Complex Carbohydrate Research Center

Complex Carbohydrate Research Center
BCMB8130: Glycobiology Feb 13, 2007 Glycans and cellular functions I - protein folding, ER processing, quality control Kelley Moremen Complex Carbohydrate Research Center 315 Riverbend Rd. University of Georgia Athens, GA

Asn-linked glycosylation: what does it do and why do we care???
4 Man 3 2 6 Asn GlcNAc Glc Precursor structure conserved from yeast to mammals and plants (+ possibly archaebacteria). Enormous cost in energy Amazing conservation in linkage pattern and necessary genes/gene products (enzymes) Standard dogma Provides hydrophilicity (solubility) to the newly synthesized protein Influences protein folding Protection from proteases Provides a scaffold for adding targeting information (i.e. lysosomal enzymes) Scaffold for recognition by endogenous lectins (carbohydrate-receptor interactions) Scaffold for exogenous recognition events (pathogen, toxin binding) BUT: Why start with this and only this structure?

Precursor synthesis The oligosaccharide is assembled sugar by sugar onto the carrier lipid dolichol High energy pyrophosphate bond

Helenius, A. and Aebi, M. (2004) Ann. Rev Biochem 73, 1019-49

(From Alberts el al (1994) Molecular Biology of the Cell, 3rd ed.)
Co-translational glycosylation is the rule from yeast to plants and animals. The steps in the synthesis of the lipid-linked precursor and the final structure are highly conserved. Even the exceptions are indicative of a highly conserved process: (From Alberts el al (1994) Molecular Biology of the Cell, 3rd ed.)

The diversity of dolichol-linked precursors to Asn-linked glycans likely results from secondary loss of sets of glycosyltransferases Samuelson et al (2005) P.N.A.S. 102, The vast majority of eukaryotes synthesize Asn-linked glycans (Alg) by means of a LL precursor (Dol-PP-GlcNAc2Man9Glc3) Characterized Alg glycosyltransferases and dolichol-PP-glycans of diverse protists.

Models for gain or loss of N-glycan biosynthesic enzymes
Samuelson etal (2005) P.N.A.S. 102, Conclusions: Common ancestry predicts the Alg glycosyltransferase inventory of each eukaryote. This inventory accurately predicts the Dol-PP-glycans observed. Alg glycosyltransferases are missing in sets from each organism. Dol-PP-GlcNAc2Man5 and Dol-PP- and N-linked GlcNAc2 have not been identified previously in some of the wild type organisms. Diversity of protist and fungal Dol-PP-linked glycans likely results from secondary loss of Alg genes from a common ancestor containing the complete set of Alg glycosyltransferases.

Transfer of the glycan to nascent polypeptide chains
Higher eukaryotes: greatest efficiency for transfer of Glc3Man9GlcNAc2, but smaller glycans can be transferred under stress conditions (i.e. type 1 CDG patients where the LLO biosynthetic enzymes are compromised, mutant cell lines that are defective in LLO biosynthesis)

Structure of the Sec61 pore demonstrates the dimensions of co-translational extrusion of the polypeptide chain through the ER membrane van den Berg et al (2004) Nature 427, 36-44

van den Berg et al (2004) Nature 427, 36-44

Unfolded protein Native structure/ multimeric complex
Upon arrival in the ER Unfolded protein Native structure/ multimeric complex

Protein folding Chaperones as general folding helpers: BiP prevents premature, incorrect folding of segments that arrive in the ER lumen Formation of Disulfide bridges: ER is an oxidative environment which promotes the formation of SS bridges. Protein Disulfide Isomerase (PDI) to help with the formation of disulfide bonds or the rearrangement of disulfide bonds PPI (peptide prolyl isomerase) to catalyze the isomerization of peptidyl-prolyl bonds, which can be rate-limiting in protein folding Enzymes of the quality control cycle: calnexin (soluble) and caltreticulin (membranes associated)

Chaperones as folding helpers
BiP (Hsp70 family member in the ER) keeps the newly arrived protein in an unfolded state to prevent premature folding of domains that have emerged earlier from the translocon Cleaved signal sequence

