED BY THE ER by ER ASSOCIATED DEGRADATION (ERAD). misfolded proteins are identified, RETURN TO THE CYTOSOl and degraded by the UBIQUITIN-PROTEASOME SYSTEM

chaperones and protein processing enzymes in the ER lumens can act as sensors of misfolded proteins

25
Q

one pathway involves the _____________, which assists glycoproteins to fold correctly.

  1. a protein folding sensor passes correctly folded glycoproteins on to the transitional ER
  2. if not folded correctly, the folding sensor will ____________, allowing it to cycle back to calreticulin for another attempt at correct folding.
  3. A severely misfolded glycoprotein is recognized by EDEM1, which removes mannose residues

the protein is returned to the cytosol through a ubiquitin ligase complex where it is marked by ubiquitylation and degraded into a proteasome

A

one pathway involves the CHAPERONE CALRETICULIN, which assists glycoproteins to fold correctly.

  1. a protein folding sensor passes correctly folded glycoproteins on to the transitional ER
  2. if not folded correctly, the folding sensor will ADD BACK A GLUCOSE RESIDUE, allowing it to cycle back to calreticulin for another attempt at correct folding.
  3. A severely misfolded glycoprotein is recognized by EDEM1, which removes mannose residues

the protein is returned to the cytosol through a ubiquitin ligase complex where it is marked by ubiquitylation and degraded into a proteasome

26
Q

in an excess of unfolded proteins accumulates, as signaling pathway called the _____________ is activated

  1. it leads to expansion of ER and _____________-
  2. if protein folding cant be adjusted to a normal level, the cell undergoes ______________-
A

in an excess of unfolded proteins accumulates, as signaling pathway called the UNFOLDED PROTEIN RESPONSE (UPR) is activated

  1. it leads to expansion of ER and PRODUCTION OF MORE CHAPERONES
  2. if protein folding cant be adjusted to a normal level, the cell undergoes PROGRAMMED CELL DEATH
27
Q

unfolded proteins activate 3 receptors in the ER membrane

  1. IRE1 cleaves pre-mRNA of a transcription factor (XBP1) Activate ________

_______ translocates to the nucleus and stimulates transcription of UPR genes

  1. ATF6 is cleaved to release the active ________ transcription factor
  2. _________ is a protein kinase that phosphorylates translation factor eLF2, which inhibits general translation and reduces the amount of protein entering the ER
A
  1. XBP1
  2. ATF6
  3. PERK
28
Q

“the smooth ER”

because they are hydrophobic, __________________ are synthesized in association with already existing membrane rather than the aqueous cytosol

_________- are synthesized in the smooth ER

A

membrane lipids

most lipids

29
Q

eukaryotic membranes are made of 3 lipid types : ________,_______,___________-

A

phospholipids

glycolipids

cholesterol

30
Q

phospholipids are synthesized on the _______ of the _________ from water- soluble precursors (glycerol)

A

CYTOSOL SIDE of the ER MEMBRANE

31
Q

synthesis of phospholipids on the cytosol side allows the hydrophobic fatty acid chains to ___________ membfrane

A

buried in

32
Q

new phospholipids are added only to the _____________ of the ER membrane . some must be transferred to the other half

A

CYTOSOLIC HALF

33
Q

this requires passage of polar head groups through the membrane, facilitated by membrane proteins called ____________

this ensures even growth of both sides of the phospholipid bilayer

A

flippases

34
Q

the ER is also a major site of synthesis of __________ and ____________

A

cholesterol and ceramide

35
Q

_______________ is converted to _______ or _____ in the ___________

A

CERAMIDE is converted to GLYCOLIPIDS, or SPHINGOMYELIN in the GOLGI APPARATUS

36
Q

SMOOTH ER is abundant in cells with __________________-

A

active lipid metabolism

37
Q

________________ are synthesized from CHOLESTEROL in the ER; abundant smooth ER is found in cells of the ________________-

A

steroid hormones

testis and ovary

38
Q

_____________________, smooth ER contains enzymes that metabolize lipid- soluble compounds

these detoxifiying enzymes INACTIVATE SOME DRUGS by converting them to water- soluble compounds that can be eliminated in the urine

