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Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 1
1. Purposes of post-translational modifications
2. Quality control in the cytoplasm
3. Quality control in the ER
4. Selective post-translational proteolysis
5. Glycosylation in the ER and beyond: N-linked vs. O-linked
6. Other post-translational modifications
7. Modifications that alter location: A. Acylation: myristoylation, palmitoylation, prenylation
B. GPI anchor formation
8. Examples from pathobiologyA. ERAD discovered through studying CMV US 11 proteinB. HIV-1 envelope undergoes critical post-translational modifications
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 2
1. Review of Translation:
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 3
1. Purposes of Post-translational Events & Modifications:
A. Quality Control: Chaperones, Glycosylation
B. Degradation of misfolded proteins: Ubiquitination, ERAD
C. Proper protein function: Glycosylation, Phosphorylation, Ubiquitination
D. Target protein to proper locations: Acylation, GPI anchors
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 4
2. Quality Control in the Cytoplasm:
A. Anfinsen's dogma:
All information needed for folding contained in the amino acid sequence:Leads to the concept of spontaneous protein folding.
B. Problems with Anfinsen's dogma (and the notion of spontaneous folding):
Features of cellular environments cause misfolding & aggregation.
1. Some proteins take a very long time to fold spontaneously.
2. Some protein domains are prone to misfolding and aggregation.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 5
Protein folding in vivo
aggregation due to exposure ofhydrophobic regions
DEAD-END PATHWAY
nascent chainfinal folded structure
PRODUCTIVE PATHWAY
2. Quality Control in the Cytoplasm:B. Problems with Anfinsen's dogma, cont. Folding in the cell differs from refolding of a
denatured protein in vitro due to:
Vectorial nature of protein synthesis in vivo.
Exposure of hydrophobic regions during synthesis.
Translation happens more slowly than folding, requiring a “delay” mechanism to allow translation to "catch up".
Highly crowded cytoplasm: 300 mg/ml prot.
Folding in vitro is inefficient (20 - 30%); in the cell, efficiency close to 100%.
Conditions of stress found in vivo exacerbate misfolding and aggregation.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 6
2. Quality Control in the Cytoplasm: C. Molecular Chaperones: Proteins that mediate correct fate of other polypeptides but are not part of the final structure.
Fate includes folding, assembly, interaction with other cellular components, transport, or degradation.
A. History: Molecular chaperones initially identified as heat shock proteins, i.e. proteins
upregulated by heat shock and other stresses.
Heat shock causes protein denaturation with exposure and aggregation of interactive surfaces.
Heat shock proteins inhibit aggregation by binding to exposed surfaces during times of stress but also during normal protein synthesis
Thus, the stress response is simply an amplification of a normal function that is used by cells under non-stress conditions.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 7
D. Features of molecular chaperones:
i. Hsp 70 family members:
70 kD protein monomers.
Include DnaJ (bacteria); BiP (ER)
Stabilize polypeptide surfaces in an unfolded state.
Bind transiently to newly-synthesized proteins: paradoxically, efficient folding requires "antifolding".
Bind permanently to misfolded protein.
Have affinity for exposed hydrophobic peptides.
Do NOT bind a specific sequence.
Present in bacteria, eukaryotes & all compartments.
Regulated by ATP hydrolysis.
Undergo cycles of binding and release
Act with cofactors (i.e. DnaJ, GrpE, Hip, Hop, Bag1).
Hsp 70
Hsp 70 stabilizesthe nascent chain
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 8
D. Features of molecular chaperones:
ii. Chaperonins (GroEL, Hsp 60, TCP-1):
Facilitate proper folding
Bind and hydrolyze ATP
Bind transiently to 10-15% proteins, but 2-3fold more w/stress
60 kD proteins that form oligomeric, stacked double rings
Bring non-native substrate protein to central cavity folding where protected from aggregation with other non-native proteins
Cycles of binding and release until the protein is properly folded
GroEL (prokaryotic hsp 60) uses a cofactor, GroES.
iii. Others: I.e. small heat shock proteins, hsp 90, etc.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 9
iv. Cytosolic chaperone co-ordination:
Chaperones act in tandem. Stabilization by Hsp 70 plus cofactors) may be followed by use of Hsp 60 for proper folding.
From Frydman, J. Annual Rev. of Biochemistry 70:603, 2001
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 10
3. Quality control in the ER:
A. Translation and translocation of proteins into the ER:
Proteins that translocate into ER of mammalian cells include secretory proteins, TM proteins, or residents of a membranous compartment.