Protein disulfide isomerase (PDI)
contains 2 cysteines that are close together and that are easily interconverted between the reduced SH form and the oxidized S-S form. NOTE: S-S bonds are only formed in the ER. Oxidized PDI facilitates the formation of SS bonds, acts as oxidant that is reduced. Reduced PDI helps with the formation of the correct disulfide bonds within the substrate (see figure). Exits unchanged from the reaction

Following protein folding in the ER
SH HS S-S E1 folding intermediates Use of viruses: shut down host protein synthesis glycoproteins that transit ER E1red “Tags” for following folding in ER: disulfide bonds glycosylation Eox1 Eox2 Semliki Forest virus infection of CHO cells E1*; p62 (cleaved to generate E2) p62 has glycosylation site near N-terminus; E1 does not Ab: BiP Cnx Crt Pulse 1’; Molinari & Helenius (2000) Science 288: 331

Benham & Braakman (2000) Crit Rev Biochem Mol. Biol. 35, 433-473

Peptidyl-prolyl-isomerase (PPI)
PPI catalyzes the rotation about peptide prolyl-bonds, which can be rate limiting in the folding of protein domains

Protein Folding in the ER:
Transport to the Golgi complex ER exit sites Classical ATP-driven Chaperones Translation is initiated in the cytoplasm Binding of SRP to the emerging polypeptide. Ribosome-peptide-SRP complex binds to the SRP receptor on the ER membrane Peptide is extruded through the Sec61 translocon in the ER membrane. Glycans are added from Dol-P-OS to the emerging peptide via OST. GRP94 BiP GRP170 CRT PDI Lectin-based Chaperones SRP-Rec ERp57 OST BiP Dol-P-OS Sec61 CNX SRP Ribosome mRNA Chaperones bind to facilitate protein folding: ATP driven chaperones bind directly to polypeptides Work in conjunction with PDI and PPIase to facilitate folding. Lectin chaperones, CNX and CRT, bind to the glycan and facilitate folding in conjunction with ERp57, a PDI homolog. Folded proteins are packaged at ER exits sites for transport to the Golgi complex

Carbohydrate structures influence glycoprotein folding in the ER
-KDEL -KDEL Calreticulin Protein Folding (other chaperonins??) Glc II Glc II Glc Trans (Parodi enzyme) (only acts on unfolded glycoproteins) Glc I Glc II Anteriograde Transport to Golgi Glc II Glc II Endoplasmic Reticulum Protein Folding (other chaperonins??) Legend P a1,2-Man OST Calnexin P ER membrane a1,3-Man a1,6-Man Dol b1,4-Man b1,4-GlcNAc Cytoplasm a1,2-Glc a1,3-Glc

Events in the calnexin (CNX) cycle.
Schrag et al (2003) TIBS 28, 49-57 Membrane-anchored CNX recognize monoglucosylated high-mannose sugars attached to the unfolded or incompletely folded glycoprotein (red line). Interactions with ERp57 facilitate the formation of proper disulfide bonds in the substrate glycoprotein. Two possible tracks (blue or green arrows) upon release from CNX or calreticulin. Folding and transport (green track) Release of properly folded protein from CNX leads to deglucosylation of the terminal glucose by Glc II (ii). ERGIC-53 recognizes the Man9GlcNAc2 glycoprotein for export out of the ER (iii). Terminal misfolding -> disposal (blue track) Terminal glucose of the glycan (pale blue) is removed by glucosidase II (Glc II) (ii). Improperly or incompletely folded glycoproteins are reglucosylated (iii) by UGGT or a mannose residue (yellow) is trimmed by ER Man I (iv). Additional cycles of binding by CNX or CRT follow reglucosylation by UGGT Mannose trimming by mannosidase marks the protein for retrotranslocation and degradation.

