Unit 6: Protein Sorting to Organelles - 1 Flashcards
A typical mammalian cell contains up to __ different proteins
10 000
A typical mammalian cell contains up to 10,000 different proteins, and each must be __
localized to the correct organelle.
protein: Na+/K+ ATPase
location: _
plasma membrane
Protein : RNA polymerase
location: ?
nucleus
protein: proteases
location:?
lysosome
protein: catalase
location? _
peroxisome
protein: ATP synthase
location: ?
mitochondria
protein: hormones
location : ?
extracellular space
most of the proteins (“nuclear DNA proteins”) synthesized in eukaryotic cells are (3):
(1) Encoded by nuclear DNA
(2) Synthesized on ribosomes in the cytosol
(3) Are delivered to the organelle of destination from the cytosol
most of the proteins synthesized in eukaryotic cells are encoded by:
nuclear DNA
most of the proteins synthesized in eukaryotic cells are synthesized on:
ribosomes in the cytosol
most of the proteins synthesized in eukaryotic cells are delivered to the organelle of destination from
the cytosol
a few proteins “organelle-specific proteins” synthesized in eukaryotic cells are (3):
(1) Are encoded by the DNA present
in MITOCHONDIRA and CHLOROPLASTS
(2) Are synthesized on ribosomes inside
mitochondria and chloroplasts
(3) Are incorporated directly into compartments within mitochondria and chloroplasts
A few proteins (“Organelle-specific proteins”) synthesized in eukaryotic cells are encoded by:
the DNA present
in mitochondria and chloroplasts
A few proteins synthesized in eukaryotic cells (“Organelle-specific proteins”) are synthesized on:
ribosomes inside
mitochondria and chloroplasts
A few proteins synthesized in eukaryotic cells (“Organelle-specific proteins”) are incorporated:
directly into compartments within mitochondria and chloroplasts
Define “Protein sorting” :
the process by which newly-made proteins are directed to the correct location
ex; proteins A and B are sorted to different organelels
Each protein has a __ that can range from 3-60 continuous amino acids.
sorting signal (signal sequence)
Each protein has a sorting signal (signal sequence) that can range from
3-60 continuous amino acids
Each protein has a sorting signal (signal sequence) that can range from 3-60 continuous amino acids. The signal sequence is often, but not always ___
removed once the protein arrives at its destination.
an element that is necessary and sufficient for protein sorting
Signal sequences
Signal sequences can be (4):
(1) Hydrophobic
(2) positively charged
(3) negatively charged
(4) polar
(signal sequence): signal for import into ER is (2) __ (either: hydrophobic/
/positively charged
/negatively charged
/polar) ?
Hydrophobic
Negatively charged
Signal sequence for import into mitochondria is (1) : (either: hydrophobic/
/positively charged
/negatively charged
/polar) ?
positively charged
Signal sequence for import into nucleus is (1) : (either: hydrophobic/
/positively charged
/negatively charged
/polar) ?
positively charged
Signal sequence for export from nucleus is (1) : (either: hydrophobic/
/positively charged
/negatively charged
/polar) ?
hydrophobic
3 steps in protein sorting
- Recognition of the signal sequence by a shuttling cytosolic receptor
- Targeting to the outer surface of the organelle membrane
- Import of the targeted protein into the membrane or transport of the protein across the membrane
what is step 1 in protein sorting?
Recognition of the signal sequence by a shuttling cytosolic receptor
What is step 2 in protein sorting?
Targeting to the outer surface of the organelle membrane
What is step 3 in protein sorting?
Import of the targeted protein into the membrane or transport of the protein across the membrane
what is a general problem for protein import into organelles?
How to transport the protein across membranes that are normally impermeable to hydrophilic molecules
What are the three main mechanisms to import proteins into a membrane-enclosed organelle:
(1) Transport through nuclear pores
(2) Transport across membranes
(3) Transport by vesicles
Three main mechanisms to import proteins into a membrane-enclosed organelle: Method 1 Transport through nuclear pores –> transport __ and proteins __
(1) transports specific proteins
(2) proteins remain folded during transport
do proteins remain folded during transport through nuclear pores?
yes proteins remain folded during transport through nuclear pores
There are three main mechanisms to import proteins into a membrane-enclosed organelle: Method 2: transport across membranes: includes transport across the membranes of which organelles (4)?
