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Antigen presentation: Peptides and proteins scramble for the exit
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R839
Antigen presentation: Peptides and proteins scramble for the exit Paul J. Lehner*, Eric W. Hewitt* and Karin Römisch†
The fate of peptides that fail to bind to major histocompatibility complex class I molecules in the endoplasmic reticulum (ER) has remained unclear. A recent study has revealed that these peptides exit the ER via the Sec61 channel and compete for this pathway with misfolded proteins. Addresses: *Division of Immunology, and †Department of Clinical Biochemistry, Wellcome Trust Centre for Molecular Mechanisms In Disease, CIMR, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2XY, UK. E-mail: [email protected]; [email protected] Current Biology 2000, 10:R839–R842 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Antigen recognition by the immune system occurs by humoral (or antibody-mediated) immunity and cell-mediated immunity. The cell-mediated response involves helper T cells, which aid antibody production, and cytotoxic T cells, which kill virus-infected and tumour cells. The cytotoxic T cells therefore need to distinguish between healthy and diseased cells and do so by sampling the protein content of a cell in the form of peptides bound to cell surface major histocompatibility complex (MHC) class I molecules. If the MHC class I molecules bind peptides derived from viral or malignant proteins, cytotoxic T cells are activated and eliminate the infected or transformed cell. The MHC class I complex is assembled in the endoplasmic reticulum (ER). Peptide binding forms an integral part of the assembly process, as bound peptide stabilises MHC class I complexes for transport from the ER to the cell surface [1]. The majority of antigenic peptides are generated in the cytosol by proteasomes and are then transported across the ER membrane via the transporter associated with antigen processing (TAP). Many peptides transported into the ER do not associate with MHC class I molecules, however. A recent study [2] has provided insights into the fate of such peptides, demonstrating an interesting link between peptide and protein export from the ER. Peptides are delivered to the ER via TAP
TAP is a member of the ATP-binding cassette (ABC) transporter superfamily, whose members translocate molecules unidirectionally across membranes, utilising energy from ATP hydrolysis [3]. TAP is formed by two integral ER membrane proteins, TAP1 and TAP2, and the peptide-binding site is formed by the membrane domains
of TAP1 and TAP2. TAP can bind and translocate a range of different-sized peptides, although peptides of 8–12 amino acids appear to be optimal. TAP substrates are therefore the same size or slightly longer than peptides that bind to MHC class I molecules (8–10 amino acids). TAP has a relatively broad specificity in terms of peptide sequence, although there are some preferences for the amino acids at the amino and carboxyl termini of the peptide. In contrast to TAP, MHC class I molecules are much more selective in their peptide binding. Selective peptide binding to MHC class I molecules is controlled by allele-specific peptide-binding motifs. Although any single MHC class I allele encodes a molecule that can bind a broad range of peptides, a significant proportion, if not the majority, of TAP-translocated peptides will not associate with MHC class I molecules. So, what happens to peptides that fail to find a suitable MHC class I binding partner? Some of the earliest studies demonstrating peptide transport by TAP found that imported reporter peptides were rapidly exported from the ER back to the cytosol [4]. It was not until Neefjes and colleagues [5] introduced the use of reporter peptides bearing N-linked glycosylation consensus motifs, which allowed peptides entering the ER to become ‘tagged’ by glycosylation, that an accumulation of peptides in the ER lumen was detected. Although the TAP-translocated peptides were found to be removed from the ER in a temperature- and ATP-dependent process [4,6,7], the nature of this export pathway remained unclear until recently. Peptides exit the ER via the protein translocation channel
There are a number of possible fates for peptides that fail to bind MHC class I molecules, including degradation in the ER lumen, TAP-mediated export back to the cytosol or export to the cytosol via another channel in the ER membrane. In a recent elegant study published in Immunity, Koopmann et al. [2] discriminate between these possibilities. This group confirmed that TAP-imported peptides rapidly exit the ER with half-times ranging from 1.5 to 4 minutes and that this export requires ATP, but is clearly independent of TAP. For proteins, there are two known exit pathways from the ER — forward via the secretory pathway or back across the ER membrane to the cytosol via the Sec61 channel in a process known as retrograde protein translocation [8]. Initially described for unassembled MHC class I heavy chains, retrograde translocation to the cytosol is now recognised as the main disposal route for misfolded soluble and membrane proteins [9,10]. Proteins exported through the Sec61 channel are subsequently degraded by proteasomes in the cytosol [8].