Domain structures of calnexin and calreticulin.
3-D structure of calnexin calnexin Lectin domain (blue) Extended arm called the P-domain containing four repeat modules, P1–P4 (cyan, green, yellow and red). Repeat modules formed by antiparallel interactions of two different proline-rich sequence motifs. Glucose-binding site in the lectin domain is indicated by a ball-and-stick model (green) Glc1Man9GlcNAc2 glycoprotein modeled by superimposing the terminal glucose on the observed position of bound glucose. Position of the model glycoprotein (RNase B, magenta) suggests that interaction with the P-domain is likely. calreticulin glycan- binding protein-substrate binding ?? Schrag et al (2003) TIBS 28, 49-57

Helenius, A. and Aebi, M. (2004) Ann. Rev Biochem 73, 1019-49

Sequence alignment of lectin domains of CNX, ERGIC-53 and VIP-36
Canine CNX and rat ERGIC-53 sequences were aligned according to the structural similarities (P-domains were omitted). Pink boxes mark those residues whose C atoms are within 3 A° of their counterparts in the other structure. Secondary structure assignments according to assignments in the Swiss-Pdb Viewer are shown (CNX in dark blue, ERGIC-53 in green). Sequence identities of equivalent residues (those in pink boxes) are in bold purple characters. Aligned residues that are farther apart than 3 A° are red. Residues important in ligand binding in ERGIC-53 and their counterparts in VIP36 are yellow. The alignment of the VIP36 amino acid sequence is based solely on visual comparison with that of ERGIC-53. Schrag et al (2003) TIBS 28, 49-57

Specific interaction of ERp57 and calnexin determined by NMR spectroscopy and an ER two-hybrid system Pollock etal (2004) EMBO J. 23, Calnexin and ERp57 act cooperatively to ensure a proper folding of proteins in the endoplasmic reticulum (ER). Calnexin contains two domains: a lectin domain and an extended arm termed the P-domain. ERp57 is a protein disulfide isomerase composed of four thioredoxin-like repeats and a short basic C-terminal tail. Here we show direct interactions between the tip of the calnexin P-domain and the ERp57 basic C-terminus by using NMR and a novel membrane yeast two-hybrid system (MYTHS) for mapping protein interactions of ER proteins. Our results prove that a small peptide derived from the P-domain is active in binding ERp57, and we determine the structure of the bound conformation of the P-domain peptide.

Recognition of local glycoprotein misfolding by the ER folding sensor UDP-glucose:glycoprotein glucosyltransferase Ritter and Helenius (2000) Nat. Structure Biol. 7, UGGT acts as a folding sensor by specifically glucosylating N-linked glycans in misfolded glycoproteins thus retaining them in the calnexin/calreticulin chaperone cycle. To investigate how GT senses the folding status of glycoproteins, we generated RNase B heterodimers consisting of a folded and a misfolded domain. Only glycans linked to the misfolded domain were found to be glucosylated, indicating that the enzyme recognizes folding defects at the level of individual domains and only reglucosylates glycans directly attached to a misfolded domain. The result was confirmed with complexes of soybean agglutinin and misfolded thyroglobulin. BS: RNaseB subtilisin treated AS: RNaseA subtilisin treated Bsc: RnaseB with scrambled disulfides Asc: RnaseA with scrambled disulfides Solid bar: denstured domain Open bar: native domain

What happens if there are folding problems?
glucosidases Folding problem Simply trapped in a malfolded conformation mutation that leads to misfolding unassembled multimer subunit glucosyltransferases Calnexin and BiP bind irreversibly to misfolded proteins Unfolded protein response pathway Increased transcription of chaperones and folding catalysts

Model for IRE1, XBP1, and ATF6 function in UPR
Yoshida (2001) Cell 107, 881

Induction of proteins during glucose starvation
[3H] leucine labeling for 16 hours of mammalian (K12) cells; 2d gel electrophoresis; autoradiography + glucose Grp94 (hsp90 of ER lumen) Grp78 (hsp70 of ER lumen - BiP) hs) Hsc70 of cytosol Ac) actin glucose Response to many stimuli - anything affects protein folding in ER: affects glycosylation (glucose starvation, inhibitors of glycosylation such as tuni- camycin); ß-mercaptoethanol; calcium ionophores UPR - Unfolded protein response (found in all eucaryotes) Lee et al (1983) J. Biol. Chem. 259: 4616