(1) ER
(2) mitochondria
(3) chloroplasts
(4) peroxisomes
There are three main mechanisms to import proteins into a membrane-enclosed organelle: Method 2: transport across membranes: Are proteins folded or unfolded in order to cross the membrane?
Proteins are unfolded in order to cross the membrane
There are three main mechanisms to import proteins into a membrane-enclosed organelle: Method 2: transport across membranes: Requires
protein
translocators
There are three main mechanisms to import proteins into a membrane-enclosed organelle: Method 3: Transport by vesicles used from __ and through
From ER onward and through endomembrane system
Transport by vesicles: collect __ and __ from the membrane
Transport vesicles collect CARGO PROTEIN and PINCH OFF from membrane
Transport by vesicles: deliver cargo by:
fusing with another compartment
Proteins remain folded / unfolded during transport by vesicles?
Proteins remain folded during transport
Nuclear import: Entry into the nucleus proceeds through
a protein structure called the nuclear pore complex (NPC)
Nuclear import : nuclear pore complex (NPC) composed of
~30 proteins
Nuclear import: Entry into the nucleus proceeds through a protein structure called the nuclear pore complex (NPC). Composed of ~30 proteins, each with ( such that the NPC contains __ proteins when assembled
Composed of ~30 proteins, each with multiple copies such that the NPC contains 500-1000 proteins when assembled.
Nuclear import: nuclear pore complex (NPC) can transport __ molecules/sec and in which direction?
1000 molecules/sec, both directions simultaneously
nuclear import:NPC transports molecules up to what size?
40 kDa
_ can move through the nuclear pore complex (NPC) by passive diffusion.
Small, water-soluble molecules and proteins up to ~40 kDa
Nuclear Localization Signal (NLS):
Targets proteins to nucleus
Nuclear import components: cytosolic fibrils :
project outwards and helps channel cargo to the npc
Nuclear import components: scaffold nucleoporins
membrane-bending, stabilize membrane curvature
nuclear import channel components: channel nucleoporins:
line the central pore, many unstructured regions containing FG repeats, makes up the meshlike nature of the NPC
nuclear import channel components: nuclear basket:
Fibrils inside the nucleus converge at their distal ends to form a basket, function is not well understood
nuclear import channel components: membrane ring proteins
anchor the NPC to the nuclear envelope
Proteins synthesized in the cytoplasm are targeted for the nucleus by
a nuclear localization signal (NLS),having basic residues.
What is the NLS receptor?
importin a/B heterodimer
The Ran “gradient” ensures __ to nuclear transport.
directionality
nuclear import: proteins with an NLS bind to
an NLS receptor ; aka ; importin a/B heterodimer
nuclear import: what happens after proteins with an NLS bind to an NLS receptor ?
the protein/importin complex associates with cytoplasmic filaments
Nuclear import: What happens after the protein/importin complex associates with cytoplasmic filaments?
the protein/ importin complex passes through the NPC
What happens after the protein/importin complex passes through the NPC?
it associates with a GTPase called Ran
Nuclear import: How is importin B transported back to cytoplasm?
The Ran●GTP-importin b complex is transported back to the cytoplasm where Ran is converted to Ran●GDP and brought back in to the nucleus
Nuclear import: How is importin a returned to the cytoplasm?
importin a is returned to the cytoplasm via a protein called exportin
The Ran “gradient” ensures directionality to nuclear transport. The GTP-bound form only exists in
the nucleus
The Ran “gradient” ensures directionality to nuclear transport:GDP-bound form only exists in
the cytosol.
Describe the 5 steps involved in nuclear import:
- Proteins with an NLS bind to an NLS receptor (importin a/b heterodimer).
- Theprotein/importincomplexassociateswithcytoplasmicfilaments.
- Theprotein/importincomplexpassesthroughtheNPC…..
- …..andassociateswithaGTPasecalledRan.
- The Ran●GTP-importin b complex is transported back to the cytoplasm where Ran is converted to Ran●GDP and brought back in to the nucleus. Importin a is returned to the cytoplasm via a protein called exportin.
Mitochondrial import:for import into the matrix, a (usually) __ is required.