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Figure 1
Transport vesicle
Peptides
4
TAP2
TAP1
1
Sec61
2
3
6
MHC class I molecule 2*
5 6*
Endoplasmic reticulum lumen Cytosol
Ribosome
Fates of peptides and proteins in the ER lumen. After import through TAP (1), antigenic peptides bind to MHC class I molecules (2); this allows packaging of the loaded MHC class I complex into ER-to-Golgi transport vesicles (3). The TAP-imported peptides that cannot bind to MHC class I molecules are exported back to the cytosol via the Sec61 channel (2*,3*). Secretory proteins enter the ER co-translationally through the Sec61 channel (4), fold with the help of chaperones (5) and are packaged into budding transport vesicles (6). Proteins that fail to fold are targeted back to the Sec61 channel and retrogradely translocated to the cytosol (6*) where they are degraded by proteasomes.
Chaperone
3*
Sec61
Proteasome
Although Sec61 allows proteins to exit the ER, the essential function of this channel is to mediate secretory protein import into the ER, which, in mammalian cells, is largely cotranslational [11]. During this process, the cytosolic end of the channel is sealed by the ribosome, which is tightly bound to Sec61p. The channel is formed by several heterotrimeric Sec61 complexes composed of Sec61α, β, and γ subunits, where Sec61α is the channel-forming subunit. The discovery that the Sec61 channel can transport proteins in either direction is relatively recent [12–14]. It remains as yet unclear how bidirectional protein transport through the Sec61 channel is regulated, whether specific accessory proteins are required for transport in a specific direction, and how the substrates for retrograde transport are recognised and targeted to the channel [10]. Some bacterial and plant toxins use the Sec61 channel to gain entry to the cytosol [15]. Cholera toxin, for example, enters mammalian cells by endocytosis, travels retrogradely through the secretory pathway back to the ER and, camouflaged as a misfolded protein, uses the Sec61 channel to enter the cytosol [16]. Another toxin, ricin, probably uses the same route [17]. The Sec61 channel can transport misfolded proteins and toxins whether or not they are N-glycosylated [10]. In a study based on a yeast cell-free system and a large number of sec61 mutants, Gillece and colleagues [18] showed recently that transport of glycopeptides from the ER lumen to the cytosol is also dependent on Sec61p. In this system, exit of glycopeptides can be
Current Biology
inhibited by ribosomes binding to the cytosolic side of the Sec61 channel. Koopmann et al. [2] now demonstrate that removal of TAP-imported peptides from the ER lumen similarly occurs via the Sec61 channel (Figure 1). This group shows that peptide export is blocked by Pseudomonas aeruginosa exotoxin, which binds Sec61α and presumably competes with peptides for retrograde translocation. In addition, Koopmann et al. [2] demonstrate that an increased concentration of peptides in the ER lumen interferes with the transport to the cytosol of an unassembled subunit of the MHC class I complex, β2 microglobulin, which most likely also exits via the Sec61 channel. It seems, therefore, that TAP-translocated peptides and proteins can compete for translocation back to the cytosol. So what happens when TAP-translocated peptides enter the ER? The intermediate steps in this process require further elucidation, but there are clues to the identities of the main players. In addition to transporting peptides into the ER, TAP acts as a molecular scaffold for the final stage of assembly of the peptide–MHC class I complex — the loading of peptide onto MHC class I molecules [1]. The proximity of MHC class I molecules to newly translocated peptides gives them ‘first go’ at peptide binding. TAPtranslocated peptides that fail to bind MHC class I molecules can be found in the ER lumen in association with a number of ER resident chaperone proteins including