Failure of quality control
-KDEL -KDEL Calreticulin Protein Folding (other chaperonins??) Glc II Glc II Glc Trans (Parodi enzyme) (only acts on unfolded glycoproteins) Glc I Glc II Anteriograde Transport to Golgi Glc II Glc II Endoplasmic Reticulum Protein Folding (other chaperonins??) Legend P a1,2-Man OST Calnexin P ER membrane a1,3-Man a1,6-Man Dol b1,4-Man b1,4-GlcNAc Cytoplasm a1,2-Glc a1,3-Glc

What happens when proteins can not fold
correctly in the ER: failure in quality control Calreticulin Protein Folding (other chaperonins??) ER-associated Degradation (ERAD) Glc II Glc Trans (Parodi enzyme) (only unfolded glycoproteins) Glc I Glc II Glc II Translocation to cytoplasm through the Sec61 pore complex Endoplasmic Reticulum Protein Folding (other chaperonins??) P OST Calnexin P ER membrane Dol Proteasome Degradation Cytoplasm + Amino acids

What determines whether a protein is terminally misfolded?
Glucose residues: determine interaction with calnexin/calreticulin The removal of a single mannose residue (by ER mannosidase I) while still in the cycle promotes association with EDEM (ER-degradation enhancing 1,2 mannosidase like protein) and targets the protein for ER associated degradation (ERAD).

Benham & Braakman (2000) Crit Rev Biochem Mol. Biol. 35, 433-473

Multiple steps in ER-associated degradation of unfolded glycoproteins
(Jakob, C. A., Burda, P., Roth, J., and Aebi, M. (1998) J. Cell Biol. 142, ) Protein Folding (other chaperonins??) Lectin? (EDEM?, Htm1p?) Step 2 ER Man I gene disruption Calreticulin Glc II dMNJ Kif Glc Trans (only unfolded glycoproteins) Glc I Glc II ER Man I Step 1 Step 3 Glc II Endoplasmic Reticulum Other Components? Protein Folding (other chaperonins??) Sec61 Translocon P Calnexin OST P ER membrane Dol Cytoplasm Cytosolic N-glycanase Proteosome Degradation Step 4 Two of the steps involve Class 1 mannosidases or mannosidase-related proteins

N-Linked oligosaccharide processing, but not association with
CNX/CRT is highly correlated with ERAD of antithrombin Glu313-deleted mutant Tokunaga et al (2003) Arch. Biochem Biophys 411, Examined the combined effects of inhibitors of glycosidases, protein synthesis, proteasome, and tyrosine phosphatase on ERAD of a Glu313-deleted mutant of antithrombin. Kifunensine suppressed ERAD (mannose trimming critical. Cycloheximide and puromycin suppress ERAD, the effects cancelled by pretreatment with Cast. Kifunensine suppresses ERAD even in Cast-treated cells, (does not require binding to CNX and/or CRT). Inhibitors of ER mannosidase I and protein synthesis suppress ERAD at different stages and processing of N-linked oligosaccharides highly correlated with the efficiency of ERAD.

Overexpression of ERManI accelerates the timing clock for disposal (even for wild type proteins!!)
ERManI overexpression accelerates AAT-Z disposal (non-proteasomal->proteasomal) ERManI overexpression targets WT AAT from fully secreted to fully degraded protein ERManI acts is the rate-limiting timer for targeting proteins for disposal Wu et al (2003) P.N.A.S. 100,

Glycoprotein Biosynthesis
Endoplasmic Reticulum Golgi Complex Endo a-Man A ER Man II Man III/IIx(??) Golgi GnTI C dMNJ dMNJ Kif Sw Kif Glc II Golgi Man IA/IB GnTI B Golgi Man II GnTII Glc II ER Man I P Glc I UGGT OST P Dol ER membrane Failure of Quality Control Cytosol Lysosome Glycosylasyaraginase Legend Chitobiase 1,2-Man  1,2-Glc Cytosolic -Man  1,3-Man  1,3-Glc B 1,2-GlcNAc Proteosome degradation  1,6-Man Lys -Man Lys 1,6- Man Lys -Man  1,4-Man  1,4-Gal Amino Acids  1,4-GlcNAc  2,6-NeuNAc Quality Control Glycoprotein Degradation (ERAD)