N-terminal sorting sequence
For import into the matrix, a (usually) N-terminal sorting sequence is required. If the protein localizes to the intermembrane space:
a second sorting sequence is needed
Matrix-targeting sequences are rich in hydrophobic, positively-charged and hydroxylated (Ser, Thr) residues, but lack
acidic residues
Matrix-targeting sequences tend to form
amphipathic helix.
mitochondrial import: targeting sequence characteristics (4):
(1) Usually N-terminal
(2) Rich in hydrophobic, positively-charged and hydroxylated residues (Ser, Thr)
(3) Lack acidic residues
(4) Tends to form amphipathic helix
Import into the mitochondria only occurs at points where
the inner and outer membranes are in close contact.
mitochondrial import:in the cytosol, Precursor proteins are kept in an unfolded state by the action of
the cytosolic chaperone Hsc70.
Precursor proteins are kept in an unfolded state by the action of the cytosolic chaperone Hsc70. This requires
energy in the form of ATP hydrolysis.
mitochondrial import: receptor in the outer mitochondrial membrane is called:
TOM 20 or TOM 22
mitochondrial import: The matrix-targeting sequence interacts with a receptor in the outer mitochondrial membrane called TOM20 or TOM22, what does this receptor do?
The receptor transfers the protein to the general import pore of the outer membrane composed of the protein TOM40.
mitochondrial import:the general import pore of the outer membrane composed of the protein:
TOM 40
Mitochondiral import: At contact sites between the inner and outer membranes, the protein passes through
the import pore of the inner membrane
mitochondrial import: At contact sites between the inner and outer membranes, the protein passes through the import pore of the inner membrane composed of the proteins (2):
(1) TIM 23
(2) TIM17
mitochondrial import:Matrix Hsc70 binds to
TIM44.
mitochondrial import: Matrix Hsc70 binds to TIM44. __ by this complex helps power translocation of the protein into the matrix.
ATP hydrolysis
mitochondrial import: As the matrix-targeting sequence emerges in the matrix, it is
cleaved by a matrix protease.
mitochondrial import: what happens before the protein can fold into its final conformation in the mitochondrial matrix?
As the matrix-targeting sequence emerges in the matrix, it is cleaved by a matrix protease.
mitochondrial import:As the matrix-targeting sequence emerges in the matrix, it is cleaved by a matrix protease.The protein can then fold into its final conformation, often (but not always), it is :
assisted by matrix chaperonins.
what is required for protein transport into the mitochondria?
The H+ electrochemical gradient generated by oxidative phosphorylation
The H+ electrochemical gradient generated by oxidative phosphorylation is also required for protein import into mitochondria. This ensures that:
only mitochondria that are actively respiring can import proteins. Hence, uncouplers block import.
mitochondrial import: Targeting of proteins to the intermembrane space requires a
second, hydrophobic targeting sequence
mitochondrial impoty: Targeting of proteins to the intermembrane space requires a second, hydrophobic targeting sequence that
does not allow the protein to completely pass through the TIM23/17 import pore.
mitochondrial import: Targeting of proteins to the intermembrane space requires a second, hydrophobic targeting sequence that does not allow the protein to completely pass through the TIM23/17 import pore.
The stalled protein is then
released from the pore into the membrane where a membrane- anchored protease cuts the protein, releasing it into the intermembrane space.
Unlike the proteins that enter the nucleus, mitochondria, chloroplasts and peroxisomes, most of the proteins that enter the endoplasmic reticulum begin to be translocated (transported) across the ER membrane ___
before the protein is completely synthesized.
Endoplasmic Reticulum (ER) Import
Unique Characteristic:
Protein begins translocation before complete synthesis
Unlike the proteins that enter the nucleus, mitochondria, chloroplasts and peroxisomes, most of the proteins that enter the endoplasmic reticulum begin to be translocated (transported) across the ER membrane before the protein is completely synthesized.
This requires that
the ribosome that is synthesizing the protein be attached to the ER membrane, giving it a rough appearance (and the name rough ER).
There are two separate populations of ribosomes in the cytosol:
- membrane-bound ribosomes (attached to the ER)
- free ribosomes
membrane-bound ribosomes are
attached to the cytosolic surface of the ER membrane and are synthesizing proteins that are translocated into the ER.
free ribosomes are
unattached to any membrane and are synthesizing all of the other proteins
ER Import: Steps 1 + 2 : 3
The emerging polypeptide with its ER signal sequence exposed is engaged by:
A complex of six proteins and an associated RNA molecule called the signal recognition particle (SRP)
ER Import: Steps 1 + 2 :
The emerging polypeptide with its ER signal sequence exposed is engaged by a complex of six proteins and an associated RNA molecule called the signal recognition particle (SRP). This binding:
halts translation and delivers the ribosome/polypeptide to the ER.