Dispatch
protein disulphide isomerase (PDI), calreticulin, gp96 and Erp57, although PDI appears to be the major peptide sink [19–21]. These proteins are important components of the ER protein folding machinery required for assembly of nascent polypeptides. Export of peptides from the ER into the cytoplasm may therefore be necessary to reduce competition between peptides and nascent proteins for these chaperone proteins. Koopmann et al. [2] find that peptide export back to the cytosol can be driven by non-hydrolysable ATP analogues, suggesting that the release of peptide from the ER may involve ATP-binding-induced conformational changes rather than ATP hydrolysis. Such conformational changes have been observed to regulate the association of proteins with a number of chaperones including BiP, gp96 and calnexin. Retrograde transport of soluble misfolded proteins through the Sec61 channel does require a substrate-specific set of ER-lumenal chaperones, which may be responsible for targeting the proteins to the channel [22]. Likewise, retrograde transport of toxins is dependent on ER-lumenal proteins [16]. In contrast, mutations in BiP and PDI that cause defects in protein export have no effect on glycopeptide transport from the yeast ER [18], so different processes seem to have different requirements for chaperone proteins. Whether ER-lumenal chaperones are actively involved in the export of TAP substrates from the mammalian ER or whether export simply requires ATP-induced release of the peptides from the chaperones remains unclear. Peptides and proteins compete for retrograde translocation
Koopmann et al. [2] clearly show that TAP-translocated peptides can compete with soluble proteins, such as β2 microglobulin and Pseudomonas exotoxin for Sec61-mediated transport back to the cytosol. This intersection of peptide and protein export pathways at the Sec61 channel has important implications. TAP is constitutively expressed in most nucleated cells and it is tempting to speculate on the consequences of TAP overexpression. TAP induction by proinflammatory mediators, including interferon-γ, will result in a rapid rise in the peptide concentration within the ER. Is there an associated mechanism to promote export of peptides from the ER? Misfolded ER proteins can induce the ‘unfolded protein response’ (UPR) in which an accumulation of unfolded proteins is sensed by the IRE1 kinase resulting in transcriptional induction of UPR target genes [23], many of which encode ER resident chaperones. It is not yet known whether the accumulation of peptides in the ER also stimulates the induction of the UPR. Induction of this response could occur as a direct consequence of peptide accumulation, or indirectly as a result of competition between peptides and proteins for retrograde translocation leading to an accumulation of unfolded proteins in the ER.
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When misfolded secretory proteins reach the cytosol they are degraded by proteasomes [10]. The fate of peptides in the cytosol is not yet known, but there must be a mechanism that promotes their degradation and prevents a second round of translocation by TAP into the ER. By identifying the Sec61 channel as the exit route for both peptides and proteins from the ER, Koopmann et al. [2] have discovered an important connection between protein and peptide homeostasis in the ER. Their results suggest that stringent controls on TAP-mediated import of peptides into the ER are required to maintain protein quality control in the ER. References 1. Pamer E, Cresswell P: Mechanisms of MHC class I-restricted antigen processing. Annu Rev Immunol 1998, 16:323-358. 