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