ER import: SRP delivers the ribosome/polypeptide to the
SRP receptor
ER import: SRP delivers the ribosome/polypeptide to the SRP receptor. This interaction is enhanced by
the binding of GTP to both SRP and its receptor.
ER import: what happens after SRP delivers the ribosome/polypeptide to the SRP receptor?
The ribosome/polypeptide is then transferred to the translocon, inducing it to open and receive the polypeptide which enters as a loop
in ER import, what enables another round of import?
Hydrolysis of GTP by SRP and its receptor free these factors for another round of import.
ER import; what happens after 2 rounds of import;
Translation resumes and the signal sequence is cleaved by a membrane-bound protease called signal peptidase
ER import: what happens. after the digestion by signal peptidase?
the rest of the protein is synthesized and enters the lumen of the ER.
ER import: steps 7 + 8: Following completion of translation
the ribosome is released causing the translocon to close. The newly-synthesized protein then folds.
Membrane proteins of the plasma membrane, Golgi, lysosome and endosomes are all inserted into
the ER membrane.
Membrane proteins of the plasma membrane, Golgi, lysosome and endosomes are all inserted into the ER membrane. From there, they are
transported to their correct location using amino acid and carbohydrate sorting signals.
membrane-anchored proteins:Type I:
- single pass,
-cleaved signal sequence at the N-terminus,
-uses SRP-SRP receptor to get to ER membrane,
-Nout-Cin
There are _ main types of membrane-anchored proteins:
6
membrane-anchored proteins:Type II:
-single pass
-no cleavable signal sequence
-uses SRP-SRP
receptor to get to ER membrane
-Nin-Cout
membrane-anchored proteins: Type III:
same as type II but Nout-Cin
-single pass
-no cleavable signal sequence
-uses SRP-SRP
receptor to get to ER membrane
membrane-anchored proteins:Tail-anchored:
-single pass,
-no cleavable signal sequence,
-hydrophobic
membrane-spanning sequence at C-terminus,
-does not use SRP-SRP receptor but the GET1/2/3 system to get to ER, posttranslational insertion, -Nin-Cout
membrane-anchored proteins:Type IV:
-multispanning
-no cleavable signal sequence
-uses SRP-SRP receptor for insertion of the first membrane-spanning domain but not subsequent ones,
IV-A are Nin-Cin,
IV-B are Nout-Cin
Difference IVa vs IVb
IV-A are Nin-Cin, IV-B are Nout-Cin
(in=cytosol, out=lumen or extracellular space)
membrane-anchored proteins:. GPI-anchored
entire protein is lumenal (out), cleaved signal sequence at the N-terminus, uses SRP-SRP receptor to get to ER membrane, anchored at C-terminus to membrane and then transferred to GPI anchor
memorize slide 26
26-38 missing
The roles of glycosylation include:
- Promote folding of proteins (e.g. protein secretion is blocked for certain proteins when tunicamycin is used or if Asn is mutated)
- Provide stability to proteins (e.g. some non-glycosylated proteins (fibronectin) are transported from the ER but are degraded faster)
- Promote cell-cell adhesion on plasma membrane proteins (e.g. leukocyte-endothelial cell attachment during inflammatory response)
- Act as a transport signal (e.g. mannose-6-phosphate directs proteins to the lysosome).
molecular chaperones assist in protein folding by preventing
aggregation of hydrophobic stretches of amino acids
Ultimately, if mannose residues are removed, the protein is targeted for
dislocation (transport out of the ER) and degradation in the cytosol.