These are targeted to the ER CO-TRANSLATIONALLY by an N-terminal signal sequence that generally gets cleaved during translocation across the ER membrane.
The Signal Hypothesis SRP and SRP Receptor
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 11
Translocation of Secretory Protein
Translocation of Single Pass TM Protein Translocation of Double Pass TM Protein
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 12
3. Quality Control in the ER:B. Features of the ER:
Proteins need to be unfolded to translocate
Until signal sequence cleaved, N terminus of protein is constrained "incorrectly”
ER lumen is topologically equivalent to extracellular space
High oxidizing potential (unlike cytoplasm which is highly reduced)
High Ca+2 concentration unlike cytoplasm
Many sugars present along with machinery for glycosylation
As in cytoplasm: high protein conc. (100 mg/ml) promotes aggregation
As in cytoplasm: delay between translation/ translocation vs. folding
Site of specific post-translational events: signal cleavage, S-S bond formation, N-linked glycosylation and GPI anchor addition
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 13
3. Quality Control in the ER: C. Specific ER chaperones:
i. HSP 70 family members: BiP/GRP78 Recognize hydrophobic sequences in nascent chains. Undergo successive rounds of ATP-dependent binding and release.
Essential for translocation of newly-synthesized proteins across the ER lumen and for retrograde transport into the cytosol (see ERAD, below).
ii. Immunophilins/ FKBP - peptidyl prolyl isomerases.
iii. Thiol-disulfide isomerases - PDI and ERp57
iv. Calnexin and Calreticulin: Unique to the ER
Are lectins (carbohydrate binding proteins) Calreticulin - lumenal; Calnexin - integral membrane protein
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 14
3. Quality Control in the ER
D. Mechanisms
To pass QC checkpoints, protein must be correctly folded (most energetically favorable, native state)
If protein fails to fold properly it must be degraded
I. Example 1: BiP
BiP (Hsp70 in ER) binds to newly-synthesized and unfolded chains.
BiP stays associated with misfolded (but not properly folded) proteins.
Retention by BiP leads to degradation (see proteolysis below).
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 15
D. Mechanisms, cont.ii. Example 2: Calnexin/calreticulin bind
to incompletely folded monoglucosylated glycans
Cycles of binding/release controlled by:
Glucosidase II: cleaves glucose from core glycan
UDP-glucose: glucosyltransferase (GT) reglucosylates incompletely-folded proteins so that they bind lectins again
Thus GT acts as a folding sensor: proteins exit the cycle when GT fails to re-glucosylate. Glucose is a tag that signifies incomplete folding
3. Quality Control in the ER
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 16
3. Quality Control in the ERD. Mechanisms, cont.
iii. Example 3: Trimming of a single mannose is a signal for degradation.
Causes association with ER degradation-enhancing mannosidase like protein (EDEM), which is a link to ER-associated degradation (see proteolysis below)
Tsai, B. et al. Nature Rev. Mol. Cell Bio. 3: 246 (2002).
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 17
4. Selective post-translational proteolysis.Selective proteolysis is critical for cellular regulation.
3 steps for proteolysis in the cytoplasm: identify protein to be degradedmark it by ubiquitinationdeliver it to the proteasome, a protease complex that degrades it
A. The Ubiquitin/Proteasome system:Ubiquitin: A highly-conserved 76 aa protein present in all eukaryotes.Covalently attached to -amino groups in lysine side chains, Can be a single ubiquitin or multiple branched ubiquitins.
Signal for ubiquitination can be:1. Programmed via hydrophobic sequence or other motif2. Acquired by 1) phosphorylation, 2) binding to adaptor protein,
or 3) protein damage due to fragmentation, oxidation or aging.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 18
4. Post-translational Quality Control: Selective proteolysis. B. Ubiquitination requires 3 enzymes:
E1 (ubiquitin-activating enzyme) activates ubiquitin (U)
E2 (ubiquitin-conjugating enzyme) acquires U via high-energy thioester
E3 (ubiquitin ligase) transfers U to target proteins
Hierarchical organization: one or few E1s exist, more E2s, many E3s.
Other functions for ubiquitination (to be discussed in plasma membrane lecture).
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 19
4. Post-translational Quality Control: Selective proteolysis
B. The Proteasome - high molecular weight (28S) protease complex that degrades ubiquitinated proteins in the cytoplasm
Present in cytoplasm and nucleus, not ER
Uses ATP
Contains a 700 kD protease core and two 900 kD regulatory domains.
Highly conserved and similar to proteases found in bacteria.
Shaped like a cylinder.