2. Koopmann JO, Albring J, Huter E, Bulbuc N, Spee P, Neefjes J, Hammerling JG, Momburg F: Export of antigenic peptides from the endoplasmic reticulum intersects with retrograde protein translocation through the Sec61p channel. Immunity 2000, 13:117-127. 3. Abele R, Tampe R: Function of the transport complex TAP in cellular immune recognition. Biochim Biophys Acta 1999, 1461:405-419. 4. Shepherd JC, Schumacher TNM, Ashton-Rickardt PG, Imaeda S, Ploegh HL, Janeway CAJ, Tonegawa S: TAP1-dependent peptide translocation in vitro is ATP dependent and peptide selective. Cell 1993, 74:577-584. 5. Neefjes JJ, Momburg F, Hammerling GJ: Selective and ATPdependent translocation of peptides by the MHC-encoded transporter. Science 1993, 261:769-771. 6. Roelse J, Gromme M, Momburg F, Hammerling GJ, Neefjes J: Trimming of TAP-translocated peptides in the endoplasmic reticulum and in the cytosol during recycling. J Exp Med 1994, 180:1591-1597. 7. Schumacher TM, Kantesaria DV, Heemels M-T, Ashton-Rickardt PG, Shepherd JC, Fruh K, Yang Y, Peterson P, Tonegawa S, Ploegh HL: Peptide length and sequence specificity of the mouse TAP1/TAP2 translocator. J Exp Med 1994, 179:533-540. 8. Ellgaard L, Molinari M, Helenius A: Setting the standards: quality control in the secretory pathway. Science 1999, 286:1882-1888. 9. Wiertz EJ, Jones TR, Sun L, Bogyo M, Geuze HJ, Ploegh HL: The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the endoplasmic reticulum to the cytosol. Cell 1996, 84:769-779. 10. Romisch K: Surfing the Sec61 channel: bidirectional protein translocation across the ER membrane. J Cell Sci 1999, 112:4185-4191. 11. Johnson AE, van Waes MA: The translocon: a dynamic gateway at the ER membrane. Annu Rev Cell Dev Biol 1999, 15:799-842. 12. Wiertz EJ, Tortorella D, Bogyo M, Yu J, Mothes W, Jones TR, Rapoport TA, Ploegh HL: Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction. Nature 1996, 384:432-438. 13. Pilon M, Schekman R, Romisch K: Sec61p mediates export of a misfolded secretory protein from the endoplasmic reticulum to the cytosol for degradation. EMBO J 1997, 16:4540-4548. 14. Plemper RK, Bohmler S, Bordallo J, Sommer T, Wolf DH: Mutant analysis links the translocon and BiP to retrograde protein transport for ER degradation. Nature 1997, 388:891-895. 15. Lord JM, Roberts LM: Toxin entry: retrograde transport through the secretory pathway. J Cell Biol 1998, 140:733-736. 16. Schmitz A, Herrgen H, Winkeler A, Herzog V: Cholera toxin is exported from microsomes by the Sec61p complex. J Cell Biol 2000, 148:1203-1212. 17. Simpson JC, Roberts LM, Romisch K, Davey J, Wolf DH, Lord JM: Ricin A chain utilises the endoplasmic reticulum-associated protein degradation pathway to enter the cytosol of yeast. FEBS Lett 1999, 459:80-84. 18. Gillece P, Pilon M, Romisch K: The protein translocation channel mediates glycopeptide export across the endoplasmic reticulum membrane. Proc Natl Acad Sci USA 2000, 97:4609-4614.
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19. Marusina K, Reid G, Gabathuler R, Jefferies W, Monaco JJ: Novel peptide-binding proteins and peptide transport in normal and TAP-deficient microsomes. Biochemistry 1997, 36:856-863. 20. Spee P, Neefjes J: TAP-translocated peptides specifically bind proteins in the endoplasmic reticulum, including gp96, protein disulfide isomerase and calreticulin. Eur J Immunol 1997, 27:2441-2449. 21. Lammert E, Stevanovic S, Brunner J, Rammensee HG, Schild H: Protein disulfide isomerase is the dominant acceptor for peptides translocated into the endoplasmic reticulum. Eur J Immunol 1997, 27:1685-1690. 22. Brodsky JL, McCracken AA: ER protein quality control and proteasome-mediated protein degradation. Semin Cell Dev Biol 1999, 10:507-513. 23. Hampton RY: Getting the UPR hand on misfolded proteins. Curr Biol 2000, 10:R518-R521.