Two types of ER chaperones:
- Classical chaperones 2. Carbohydrate-binding chaperones
Classical chaperones examples
Hsp70 (BiP), Hsp90, GRP94)
Carbohydrate-binding chaperones examples:
calnexin, calreticulin
Carbohydrate-binding chaperones bind to
polypeptides that are monoglucosylated. Terminal glucose is removed
and if folded the protein can exit the ER. If not, a glucosyltransferase adds one glucose back and the cycle repeats.
type 1 membrane anchored protein: signal sequence:
yes
type 1 membrane anchored protein: SRP/SRP receptor:
yes
Type 2 membrane anchored protein: signal sequence:
no
type 3 membrane anchored protein: signal sequence:
no
tail anchored membrane anchored protein: signal sequence:
no
type 4 membrane anchored protein: signal sequence:
no
GPI anchored membrane anchored protein: signal sequence:
yes
type 2 membrane anchored protein: SRP/SRP receptor:
yes
type 3 membrane anchored protein: SRP/SRP receptor:
yes
tail anchored membrane anchored protein: SRP/SRP receptor:
no
type 4 membrane anchored protein: SRP/SRP receptor:
yes
GPI anchored membrane anchored protein: SRP/SRP receptor:
yes
type 1 membrane proteins use a
cleavable signal sequence and a stop-transfer anchor (STA) sequence that acts as the membrane spanning domain. The translocon opens to release this hydrophobic stretch into the membrane.
Type II and III membrane proteins use a
signal-anchor (SA) sequence
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a
a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
The orientation is determined by
the positioning of the SA sequence within the translocon
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
The orientation is determined by the positioning of the SA sequence within the translocon.
This is in turn determined by the positioning of
positively-charged residues relative to the SA sequence:
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
The orientation is determined by the positioning of the SA sequence within the translocon.
This is in turn determined by the positioning of positively-charged residues relative to the SA sequence: if they are between the N- terminus and the SA, then it will be
Type II
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
The orientation is determined by the positioning of the SA sequence within the translocon.
This is in turn determined by the positioning of positively-charged residues relative to the SA sequence:if between SA and C- terminus, then it will be
Type III.
Type II and III membrane proteins use a signal-anchor (SA) sequence that acts as a dual signal sequence (directing the protein to the ER by the SRP) and the anchor or membrane-spanning sequence.
The orientation is determined by the positioning of the SA sequence within the translocon.
This is in turn determined by the positioning of positively-charged residues relative to the SA sequence: These positive residues remain
cytosolic.
Tail-anchored proteins are inserted into the ER __
after translation is completed
Tail-anchored proteins are inserted into the ER after translation is completed since
the hydrophobic stretch that is inserted into the bilayer needs to fully emerge from the ribosome.
slide 29 missing diagram slides 1-4
GPI (glycosylphosphatidylinositol)-anchored proteins insert into the ER like
Type I membrane protein
GPI (glycosylphosphatidylinositol)-anchored proteins insert into the ER like Type I membrane protein using a
stop-transfer anchor (STA) sequence
GPI (glycosylphosphatidylinositol)-anchored proteins insert into the ER like Type I membrane protein using a stop-transfer anchor (STA) sequence. An enzyme then (i) cleaves the protein within the lumen of the ER and (ii) transfers it to the assembled GPI anchor. – what is this enzyme?
transamidase
GPI (glycosylphosphatidylinositol)-anchored proteins insert into the ER like Type I membrane protein using a stop-transfer anchor (STA) sequence. An enzyme (transamidase) then (2):
(i) cleaves the protein within the lumen of the ER and (ii) transfers it to the assembled GPI anchor.
GPI (glycosylphosphatidylinositol)-anchored proteins:The purpose of transferring one lipid anchor for another (2) :
- The GPI anchor more readily diffuses in the membrane
- GPI can act as a targeting signal (e.g. apical localization versus
basolateral).
Type IV membrane proteins use combinaitions of:
stop-transfer anchor (STA) and signal-anchor (SA) sequences.
Type IV membrane proteins use combinations of stop-transfer anchor (STA) and signal-anchor (SA) sequences.
If the first SA sequence is a Type II SA (i.e. N-term+++++SA), then the protein will be
Nin (like a Type II membrane protein).
Type IV membrane proteins use combinations of stop-transfer anchor (STA) and signal-anchor (SA) sequences.If the first SA sequence is a Type III SA, then the protein will be
Nout (like a Type III membrane protein).
Hydropathic plots can help:
determine the type of membrane protein.
Hydropathic plots can help determine the type of membrane protein.
