Ch.12 - Protein Sorting and Transport Flashcards
What does the vesicular transport connect?
- ER
- golgi
- endosomes
- lysosomes
ER extends from what, throughout what?
from the nuclear membrane throughout the cytoplasm
ER is a network of what?
membrane-enclosed tubules and sacs (cisternae)
What is typically the largest organelle?
ER
What membrane is continuous?
ER
Which ER makes lipids?
smooth ER
What does the rough ER make?
transmembrane and secreted proteins
What is the rough ER covered by?
ribosomes
what is the role of the ER?
protein processing and sorting
The role of the ER was first demonstrated by Palade and colleagues in the ______s
1960s
What did the first demonstration of protein processing and sorting study?
pancreatic acinar cells that secrete digestive enzymes into the small intestine
What did autoradiography determine?
the location of radiolabeled proteins
After labeling the proteins in the ER experiment, what happened next?
a chase (incubation w/non-labeled amino acids for different lengths of time)
What did a chase allow the tracking of?
the labeled proteins throughout the ER, golgi, secretory vesicles, and then outside the cell
What did the experiments in the 1960s (about the ER) define?
secretory pathway
What is the secretory pathway?
rough ER –> golgi –> secretory vesicles –> cell exterior
What do proteins synthesized on free ribosomes stay in?
cytosol or transported to nucleus/other organelles
Proteins synthesized on membrane-bound ribosomes are taken where?
directly to the ER
Which type of translation uses energy from the ribosome?
cotranslational translation
Proteins moving into the ER DURING their synthesis on membrane-bound ribosomes. Which translocation is this?
cotranslational translocation
Proteins move into the ER AFTER translation has been completed on free ribosomes. Which translocation is this?
posttranslational translocation
How are ribosomes targeted to the ER?
by a signal sequence at the amino terminus
When is the signal sequence removed?
when the growing polypeptide chain enters the ER
How are signal sequences determined?
by in vitro preparations of rough ER
What happens to the ER when cells are disrupted?
the ER breaks up into microsomes (small vesicles)
About how long are signal sequences?
20 amino acids
Signal sequences include a stretch of what?
hydrophobic residues
Where are the hydrophobic residues usually located in signal sequences?
at the N-terminus
What do SRPs consist of?
6 polypeptides + a small cytoplasmic RNA (SRP RNA)
What is a complex of 3 transmembrane proteins called?
Sec61 proteins
What do Sec61 proteins do?
translocate secreted polypeptides in bacteria
Which type of translocation is more common in yeast?
posttranslational
What is the function of Hsp70 and Hsp40 chaperones ?
keep the peptide chains from folding so they can enter the translocon
What is the Hsp70 chaperone that acts as a ratchet to pull the polypeptide chain through the channel into the ER
BiP (binding protein)
What does BiP bind to?
hydrophobic regions
Proteins destined for incorporation into membranes are initially inserted where?
ER membrane
Membrane-spanning regions of integral regions of integral membrane proteins typically have __________ regions with _______ amino acids.
α helical regions with hydrophobic amino acids
The ER lumen is topologically equivalents to what?
cell exterior
Domains of membrane proteins that are exposed on the cell surface correspond to what?
regions of polypeptide chains that are translocated into the ER lumen
How are many proteins directly inserted into the ER membrane?
internal transmembrane sequences
What are the internal transmembrane sequences recognized by?
SRP (but not cleaved by it)
Transmembrane α helices exit the translocon laterally and do what?
anchor proteins in the ER membrane
What direction are the polypeptides oriented across the ER membrane (N and C terminus)?
either direction
What does the Transmembrane α helix do?
Halts translocation and anchors the polypeptide in the membrane
Where is the transmembrane α helix located in the protein?
middle
Where does the carboxy terminal portion of polypeptide remain? (ER lumen or cytosol)
cytosol
What does the signal peptidase do?
cut the signal sequence from the primary polypeptide
Proteins that span the membrane multiple times are inserted by what?
an alternating series of internal signal sequences and transmembrane stop-transfer sequences (N terminus is in cytosol)
Some proteins have a transmembrane sequence at the carboxy terminus that cannot what?
cannot be recognized by SRP until translation is complete
What is the transmembrane sequence recognized by (when it is at the carboxy terminus)?
targeting factor TRC40
What does the targeting factor TRC40 specifically do?
brings the protein to a GET1-GET2 receptor in the ER membrane
Where does protein folding and processing occur (2 places)?
either during translocation across the ER membrane or in the ER lumen
what is the primary role of lumen proteins?
assist folding and assembly of newly translocated peptides
Hsp70 chaperone BiP helps proteins do what?
not fold
Hsp70 chaperone BiP binds to what as they cross the ER membrane?
unfolded polypeptides
Why is BiP not needed for cotranslational translocation in the ER?
the polypeptide is pushed through the translocator by the energy of the ribosome
What type of bonds are important in protein folding?
disulfide bonds
What type of environment is the cytosol?
reducing
Most cystine residues in the cytosol are in the _____ state.
reduced (-SH)
What type of environment is the ER?
oxidizing
What part of the cell promotes disulfide (S-S) bond formation?
ER
What facilitates the disulfide bond formation?
protein disulfide isomerase
