Chapter 11 Flashcards
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
endoplasmic reticulum, golgi apparatus, endosomes, lysosomes
endoplasmic reticulum (ER)
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 __________
through the ER, GOLGI APPARATUS, SECRETORY VESICLES, and then OUTSIDE THE CELL
these experiments define the __________
rough ER–> Golgi–>secretory vesicles –> cell exterior
further studies showed a similar pathway for non-secreted proteins
secretory pathway
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
free ribosomes
membrane bound ribosomes
____________________:
proteins move into the ER during their synthesis on _________:____________
cotranslational translocation
proteins move into the ER during their synthesis on MEMBRANE BOUND RIBOSOMES
_____________________:
proteins move into the ER after translation has been completed on ________________
postranslational translocation
proteins move into the ER after translation has been completed on FREE RIBOSOME
protein synthesis starts on ________________.
ribosomes that are free in the cytosol
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
SIGNAL SEQUENCE
when cells are distrupted and the nuclei centrifuged out, the Er breaks up into small vesicles called ___________
microsomes
___________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 ________________-
Blobel and Sabatini
proteolytic cleavage
signal sequences (about 20 amino acids) include a stretch of hydrophobic residues, and are usually located at the ______________ of the polypeptidechain
amino terminus
COTRANSLATIONAL TARGETING
- as they emerge on the ribosome, signal sequences are bound by a _________________
- SRPs consists of six polypeptides and small cytoplasmic RNA (________)
- the SRP binds the ribosome and the signal sequence, inhibing further translation
- the entire complex binds to a ________________ on the rough ER membrane
- the SRP is then released, and the ribosome binds to a membrane channel or _________________
- the signal sequence is inserted into the translocon, and ______________
- the signal sequence is cleaved by __________ and released into the ER lumen
COTRANSLATIONAL TARGETING:
- as they emerge from the ribosome, signal sequences are bound by a SIGNAL RECOGNITION PARTICLE (SRP)
- SRPs consist of six polypeptides and a small cytoplasmic RNA (SRP RNA)
- the SRP binds the ribosome and the signal sequence, inhibiting further translation
- the entire complex binds to a SRP RECEPTOR on the rough ER membrane
- the SRP is then released, and the ribosome binds to a membrane channel or TRANSLOCON
- the signal sequence is inserted into the translocon, and TRANSLATION RESUME
- the signal sequence is cleaved by SIGNAL PEPTIDASE and released into the ER lumen
- 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
- Proteins are synthesized on ___________________
- signal sequence are recognized by receptor protein *the Sec62/63 complex) associated with the translocon in the ER membrane
- _____ and _______ chaperones keep the polypeptides chains unfolded so hey can enter the translocon
- another Hsp70 chaperone in the Er _______ acts as a ratchet to pull the polypeptide chain through the channel and into the ER
SIGNAL SEQUENCE
- 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
- Proteins are synthesized on FREE CYTOSOLIC-BOUND RIBOSOMES
- signal sequence are recognized by receptor protein *the Sec62/63 complex) associated with the translocon in the ER membrane
- HSP 70 and HSP 40 chaperones keep the polypeptides chains unfolded so hey can enter the translocon
- another Hsp70 chaperone in the Er (BIP) acts as a ratchet to pull the polypeptide chain through the channel and into the ER
_______________________
1. initially inserted into ER membrane instead of being released into lumen
- they are transported along secretory pathway as membrane components rather than as soluble proteins
- the _________________ of internal membrane proteins: usually alpha _________ with hydrophobic amino acids
- orientations vary-the amino (N) or the carboxyl (C) terminus is on the cytosolic side
- some proteins have multiple membrane-spanning regions
PROTEINS DESTINED FOR INCORPORATION INTO MEMBRANES
the MEMBRANE-SPANNING REGIONS of integral membrane proteins: usually alpha HELICAL REGIONS with hydrophobic amino acids
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
the lumen of the ER is topologically equivalent to the EXTERIOR OF THE CELL
many proteins are inserted directly into the ER membrane by ________________
these sequences are recognized by SRP, but not cleaved by signal peptidase
many proteins are inserted directly into the ER membrane by INTERNAL TRANSMEMBRANE SEQUENCES
the transmembrane alpha helices ________________________________
the proteins can often be oriented in either direction across the membrane
the transmembrane alpha helices EXIT THE TRANSLOCON LATERALLY AND ANCHOR PROTEINS IN THE ER MEMBRANE
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
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
protein folding and processing can occur
1.?
2?
- during translocation across the ER membrane
2. within the ER lumen
the primary role of lumenal ER proteins is to ?
1?
2?