Proteins enter the cavity, and are cleaved into small peptides.
Most but not all proteasome substrates are ubiqutinated.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 20
4. Post-translational Quality Control: Selective Proteolysis C. Misfolding in the ER results in:
ER-associated degradation (see below) Lysosomal degradation (next lecture)
ER-Associated Protein Degradation (ERAD):
Earlier notion was that ER had proteases.However, in fact most ER proteins targeted for degradation undergo
retrograde translocation into cytosol and delivery to the proteasome.
ER lumenmisfolded protein
cytoplasm
translocon
hsp 70 (BiP)
proteasome
U
U ATP
Ucytoplasm
ER lumen
ER-Associated Degradation (ERAD)
U
U ubiquitin
UU
U
UU
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 21
5. Glycosylation in the ER and beyond:
Role of sugars in the ER: bulky hydrophilic groups that maintain proteins in solution, affect protein conformation, and allow lectins to facilitate folding and exert quality control.
A. N-linked glycosylation - co-translational; recognizes Asn-x-Ser/Thr on nascent
chainCatalyzed by oligosaccharyltransferases - integral
membrane proteins with active site in the lumen. Transfers a preformed "high mannose" 14-residue sugar(Glc3Man9GlcNAc2) en bloc to asparagine residues on the acceptor nascent polypeptide chains. Highly conserved in mammals, plants, fungi. i. Donor molecule is dolichol-P-P-Glc3Man9GlcNAc2.
Dolichol is a very long lipid carrier. ii. Subsequent trimming of residues (also called
processing) off core sugar attached to protein occurs in the ER via glucosidases and mannosidases.
N glycosylation can be prevented using: Tunicamycin: inhibits formation of the dolichol-P-P
precursor.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 22
5. Glycosylation in the ER and beyond:
A. N-linked glycosylation, cont.iii. -Glucosyltransferase recognizes
misfolded glycoproteins and reglycosylates them.
iv. Calreticulin and calnexin serve as sensors by binding to mono-glucosylated proteins, facilitating their folding and assembly.
v. Only glycoproteins that have been correctly folded (by calnexin and calreticulin), get packaged into ER-to-Golgi transport vesicles.
vi. In the cis Golgi, further processing & addition of GlcNac's to form branched structures
vii. Addition of more sugar residues in the trans-Golgi (I.e. fucose and sialic acid) to produce the diversity that is seen in mature glycans.
Bacteria: no N-glycosylation via dolichol
Yeast: have only oligomannose type N-glycans, because they don't have the ability to add GlcNac in the trans Golgi
Since bacteria & yeast lack Glc-Nac transferase enzyme, this enzyme demarcates a fundamental evolutionary boundary between uni- and multicellular organisms.
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 23
2. trimmingof glucose
residues in ER
1. core sugar added en bloc
co-translationally to asparagine
residues in nascent chains
(from dolicholdonor)
3 glucosyltransferase adds back
glucose in ER to unfolded
glycoproteins
4. monoglucosylated proteins are bound
and folded by calnexinand calreticulin
cis-Golgimedial-Golgi
6. in the medial andtrans-Golgi
moreN-acetylglucosamines
and fucose are added aswell as galactoses and
sialic acid (terminalglycosylation)using GlcNac
transferase
5. in theGolgi,
trimming of mannose
residuesoccurs
= Sialic Acid
= GlcNac
= Mannose
= Glucose
= Galactose
Simplified view of N-glycosylation
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 24
5. Glycosylation in the ER and beyond:
B. O-linked glycosylationMany different types of sugars are added onto -OH of serine
or threonine residues.
Most of these are added in ER or Golgi
However, addition of N-acetylglucosamine (GlcNac) can occur in cytoplasm on many different types of proteins
May play an important role in signaling, much like phosphorylation
May act in signaling to oppose phosphorylation
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 25
6. Other post-translational modifications:A. Disulfide bond formation in the ERProtein disulfide isomerase (PDI): in the ER: catalyzes oxidation of disulfide bonds
in the cytosol and at the plasma membrane: reduces disulfide bonds
Other proteins that act like PDI may be even more important in disulfide bond formation
Requires action of a regenerating molecule (i.e. glutathione); NADPH is the source of redox equivalents.
substrate
substrate
S
S
SH
SH
PDI
S
S
PDI
SH
SH
redoxregenerator
SH
SH
S
S
redoxregenerator
Disulfide Bond Formation
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 26
6. Other post-translational modifications, cont.B. Phosphorylation
Kinases phosphorylate proteins at the hydroxyl groups of serine, threonine, and tyrosine
Occurs in cytoplasm and nucleus
C. Intracellular Proteolytic CleavageFurin - protease that cleaves specific sites, located in the trans-Golgi
network and in endosomes.