* The more hydrophobic an amino acid is, the more ___ the hydropathic
index.
positive
Hydropathic plots can help determine the type of membrane protein,The more hydrophilic the amino acid, the more __ the hydropathic index
negative
Besides proteins that reside in the ER, this organelle is the starting point for (3):
- Soluble proteins that will be secreted from the cell (e.g.
hormones) - Soluble proteins that are destined for the Golgi, lysosome or
endosomes (e.g. acid hydrolases) - Membrane proteins that will embed in the Golgi, lysosome,
endosomes or plasma membrane (e.g. Na+/K+-ATPase).
Besides proteins that reside in the ER, this organelle is the starting point for 3 other categories of proteins with other destinations, in order to export these proteins, the ER ensures that they are properly modified, folded and assembled by a process known as
quality control.
Four principle modifications that occur in the ER:
- Disulfide bond formation
- Glycosylation (the addition and processing of carbohydrates) 3. Folding of polypeptides chains and assembly of multisubunit
complexes - Proteolytic cleavage of amino-terminal signal sequences
what is glycosylation
the addition and processing of carbohydrates
ER modification: describe Disulfide bond formation
covalent bond formation between thiol groups of cysteine residues either on the same protein (intramolecular) or on two different proteins (intermolecular)
Disulfide bond formation is dependent upon
ER resident enzyme protein disulfide isomerase (PDI).
Disulfide bond formation is dependent upon the ER resident enzyme protein disulfide isomerase (PDI). Thus, only __ undergo this modification.
(i) secreted proteins or (ii) lumenal or extracellular domains of membrane proteins
Disulfide bonds __ protein structure
stabilize
Disulfide bonds stabilize protein structure – important for proteins that
will be subjected to either extremes in pH or environments with high levels of proteases.
- Glycosylation – begins with
the addition of a common oligosaccharide addition to asparagine residues in the consensus sequence Asn-X-Ser/Thr.
- Glycosylation – begins with the addition of a common oligosaccharide addition to asparagine residues in the consensus sequence Asn-X-Ser/Thr. Referred to as
N-linked glycosylation
- Glycosylation – begins with the addition of a common oligosaccharide addition to asparagine residues in the consensus sequence Asn-X-Ser/Thr. Referred to as N-linked glycosylation since
the oligosaccharide is added to the amine group of asparagine.
Glycosylation:The precursor oligosaccharide is transferred to the protein as the consensus sequence emerges from the translocon. Requires an ER membrane – bound enzyme complex called
oligosaccharyl transferase.
Glycosylation; The precursor oligosaccharide is assembled in a step-wise fashion on a lipid molecule called __
dolichol
glycosylation: dolichol contains __ carbons
75-95 carbons
glycosylation: Assembly of the 2 N-acetylglucosamine (GlcNAc) residues and the first 5 mannose residues takes place on
the cytosolic surface of the ER.
glycosylation: Assembly of the 2 N-acetylglucosamine (GlcNAc) residues and the first 5 mannose residues takes place on the cytosolic surface of the ER. The dolichol containing the seven sugar residues then
flips (using a transporter) to display the oligosaccharide in the lumen of the ER. The remaining mannose residues and glucose residues are added one at a time until the complete precursor is made.
glycosylation: Attachment of sugars to dolichol is mediated by
nucleotides (UDP- GlcNAc, UDP-glucose, GDP-mannose).
glycosylation: Tunicamycin is
a molecule that blocks the attachment of the first GlcNAc residue to dolichol
glycosyaltion: Tunicamycin is a molecule that blocks the attachment of the first GlcNAc residue to dolichol. This results in
non-glycosylated proteins
Since glycosylation is used as a sign of protein folding, and folding is required for export from the ER,tunicamycin increases
he level of unfolded proteins in the ER inducing the unfolded protein response (UPR).
molecular chaperones assist in protein folding by
preventing aggregation of hydrophobic stretches of amino acids.
Two types of ER chaperones:
- Classical chaperones
- Carbohydrate-binding chaperones
ER classical chaperones include:
Hsp70 (BiP), Hsp90, GRP94
ER carbohydrate binding chaperones (2):
calnexin, calreticulin
ER chaperones: arbohydrate-binding chaperones (calnexin, calreticulin) – bind to
polypeptides that are monoglucosylated. Terminal glucose is removed
and if folded the protein can exit the ER. If not, a glucosyltransferase adds one glucose back and the cycle repeats.
if mannose residues are removed, the protein is targeted for
dislocation (transport out of the ER) and degradation in the cytosol.