Howell and Herscovics labs
Structure of the yeast ER a-mannosidase I Howell and Herscovics labs (Vallee, F., Lipari, F., Yip, P., Sleno, B., Herscovics, A., and Howell, P. L. (2000) EMBO J. 19, ) Side view of (aa)7 barrel End view of (aa)7 barrel

Howell and Herscovics labs units in the crystal lattice
Structure of the yeast ER a-mannosidase I Howell and Herscovics labs (Vallee, F., Lipari, F., Yip, P., Sleno, B., Herscovics, A., and Howell, P. L. (2000) EMBO J. 19, ) Protein unit 1 N-glycan 1 Protein unit 2 N-glycan2 Display of two protein units in the crystal lattice Active site in core of barrel

Domain structures of Class 1 mannosidases
EDEM1 EDEM2 EDEM3 KDEL ER MAN I Protease-associated domain Golgi Man IA 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Threading of yeast HTM1 results in close fit with ER Man I
Human ER Man I + DMJ Yeast HTM1 Overlap of side chains in catalytic core Ca backbone overlap

EDEM2 overexpression accelerates mutant 1antitrypsin disposal
EDEM2 overexpression can accelerate the degradation of AAT mutants PI Z or NHK Acceleration of ERAD by EDEM2 requires a glycosylated ERAD substrate Truncation of the COOH-terminal extension eliminates EDEM function in ERAD EDEM2 in cell extracts can be found in a complex with calnexin by co-IP (data not shown) Partially purified EDEM2 has been tested for cleavage of Man9-5GlcNAc2: no cleavage detected with any oligosaccharide structure Mast et al (2005) Glycobiology 15, Steve Mast

Analogies for ERAD Sifers, R.N. (2003) Science 299,

Cellular response to unfolded proteins in the ER
Only let correctly folded proteins out of the ER (Quality Control) Degrade the unfolded proteins before they accumulate in the ER (ER-associated degradation, ERAD)

Two routes for exit from the ER:
gating the exit of correctly folded proteins down the secretory pathway -KDEL Calreticulin Protein Folding Glc II to Golgi Complex Glc II Glc Trans (Parodi enzyme) (only unfolded glycoproteins) Glc I Glc II ER-associated degradation (ERAD) 9 12 11 10 8 7 6 5 4 3 2 1 Glc II Endoplasmic Reticulum Timing? Recognition? P OST Calnexin P ER membrane Dol Proteasome Degradation Cytoplasm + Amino acids

Cholera Toxin Is Exported from Microsomes by the Sec61p Complex
Schmitz et al (2000) J. Cell Biol. 148, Cholera toxin is transported from the plasma membrane to the ER The A1 subunit (CTA1) is transferred to the cytosol. Export of CTA1 from the ER to the cytosol was investigated in a cell-free assay using microsomes loaded with CTA1. Export of CTA1 from the microsomes was time- and adenosine triphosphate-dependent and required lumenal ER proteins. By co-IP CTA1 was shown to be associated during export with the Sec61p complex Export of CTA1 was inhibited when the Sec61p complexes were blocked by nascent polypeptides arrested during import, thus export of CTA1 depended on translocation competent Sec61p complexes. Export of CTA1 indicated insertion of the toxin into the Sec61p complex from the lumenal side. Suggests that Sec61p complex–mediated protein export from the ER is not restricted to ER-associated protein degradation but is also use by bacterial toxins for entry into the cytosol of the target cell.