R839
Antigen presentation: Peptides and proteins scramble for the exit Paul J. Lehner*, Eric W. Hewitt* and Karin Römisch†
The fate of peptides that fail to bind to major histocompatibility complex class I molecules in the endoplasmic reticulum (ER) has remained unclear. A recent study has revealed that these peptides exit the ER via the Sec61 channel and compete for this pathway with misfolded proteins. Addresses: *Division of Immunology, and †Department of Clinical Biochemistry, Wellcome Trust Centre for Molecular Mechanisms In Disease, CIMR, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2XY, UK. E-mail: [email protected]; [email protected] Current Biology 2000, 10:R839–R842 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Antigen recognition by the immune system occurs by humoral (or antibody-mediated) immunity and cell-mediated immunity. The cell-mediated response involves helper T cells, which aid antibody production, and cytotoxic T cells, which kill virus-infected and tumour cells. The cytotoxic T cells therefore need to distinguish between healthy and diseased cells and do so by sampling the protein content of a cell in the form of peptides bound to cell surface major histocompatibility complex (MHC) class I molecules. If the MHC class I molecules bind peptides derived from viral or malignant proteins, cytotoxic T cells are activated and eliminate the infected or transformed cell. The MHC class I complex is assembled in the endoplasmic reticulum (ER). Peptide binding forms an integral part of the assembly process, as bound peptide stabilises MHC class I complexes for transport from the ER to the cell surface [1]. The majority of antigenic peptides are generated in the cytosol by proteasomes and are then transported across the ER membrane via the transporter associated with antigen processing (TAP). Many peptides transported into the ER do not associate with MHC class I molecules, however. A recent study [2] has provided insights into the fate of such peptides, demonstrating an interesting link between peptide and protein export from the ER. Peptides are delivered to the ER via TAP
TAP is a member of the ATP-binding cassette (ABC) transporter superfamily, whose members translocate molecules unidirectionally across membranes, utilising energy from ATP hydrolysis [3]. TAP is formed by two integral ER membrane proteins, TAP1 and TAP2, and the peptide-binding site is formed by the membrane domains
of TAP1 and TAP2. TAP can bind and translocate a range of different-sized peptides, although peptides of 8–12 amino acids appear to be optimal. TAP substrates are therefore the same size or slightly longer than peptides that bind to MHC class I molecules (8–10 amino acids). TAP has a relatively broad specificity in terms of peptide sequence, although there are some preferences for the amino acids at the amino and carboxyl termini of the peptide. In contrast to TAP, MHC class I molecules are much more selective in their peptide binding. Selective peptide binding to MHC class I molecules is controlled by allele-specific peptide-binding motifs. Although any single MHC class I allele encodes a molecule that can bind a broad range of peptides, a significant proportion, if not the majority, of TAP-translocated peptides will not associate with MHC class I molecules. So, what happens to peptides that fail to find a suitable MHC class I binding partner? Some of the earliest studies demonstrating peptide transport by TAP found that imported reporter peptides were rapidly exported from the ER back to the cytosol [4]. It was not until Neefjes and colleagues [5] introduced the use of reporter peptides bearing N-linked glycosylation consensus motifs, which allowed peptides entering the ER to become ‘tagged’ by glycosylation, that an accumulation of peptides in the ER lumen was detected. Although the TAP-translocated peptides were found to be removed from the ER in a temperature- and ATP-dependent process [4,6,7], the nature of this export pathway remained unclear until recently. Peptides exit the ER via the protein translocation channel
There are a number of possible fates for peptides that fail to bind MHC class I molecules, including degradation in the ER lumen, TAP-mediated export back to the cytosol or export to the cytosol via another channel in the ER membrane. In a recent elegant study published in Immunity, Koopmann et al. [2] discriminate between these possibilities. This group confirmed that TAP-imported peptides rapidly exit the ER with half-times ranging from 1.5 to 4 minutes and that this export requires ATP, but is clearly independent of TAP. For proteins, there are two known exit pathways from the ER — forward via the secretory pathway or back across the ER membrane to the cytosol via the Sec61 channel in a process known as retrograde protein translocation [8]. Initially described for unassembled MHC class I heavy chains, retrograde translocation to the cytosol is now recognised as the main disposal route for misfolded soluble and membrane proteins [9,10]. Proteins exported through the Sec61 channel are subsequently degraded by proteasomes in the cytosol [8].