D. Modified amino acids: hydroxyproline, hydroxylysine, 3-methylhistidine
E. Lipidation
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 27
7. Post-translational Modifications that Alter Location:
A. Acylation - Lipid attachments that anchor proteins to the membranes:
Include myristoylation, palmitoylation, prenylation
Involves addition to protein of fatty acids (long hydrocarbon ending in COOH)
Allows proteins to target to the cytoplasmic faces of membrane compartments
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 28
7. Post-translational Modifications that Alter Location: i. Myristoylation: addition of C-14 FA myristate to N-terminus in cytoplasm
Donor is myristoyl CoAOccurs co-translationally in the cytoplasm; can occur post-translationally when hidden motif is revealed by protein cleavage (i.e. pro-apoptotic protein BID)Enzyme NMT recognizes consensus sequence at N-terminus often revealed by aconformational change (myristoyl switch).Promotes weak but typically irreversible interaction with cytosolic membrane faceMyristoylated proteins traffic through the cytoplasmMyristoylation necessary but not sufficient for membrane bindingSecond signal needed for membrane binding: myristate plus basic (basic aa’s interact with acidic phospholipids PS and PI), or myristate plus palmitate
Myristoylation GlyMet
Gly
O
Gly
N-myristoyl transferase (NMT)
CH3
C-N-CH2-C
H
Removal of initiating methionine
Addition of myristate to N-terminal
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 29
7. Post-translational Modifications that Alter Location: ii. Palmitoylation - addition of a C-16 fatty acid to the thiol side chain of an internal cysteine
residue.Promotes a reversible interaction with membranePalmitoylated proteins traffic to membrane via cytoplasm or via secretory pathwayEnzymes not well understood
Myristoylated and palmitoylated proteins are enriched in caveolae and rafts
Palmitoylation
Cys
SH2
C
O
CH 3SH
Cys
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 30
7. Post-translational Modifications that Alter Location: iii. Prenylation - addition of prenyl groups (two types) to S in internal cysteine a. Farnesylation - C15 fatty acid to C terminus by thioester linkage
Occurs at CAAX sequences: cys, 2 aliphatic residues and C-terminal residue
After attachment, last 3 residues are removed and new C terminal methylated
Creates a highly hydrophobic C terminus
b. Geranylgeranylation - similar to above but addition of C-20 to C terminal Cys
Cys
S
AA X
Cys
SH
AA X
Cys
S
Cys
S -O-CH3
addition of farnesyl group
proteolysis
methylation
Farnesylation
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 31
7. Post-translational Modifications that Alter Location: iii. Examples of acylated proteins important for pathogenesis: Myristoylated proteins: HIV-1 Gag, HIV-1 Nef which target to the PM; Arfs
involved in coat protein binding to vesicles (see ER-Golgi lecture)
Palmitoylated proteins: caveolin (see PM lecture)
Dual acylated proteins (myr plus palm): found in Src tyrosine kinases, i.e. Lyn, Fyn, Hck, etc. (see Signaling overview lecture)
Met-Gly-Cys signal for dual acylation
Farnesylation: Ras, does not insert into the membrane or act in signal transduction unless farnesylated.
Geranylgeranylation: Rab GTP-binding proteins that mediate initial vesicle targeting events (see PM lecture)
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 32
7. Post-translational Modifications that Alter Location:B. GPI anchors - Glycophosphatidyl inositol (GPI) attached to the C terminusComposed of oligosaccharides and inositol phospholipidsProvides a mechanism for anchoring cell-surface proteins to the membrane
as a flexible leash that allows the entire protein (except for anchor) to be in extracellular space (unlike a transmembrane protein)
Added to translocated proteins in ERTargets to PM via secretory pathwayUnlike with N- or O-glycosylation, no more than ONE GPI anchor per proteinUnlike acylation, targets proteins to outer leaflet of plasma membrane Can be cleaved by PI-phospholipase C (PI-PLC)Are minor components on mammalian cells but abundant on surfaces of parasitic
protozoa (i.e. trypanosomes and Leishmania) and yeastsConcentrated in lipid rafts
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 33
C=O
CH2
CH2
NH
P
CH2CH2NH3
MannoseN-Acetylgalactosamine
Inositol head ofPHOSPHATIDYLINOSITOL
ETHANOLAMINE
Protein
C-terminus
Lipid Bilayer
Glucosamine
OOLIGOSACCHARIDELIGOSACCHARIDE
P
Structure of a GPI anchor:
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 34
7. Post-translational Modifications that Alter Location:
B. GPI anchors - Functions:Stronger anchoring to PM
than acylationSome GPI anchors can be
replaced with TM anchors and be functional; others cannot
Crosslinking results in signal transdcution across bilayer, including Ca influx, tyrosine phosphorylation, cytokine secretion, etc.