ER-Golgi Traffic Is a Prerequisite for Efficient ER Degradation
Taxis, Vogel, and Wolf (2002) Mol. Biol. Cell 13, Efficient degradation of soluble malfolded proteins in yeast requires a fully competent early secretory pathway (i.e. mutant forms of sec12, sec23, sec18, ufe1, or sed5 all caused delays in degradation of CPY*). Mutations in proteins essential for ER-Golgi protein traffic severely inhibit ER degradation of the model substrate CPY*. ER localization of CPY* was found in WT cells, but no other specific organelle for ER degradation could be identified by electron microscopy studies. CPY* is degraded in COPI coat mutants, but only a minor fraction of CPY* or some other proteinaceous factor that is required for degradation seems to enter the recycling pathway between ER and Golgi. They propose that the disorganized structure of the ER and/or the mislocalization of Kar2p, observed in early secretory mutants, is responsible for the reduction in CPY* degradation. Also, mutations in proteins directly involved in degradation of malfolded proteins (Der1p, Der3/Hrd1p, and Hrd3p) lead to morphological changes of the endoplasmic reticulum and the Golgi, escape of CPY* into the secretory pathway and a slower maturation rate of wild-type CPY.

Quality control in the endoplasmic reticulum protein factory
Sitia and Braakman (2003) 18,

Gardinier lecture “Principles in Molecular and Cell Biology

Polyubiquitin-proeasome cycle
Amino acids

Two ER-associated ubiquitin ligase complexes.
Hrd1p/Hrd3p complex (left) and Doa10p ligase (right). Associated ubiquitin conjugating enzymes (E2s) are also shown Cue1p anchors Ubc7p to the lumenal face of the ER membrane. Both multi-spanning RING-domain proteins (Hrd1p or Doa10p) are key components of the complex. In all likelihood, still other ubiquitin E3s are involved in ER degradation of some substrates. Hampton, R.Y. (2002) Curr Opin. Cell Biol. 14,

Distinct ERAD pathways lead to protein degradation.
Black stars represent misfolded domains Black triangles are N-linked core glycans All ERAD substrates tested in vivo require the proteasome and the Cdc48–Ufd1p–Npl4p complex. Variety and distinctions among the rest of the ERAD machinery. Soluble substrates interact with lumenal chaperones to remain aggregation-free (a), Transmembrane proteins with prominent cytosolic domains require cytosolic chaperones (e.g. Hsp70 or Ssa1p) for degradation. Misfolded ER-lumenal and transmembrane proteins can be transported from the ER and retrieved from the Golgi before being degraded by the ERAD-lumenal (ERAD-L) pathway. Degradation requires Der1p, EDEM (Mnl1p), and ubiquitin-conjugating enzymes (Ubc1p and Ubc7p) and ligases (Der3p or Hrd1p). Transmembrane proteins with misfolded domains in the cytosol (b) are retained in the ER and degraded by the ERAD-cytosolic (ERAD-C) pathway, which also requires a ubiquitin-conjugating enzyme (Ubc7p) and ubiquitin ligase (Doa10p). Ahner & Brodsky (2004) Trend Cell Biol. 14,

Cytosolic complex involved in ERAD: Cdc48-Npl4-Ufd1
The AAA ATPase Cdc48/p97 and its partners transport proteins from the ER into the cytosol Ye et al (2001) Nature 414, Members of the AAA ATPase family are involved in extracting and degrading membrane proteins. Another member of this family, Cdc48 in yeast and p97 in mammals, is required for the export of ER proteins into the cytosol. Cdc48/p97 was previously known to function in a complex with the cofactor p47 in membrane fusion Its role in ER protein export requires the interacting partners Ufd1 and Npl4. The AAA ATPase interacts with substrates at the ER membrane Is required to release them as polyubiquitinated species into the cytosol. Propose that the Cdc48/p97±Ufd1±Npl4 complex extracts proteins from the ER membrane for cytosolic degradation. The conserved Npl4 protein complex mediates proteasome-dependent membrane-bound transcription factor activation Hichcock et al (2001) Mol Biol Cell 12, A membrane associated complex containing Npl4p, Ufd1p, and Cdc48p mediates proteasome-regulated cleavage of Mga2p and Spt23p, two model proteins examined in this study. Mutations in NPL4, UFD1, and CDC48 cause a block in Mga2p and Spt23p processing Our data indicate that the Npl4 complex may serve to target the proteasome to the ubiquitinated ER membrane-bound proteins Given that NPL4 is allelic to the ERAD gene HRD4, we further propose that this NPL4 function extends to all ER-membrane–associated targets of the proteasome. Co-IP of Cdc48p and Ufd1p with PA-tagged Npl4p