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Current Biology Vol 10 No 22
Figure 1
Transport vesicle
Peptides
4
TAP2
TAP1
1
Sec61
2
3
6
MHC class I molecule 2*
5 6*
Endoplasmic reticulum lumen Cytosol
Ribosome
Fates of peptides and proteins in the ER lumen. After import through TAP (1), antigenic peptides bind to MHC class I molecules (2); this allows packaging of the loaded MHC class I complex into ER-to-Golgi transport vesicles (3). The TAP-imported peptides that cannot bind to MHC class I molecules are exported back to the cytosol via the Sec61 channel (2*,3*). Secretory proteins enter the ER co-translationally through the Sec61 channel (4), fold with the help of chaperones (5) and are packaged into budding transport vesicles (6). Proteins that fail to fold are targeted back to the Sec61 channel and retrogradely translocated to the cytosol (6*) where they are degraded by proteasomes.
Chaperone
3*
Sec61
Proteasome
Although Sec61 allows proteins to exit the ER, the essential function of this channel is to mediate secretory protein import into the ER, which, in mammalian cells, is largely cotranslational [11]. During this process, the cytosolic end of the channel is sealed by the ribosome, which is tightly bound to Sec61p. The channel is formed by several heterotrimeric Sec61 complexes composed of Sec61α, β, and γ subunits, where Sec61α is the channel-forming subunit. The discovery that the Sec61 channel can transport proteins in either direction is relatively recent [12–14]. It remains as yet unclear how bidirectional protein transport through the Sec61 channel is regulated, whether specific accessory proteins are required for transport in a specific direction, and how the substrates for retrograde transport are recognised and targeted to the channel [10]. Some bacterial and plant toxins use the Sec61 channel to gain entry to the cytosol [15]. Cholera toxin, for example, enters mammalian cells by endocytosis, travels retrogradely through the secretory pathway back to the ER and, camouflaged as a misfolded protein, uses the Sec61 channel to enter the cytosol [16]. Another toxin, ricin, probably uses the same route [17]. The Sec61 channel can transport misfolded proteins and toxins whether or not they are N-glycosylated [10]. In a study based on a yeast cell-free system and a large number of sec61 mutants, Gillece and colleagues [18] showed recently that transport of glycopeptides from the ER lumen to the cytosol is also dependent on Sec61p. In this system, exit of glycopeptides can be
Current Biology
inhibited by ribosomes binding to the cytosolic side of the Sec61 channel. Koopmann et al. [2] now demonstrate that removal of TAP-imported peptides from the ER lumen similarly occurs via the Sec61 channel (Figure 1). This group shows that peptide export is blocked by Pseudomonas aeruginosa exotoxin, which binds Sec61α and presumably competes with peptides for retrograde translocation. In addition, Koopmann et al. [2] demonstrate that an increased concentration of peptides in the ER lumen interferes with the transport to the cytosol of an unassembled subunit of the MHC class I complex, β2 microglobulin, which most likely also exits via the Sec61 channel. It seems, therefore, that TAP-translocated peptides and proteins can compete for translocation back to the cytosol. So what happens when TAP-translocated peptides enter the ER? The intermediate steps in this process require further elucidation, but there are clues to the identities of the main players. In addition to transporting peptides into the ER, TAP acts as a molecular scaffold for the final stage of assembly of the peptide–MHC class I complex — the loading of peptide onto MHC class I molecules [1]. The proximity of MHC class I molecules to newly translocated peptides gives them ‘first go’ at peptide binding. TAPtranslocated peptides that fail to bind MHC class I molecules can be found in the ER lumen in association with a number of ER resident chaperone proteins including