Can interact with TM proteins capable of intracellular signaling
Can indirectly modulate activity of cytosolic signaling molecules assoc. w/ lipid rafts
ER lumen
cytoplasm
ER
GPI
ER lumen
cytoplasm
ER
=C terminalGPI signal
ER lumen
cytoplasm
ER
GPI
cleavage of hydrophobic
C terminalsequence and
transfer ofpreformed GPI
moiety
GPI Anchor Formation
GPI
ER lumen
cytoplasm
ER
vesicleformation
vesicletransport
proteintranslation
and translocation
vesiclefusion
cytoplasm
extracellular space
PM
GPI
cytoplasm
extracellular space
PM
=N terminal signal sequence
GPI anchored protein tethered to outer leaflet
of PM
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 35
8. Examples from Pathobiology:A. ERAD discovered through study of CMV US11 (Wiertz et al., Cell 84: 769, 1996).
1. MHC class I, a TM protein, binds viral peptides produced in cells and presents them at the cell surface to cytotoxic T cells.
2. CMV evades the immune system by targeting MHC class I for destruction soon after it is synthesized and translocated into the ER. How does it do this?
3. CMV US11 protein expressed alone causes MHC class I destruction.
4. US 11 effect is sensitive to proteasome inhibitors and involves MHC class I localization to cytoplasm, implying movemnt of US 11 out of ER into cytoplasm for degradation.
5. Before this paper, only forward movement thru translocon was thought to occur; this paper by Ploegh’s group studying a CMV protein raised the possibility of retrograde movement thru translocon.
6. Subsequently, retrograde movement thru translocon for degradation (ERAD) was shown to be a common in non-infected cells.
7. Note that MHC class I needs to be poly-ubiquitinated for retrograde transport to occur, implying a role for ubiqutination in retrolocation, not just in targeting for degradation.
8. Additional studies reveal that other pathogens use this mechanism: I.e. HIV-1 accessory protein Vpu promotes degradation of CD4 by ERAD.
ERAD:
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 36
8. Examples from Pathobiology:B. HIV-1 envelope protein undergoes many
critical post-translational modifications
1. HIV env consists of gp120 soluble portion bound non-covalently to TM gp41.
Role is to bind CD4 and chemokine receptors during HIV-1 entry.
2. Co-translationally translocated into ER as gp160.
3. Has ~30 potential sites for N-linked glycosylation in ER.
If non-glycosylated: won’t bind CD4.
Some glycosylations are dispensible for proper folding; others are needed.
4. Forms 10 disulfide bonds in ER (9 are in gp120 portion).
5. Trimerization of HIV-1 env in ER
6. Proper folding/trimerization equires BiP, calnexin, calreticulin, and PDI.
7. In Golgi: protease-mediated cleavage of gp160 to gp120 and gp41.
Land, A. and I. Braakman, Biochimie 83: 783 (2001).
Post-translational Modifications:
March 28, 2006 J. R. Lingappa, Pabio 552, Lecture 1 37
Additional Reading:
*Tsai, B. et al. Retro-translocation of proteins from the endoplasmic reticulum into the cytosol. Nature Rev. Mol. Cell Bio. 3: 246 (2002).
Freiman, R. N. and R. Tijan. Regulating the regulators: Lysine modifications make their mark. Cell 112: 11 - 17 (2003).
Resh, M. Fatty acylation of proteins: new insights into membrane targeting of myristoylated and palmitoylated proteins. BBA 1451: 1 (1999).
Land, A. and I. Braakman. Folding of the human immunodeficiency virus type I envelope glycoprotein in the endoplasmic reticulum. Biochimie 83: 783 (2001).
Chatterjee, S. and S. Mayor. The GPI-anchor and protein sorting. Cell Mol. Life Sci 58: 1969 (2001).
McClellan A et al. Protein quality control: chaperones culling corrupt conformations. Nat Cell Biol. 2005 Aug;7(8):736-41.
Gill, G. SUMO and ubiquitin in the nucleus: different functions, similar mechanisms? Genes Dev. 2004 Sep 1;18(17):2046-59. Review.