A

in the liver

39
Q
  1. Proteins and phospholipids
    - are exported from the ER in vesicles that bud from a specialized region of the ER, the ER exit site (ERES)
    - the vesicles fuse to form the ER-Golgi _____________________
    - then move to the golgi apparatus
A

the vesicles fuse to form the ER-Golgi INTERMEDIATE COMPARTMENT (ERGIC)

40
Q

proteins in the lumen of ________________ are packaged into ___________, then released to the lumen of the ___________ following vesicle fusion

A

proteins in the lumen of ONE ORGANELLE are packaged into BUDDING TRANSPORT VESICLES, then released to the lumen of the RECIPIENT ORGANELLE following vesicle fusion

41
Q

membrane proteins and lipids are transported in a similar way; their _______________ is maintained

A

TOPOLOGICAL ORIENTATION

42
Q

proteins targeted for export have peptide and carbohydrate signals that direct their packaging into transport vesicles

unmarked proteins in the ER can also be packaged and transported to the Golgi by a ___________________

A

default pathway

43
Q

proteins that function in the ER are recognized in the ___ or ___________ and transported back to the ER

these proteins, such as BiP, have a targeting sequence (__________) at a carboxy terminus that __________

A

ERGIC or GOLGI

these proteins, such as BiP, have a targeting sequence (KDEL or KKXX) at a carboxy terminus that DIRECTS RETRIEVAL BACK TO THE ER

44
Q

______________________________

proteins from the ER are processed and sorted for transport to endosomes, lysosomes, the plasma membrane, or secretion

A

Golgi apparatus (golgi complex)

45
Q

_____________ and _____________ are synthesized in the golgi, and complex cell wall polysaccharides in plant cells

A

most glycolipids and sphingomyelin

46
Q

The Golgi: 1. flattened mambrane-enclosed sacs (cisternae); 2. associated vesicles.

Proteins from the ER enter at the convex _____________

They are transported from the Golgi and exit from the concave ___________

A

Proteins from the ER enter at the convex CIS FACE (ENTRY FACE)

They are transported from the Golgi and exit from the concave TRANS FACE (EXIT FACE)

47
Q

the Golgi has 4 regions

what are they?

A

Cis compartment- recieves molecules from the ERGIC

medial and trans compartments- most modifications are done here

trans-Golgi network- the sorting and distributing center

48
Q

The mechanism of protein movement through the Golgi is an area of controversy

the _________: proteins are carried between cisternae in transport vesicles

the ______________: proteins are carried within the cisterae, which gradually mature and progressively move through the Golgi in the cis to trans direction

vesicles return golgi resident protiens back to earlier Golgi compartment

A

stable cisternae model

cisternal maturation model

49
Q

the _____________ portionsof the glycoproteins are extensively modified in the Golgi

A

carbohydrates

50
Q

____________ linked oligosaccharides that were added in the ER are modified by a sequence of reactions catalyzed by enzymes in different compartments

A

N-linked oligosaccharides

51
Q

O-linked glycosylation (__________________________)

processing of proteoglycans involves addtion of 100 or more carbohydrates chains to a polypeptide that is further modified by addition of _____________-

A

carbohydrates added to side chains of serine and threonine

sulfate groups

52
Q

________________ and________________ are synthesized from ____________ in the golgi

A

GLYCOLIPIDS and SPHINGOMYELIN are synthesized from CERAMIDE

53
Q

____________________ is synthesized by transfer of __________________ from phosphatidylcholine to cermamide

addition of carbohydrates to ceramide yields a variety of different glycolipids

A

SPHINGOMYELIN is synthesized by transfer of a PHOSPHORYLCHOLINE GROUP from phosphatidylcholine to ceramide

54
Q

“protein sorting and export”

In the _____ golgi netowrk, molecules are sorted and packaged into transport vesicles

proteins that need to stay in the Golgi are associated with the membrane, and contain signals that prevent packaging and transport

A

Trans Golgi network

55
Q

Transport from the Golgi to the cell surface can occur by three routes:

A

direct transport to the plasma membrane

recycling endosomes

regulated secretory pathways

HDSO Cell Biology (16 decks)

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