Dispatch
protein disulphide isomerase (PDI), calreticulin, gp96 and Erp57, although PDI appears to be the major peptide sink [19–21]. These proteins are important components of the ER protein folding machinery required for assembly of nascent polypeptides. Export of peptides from the ER into the cytoplasm may therefore be necessary to reduce competition between peptides and nascent proteins for these chaperone proteins. Koopmann et al. [2] find that peptide export back to the cytosol can be driven by non-hydrolysable ATP analogues, suggesting that the release of peptide from the ER may involve ATP-binding-induced conformational changes rather than ATP hydrolysis. Such conformational changes have been observed to regulate the association of proteins with a number of chaperones including BiP, gp96 and calnexin. Retrograde transport of soluble misfolded proteins through the Sec61 channel does require a substrate-specific set of ER-lumenal chaperones, which may be responsible for targeting the proteins to the channel [22]. Likewise, retrograde transport of toxins is dependent on ER-lumenal proteins [16]. In contrast, mutations in BiP and PDI that cause defects in protein export have no effect on glycopeptide transport from the yeast ER [18], so different processes seem to have different requirements for chaperone proteins. Whether ER-lumenal chaperones are actively involved in the export of TAP substrates from the mammalian ER or whether export simply requires ATP-induced release of the peptides from the chaperones remains unclear. Peptides and proteins compete for retrograde translocation
Koopmann et al. [2] clearly show that TAP-translocated peptides can compete with soluble proteins, such as β2 microglobulin and Pseudomonas exotoxin for Sec61-mediated transport back to the cytosol. This intersection of peptide and protein export pathways at the Sec61 channel has important implications. TAP is constitutively expressed in most nucleated cells and it is tempting to speculate on the consequences of TAP overexpression. TAP induction by proinflammatory mediators, including interferon-γ, will result in a rapid rise in the peptide concentration within the ER. Is there an associated mechanism to promote export of peptides from the ER? Misfolded ER proteins can induce the ‘unfolded protein response’ (UPR) in which an accumulation of unfolded proteins is sensed by the IRE1 kinase resulting in transcriptional induction of UPR target genes [23], many of which encode ER resident chaperones. It is not yet known whether the accumulation of peptides in the ER also stimulates the induction of the UPR. Induction of this response could occur as a direct consequence of peptide accumulation, or indirectly as a result of competition between peptides and proteins for retrograde translocation leading to an accumulation of unfolded proteins in the ER.
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When misfolded secretory proteins reach the cytosol they are degraded by proteasomes [10]. The fate of peptides in the cytosol is not yet known, but there must be a mechanism that promotes their degradation and prevents a second round of translocation by TAP into the ER. By identifying the Sec61 channel as the exit route for both peptides and proteins from the ER, Koopmann et al. [2] have discovered an important connection between protein and peptide homeostasis in the ER. Their results suggest that stringent controls on TAP-mediated import of peptides into the ER are required to maintain protein quality control in the ER. References 1. Pamer E, Cresswell P: Mechanisms of MHC class I-restricted antigen processing. Annu Rev Immunol 1998, 16:323-358. 