McCracken and Brodsky (2003) BioEssays 25, 868-877

Fbs proteins interact with innermost chitobiose in N-glycans
Glycoprotein-specific ubiquitin ligases recognize N-glycans in unfolded substrates Yoshida et al (2005) EMBO Reports 6, 1-6 Previous reports that SCF-Fbs1,2 ubiquitin-ligase complexes contribute to ubiquitination of ERAD substrates. SCF-Fbs1,2 complexes are shown to interact with unfolded glycoproteins. The SCF-Fbs1 complex associated with p97 bound to integrin-1 in a manner dependent on Cdc48/p97 ATPase activity. Both Fbs1 and Fbs2 proteins interacted with denatured glycoproteins (both high-mannose and complex-type) more efficiently than native proteins. Fbs proteins interact with innermost chitobiose in N-glycans Fbs proteins distinguish native from unfolded glycoproteins by sensing the exposed chitobiose structure.

Model for regulated degradation of Hmg2p (HMG-CoA reductase).
High levels of FPP -> Hmg2p is recognized as an ERAD substrate by the HRD/DER machinery. Lowering FPP levels or glycerol causes rapid, reversible stabilization of Hmg2p. Subtle determinants may be presented when FPP is high, triggering ERAD. Regulated transitions to ERAD have broad potential as undiscovered mechanisms in normal cellular regulation (and targets for pharmacological modulation). Hampton, R.Y. (2002) Curr Opin. Cell Biol. 14,

Where do we go from here? Many proteins (even wt proteins) have inefficient folding kinetics (i.e. slow conformational maturation of wt CFTR results in >60% disposal) Mutant forms can delay folding kinetics but not eliminate function (i.e. F508 mutant of CFTR: kinetics of folding is slowed, quantitatively degraded, chloride transporter is functional if disposal is delayed). Many loss-of-function human genetic diseases have at least one allele with delayed folding kinetics (but can produce functional protein), degraded before folding is complete. Some protein misfolding disorders can present as either dominant or recessive gain-of-toxic-function (i.e. liver disease in AAT-Z). Growing literature on strategies to increase the kinetics of folding of mutant proteins: employ substrate analogs (inhibitors) that nucleate protein folding (“chemical chaperones”) Selective chemical chaperones may be effective therapeutics for genetic disease if they can rescue folding and function prior to disposal. Goal: selectively accelerate protein folding

Kim and Arvan (1998) Endocrine Rev. 19, 173-202

Pharmacologic rescue of conformationally-defective
proteins: implications for the treatment of human disease Pharmacologic chaperones (or ‘‘pharmacoperones’’): chemical mimics of in vivo ligands, arrest or reverse loss-of-function diseases by inducing mutant proteins to adopt native type- like conformations leading to normal pattern of cellular localization and function (focus on gonadotropin-releasing hormone receptor). Ulloa-Aguirre et al (2004) Traffic 5,

Pharmacologic Rescue of Conformationally-Defective
Proteins: Implications for the Treatment of Human Disease Ulloa-Aguirre et al (2004) Traffic 5,

Pharmacological rescue of WT and mutant human GnRH receptors
Ulloa-Aguirre et al (2004) Traffic 5, Figure on the right shows inositol phosphate production (IP) by COS-7 cells transiently expressing each receptor Hydrophobic peptidomimetics penetrate cells and interact specifically with protein targets, including antagonist of GnRH such as IN3 (132–134) Pharmacological chaperones tested were the indoles IN30, IN31b, and IN3 (left figure). All peptidomimetics studied with an IC50 value for the human GnRHRof 2.3 nM displayed a measurable efficacy in rescuing GnRHR mutants Results from four rat GnRHRs and C278A rat GnRHR are also shown.

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