- assist in folding
2. assembly of newly translocated polypeptides
the __________________- is thought to bind to an unfolded polypeptide chain as it crosses the membrane, then mediate folding and assembly of multisubunit protiens
Hsp 70 chaperone BiP
- 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 _____________–
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
- 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
proteins are glycosylated on specific ASPARAGINE RESIDUES as they are translocated into the ER
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
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
one pathway involves the _____________, which assists glycoproteins to fold correctly.
- a protein folding sensor passes correctly folded glycoproteins on to the transitional ER
- if not folded correctly, the folding sensor will ____________, allowing it to cycle back to calreticulin for another attempt at correct folding.
- 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
one pathway involves the CHAPERONE CALRETICULIN, which assists glycoproteins to fold correctly.
- a protein folding sensor passes correctly folded glycoproteins on to the transitional ER
- 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.
- 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
in an excess of unfolded proteins accumulates, as signaling pathway called the _____________ is activated
- it leads to expansion of ER and _____________-
- if protein folding cant be adjusted to a normal level, the cell undergoes ______________-
in an excess of unfolded proteins accumulates, as signaling pathway called the UNFOLDED PROTEIN RESPONSE (UPR) is activated
- it leads to expansion of ER and PRODUCTION OF MORE CHAPERONES
- if protein folding cant be adjusted to a normal level, the cell undergoes PROGRAMMED CELL DEATH
unfolded proteins activate 3 receptors in the ER membrane
- IRE1 cleaves pre-mRNA of a transcription factor (XBP1) Activate ________
_______ translocates to the nucleus and stimulates transcription of UPR genes
- ATF6 is cleaved to release the active ________ transcription factor
- _________ is a protein kinase that phosphorylates translation factor eLF2, which inhibits general translation and reduces the amount of protein entering the ER
- XBP1
- ATF6
- PERK
“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
membrane lipids
most lipids
eukaryotic membranes are made of 3 lipid types : ________,_______,___________-
phospholipids
glycolipids
cholesterol
phospholipids are synthesized on the _______ of the _________ from water- soluble precursors (glycerol)
CYTOSOL SIDE of the ER MEMBRANE
synthesis of phospholipids on the cytosol side allows the hydrophobic fatty acid chains to ___________ membfrane
buried in
new phospholipids are added only to the _____________ of the ER membrane . some must be transferred to the other half
CYTOSOLIC HALF
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
flippases
the ER is also a major site of synthesis of __________ and ____________
cholesterol and ceramide
_______________ is converted to _______ or _____ in the ___________
CERAMIDE is converted to GLYCOLIPIDS, or SPHINGOMYELIN in the GOLGI APPARATUS
SMOOTH ER is abundant in cells with __________________-
active lipid metabolism
________________ are synthesized from CHOLESTEROL in the ER; abundant smooth ER is found in cells of the ________________-
steroid hormones
testis and ovary
_____________________, 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
in the liver
- 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
the vesicles fuse to form the ER-Golgi INTERMEDIATE COMPARTMENT (ERGIC)
proteins in the lumen of ________________ are packaged into ___________, then released to the lumen of the ___________ following vesicle fusion
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
membrane proteins and lipids are transported in a similar way; their _______________ is maintained
TOPOLOGICAL ORIENTATION
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 ___________________
default pathway
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 __________
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
______________________________
proteins from the ER are processed and sorted for transport to endosomes, lysosomes, the plasma membrane, or secretion
Golgi apparatus (golgi complex)
_____________ and _____________ are synthesized in the golgi, and complex cell wall polysaccharides in plant cells
most glycolipids and sphingomyelin
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 ___________
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)
the Golgi has 4 regions
what are they?
Cis compartment- recieves molecules from the ERGIC
medial and trans compartments- most modifications are done here
trans-Golgi network- the sorting and distributing center
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
stable cisternae model
cisternal maturation model
the _____________ portionsof the glycoproteins are extensively modified in the Golgi
carbohydrates
____________ linked oligosaccharides that were added in the ER are modified by a sequence of reactions catalyzed by enzymes in different compartments
N-linked oligosaccharides
O-linked glycosylation (__________________________)
processing of proteoglycans involves addtion of 100 or more carbohydrates chains to a polypeptide that is further modified by addition of _____________-
carbohydrates added to side chains of serine and threonine
sulfate groups
________________ and________________ are synthesized from ____________ in the golgi
GLYCOLIPIDS and SPHINGOMYELIN are synthesized from CERAMIDE
____________________ is synthesized by transfer of __________________ from phosphatidylcholine to cermamide
addition of carbohydrates to ceramide yields a variety of different glycolipids
SPHINGOMYELIN is synthesized by transfer of a PHOSPHORYLCHOLINE GROUP from phosphatidylcholine to ceramide
“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
Trans Golgi network
Transport from the Golgi to the cell surface can occur by three routes:
direct transport to the plasma membrane
recycling endosomes
regulated secretory pathways