2. Koopmann JO, Albring J, Huter E, Bulbuc N, Spee P, Neefjes J, Hammerling JG, Momburg F: Export of antigenic peptides from the endoplasmic reticulum intersects with retrograde protein translocation through the Sec61p channel. Immunity 2000, 13:117-127. 3. Abele R, Tampe R: Function of the transport complex TAP in cellular immune recognition. Biochim Biophys Acta 1999, 1461:405-419. 4. Shepherd JC, Schumacher TNM, Ashton-Rickardt PG, Imaeda S, Ploegh HL, Janeway CAJ, Tonegawa S: TAP1-dependent peptide translocation in vitro is ATP dependent and peptide selective. Cell 1993, 74:577-584. 5. Neefjes JJ, Momburg F, Hammerling GJ: Selective and ATPdependent translocation of peptides by the MHC-encoded transporter. Science 1993, 261:769-771. 6. Roelse J, Gromme M, Momburg F, Hammerling GJ, Neefjes J: Trimming of TAP-translocated peptides in the endoplasmic reticulum and in the cytosol during recycling. J Exp Med 1994, 180:1591-1597. 7. Schumacher TM, Kantesaria DV, Heemels M-T, Ashton-Rickardt PG, Shepherd JC, Fruh K, Yang Y, Peterson P, Tonegawa S, Ploegh HL: Peptide length and sequence specificity of the mouse TAP1/TAP2 translocator. J Exp Med 1994, 179:533-540. 8. Ellgaard L, Molinari M, Helenius A: Setting the standards: quality control in the secretory pathway. Science 1999, 286:1882-1888. 9. Wiertz EJ, Jones TR, Sun L, Bogyo M, Geuze HJ, Ploegh HL: The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the endoplasmic reticulum to the cytosol. Cell 1996, 84:769-779. 10. Romisch K: Surfing the Sec61 channel: bidirectional protein translocation across the ER membrane. J Cell Sci 1999, 112:4185-4191. 11. Johnson AE, van Waes MA: The translocon: a dynamic gateway at the ER membrane. Annu Rev Cell Dev Biol 1999, 15:799-842. 12. Wiertz EJ, Tortorella D, Bogyo M, Yu J, Mothes W, Jones TR, Rapoport TA, Ploegh HL: Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction. Nature 1996, 384:432-438. 13. Pilon M, Schekman R, Romisch K: Sec61p mediates export of a misfolded secretory protein from the endoplasmic reticulum to the cytosol for degradation. EMBO J 1997, 16:4540-4548. 14. Plemper RK, Bohmler S, Bordallo J, Sommer T, Wolf DH: Mutant analysis links the translocon and BiP to retrograde protein transport for ER degradation. Nature 1997, 388:891-895. 15. Lord JM, Roberts LM: Toxin entry: retrograde transport through the secretory pathway. J Cell Biol 1998, 140:733-736. 16. Schmitz A, Herrgen H, Winkeler A, Herzog V: Cholera toxin is exported from microsomes by the Sec61p complex. J Cell Biol 2000, 148:1203-1212. 17. Simpson JC, Roberts LM, Romisch K, Davey J, Wolf DH, Lord JM: Ricin A chain utilises the endoplasmic reticulum-associated protein degradation pathway to enter the cytosol of yeast. FEBS Lett 1999, 459:80-84. 18. Gillece P, Pilon M, Romisch K: The protein translocation channel mediates glycopeptide export across the endoplasmic reticulum membrane. Proc Natl Acad Sci USA 2000, 97:4609-4614.
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19. Marusina K, Reid G, Gabathuler R, Jefferies W, Monaco JJ: Novel peptide-binding proteins and peptide transport in normal and TAP-deficient microsomes. Biochemistry 1997, 36:856-863. 20. Spee P, Neefjes J: TAP-translocated peptides specifically bind proteins in the endoplasmic reticulum, including gp96, protein disulfide isomerase and calreticulin. Eur J Immunol 1997, 27:2441-2449. 21. Lammert E, Stevanovic S, Brunner J, Rammensee HG, Schild H: Protein disulfide isomerase is the dominant acceptor for peptides translocated into the endoplasmic reticulum. Eur J Immunol 1997, 27:1685-1690. 22. Brodsky JL, McCracken AA: ER protein quality control and proteasome-mediated protein degradation. Semin Cell Dev Biol 1999, 10:507-513. 23. Hampton RY: Getting the UPR hand on misfolded proteins. Curr Biol 2000, 10:R518-R521.