Characterization of class I MHC folding intermediates and their disparate interactions with peptide and beta 2-microglobulin

Decoupling the role of ubiquitination for the dislocation versus degradation of major histocompatibility complex (MHC) class I proteins during endoplasmic reticulum-associated degradation (ERAD)

Aberrantly or excessively expressed proteins in the endoplasmic reticulum are identified by quality control mechanisms and dislocated to the cytosol for proteasome-mediated, ubiquitin-dependent degradation by a process termed endoplasmic reticulum-associated degradation (ERAD). In addition to its role in degradation, ubiquitination has also been implicated in substrate dislocation, although whether direct ubiquitin conjugation of ERAD substrates is required for dislocation has been difficult to ascertain. An obstacle in probing the mechanism of quality control-induced ERAD is the paucity of ERAD substrates being dislocated and detected at any given time. To obviate this problem, we report here the use of a sensitive biotinylation system to probe the dislocation of major histocompatibility complex I (MHCI) heavy chain substrates in the absence of immune evasion proteins. Using this assay system the dislocation of MHCI heavy chains was found not to require potential ubiquitin conjugation sites in the cytoplasmic tail or Lys residues in the ectodomain. By contrast, dislocation of MHCI heavy chains did require deubiquitinating enzyme activity and rapid proteasome-mediated degradation required Lys residues in MHCI heavy chain ectodomain. These combined findings support the model that the endoplasmic reticulum quality control-induced dislocation of MHCI heavy chains may not require direct ubiquitination/deubiquitination as is required for proteasome-mediated degradation post dislocation.

Newly discovered viral E3 ligase pK3 induces endoplasmic reticulum-associated degradation of class I major histocompatibility proteins and their membrane-bound chaperones

Viral immune invasion proteins are highly effective probes for studying physiological pathways. We report here the characterization of a new viral ubiquitin ligase pK3 expressed by rodent herpesvirus Peru (RHVP) that establishes acute and latent infection in laboratory mice. Our findings show that pK3 binds directly and specifically to class I major histocompatibility proteins (MHCI) in a transmembrane-dependent manner. This binding results in the rapid degradation of the pK3/MHCI complex by a mechanism dependent upon catalytically active pK3. Subsequently, the rapid degradation of pK3/MHCI secondarily causes the slow degradation of membrane bound components of the MHCI peptide loading complex, tapasin, and transporter associated with antigen processing (TAP). Interestingly, this secondary event occurs by cellular endoplasmic reticulum-associated degradation. Cumulatively, our findings show pK3 uses a unique mechanism of substrate detection and degradation compared with other viral or cellular E3 ligases. More importantly, our findings reveal that in the absence of nascent MHCI proteins in the endoplasmic reticulum, the transmembrane proteins TAP and tapasin that facilitate peptide binding to MHCI proteins are degraded by cellular quality control mechanisms.

Analysis of major histocompatibility complex class I folding: novel insights into intermediate forms

Folding around a peptide ligand is integral to the antigen presentation function of major histocompatibility complex (MHC) class I molecules. Several lines of evidence indicate that the broadly cross-reactive 34-1-2 antibody is sensitive to folding of the MHC class I peptide-binding groove. Here, we show that peptide-loading complex proteins associated with the murine MHC class I molecule K(d) are found primarily in association with the 34-1-2(+) form. This led us to hypothesize that the 34-1-2 antibody may recognize intermediately, as well as fully, folded MHC class I molecules. To further characterize the form(s) of MHC class I molecules recognized by 34-1-2, we took advantage of its cross-reactivity with L(d) . Recognition of the open and folded forms of L(d) by the 64-3-7 and 30-5-7 antibodies, respectively, has been extensively characterized, providing us with parameters against which to compare 34-1-2 reactivity. We found that the 34-1-2(+) L(d) molecules displayed characteristics indicative of incomplete folding, including increased tapasin association, endoplasmic reticulum retention, and instability at the cell surface. Moreover, we show that an L(d) -specific peptide induced folding of the 34-1-2(+) L(d) intermediate. Altogether, these results yield novel insights into the nature of MHC class I molecules recognized by the 34-1-2 antibody.

A role for protein disulfide isomerase in the early folding and assembly of MHC class I molecules

Proper folding and assembly of major histocompatibility complex (MHC) class I complexes are essential for optimal peptide loading and subsequent antigen presentation. MHC class I folding involves the coordinated formation of multiple disulfide bonds within MHC class I molecules. However, the regulation of disulfide bond formation during the early process of MHC class I folding is uncharacterized. Here, we show that protein disulfide isomerase (PDI) catalyzes the disulfide bond formation of MHC class I molecules and thereby facilitates the assembly of MHC class I heavy chain with beta(2)-microglobulin (beta(2)m). Depletion of PDI but not ERp57 by RNAi interfered with the disulfide bond formation in the MHC class I molecules. In the absence of PDI, the association of free class I heavy chain with calnexin increased, whereas the assembly of MHC class I heavy chain-beta(2)m heterodimers was delayed. These observations suggest that PDI-catalyzed disulfide bond formation of MHC class I molecules is an event downstream of the interaction of class I molecules with calnexin and upstream of their interaction with beta(2)m. Thus, our data establish a critical function for PDI in the early assembly of MHC class I molecules.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Amyloid precursor-like protein 2 increases the endocytosis, instability, and turnover of the H2-K(d) MHC class I molecule

The defense against the invasion of viruses and tumors relies on the presentation of viral and tumor-derived peptides to CTL by cell surface MHC class I molecules. Previously, we showed that the ubiquitously expressed protein amyloid precursor-like protein 2 (APLP2) associates with the folded form of the MHC class I molecule K(d). In the current study, APLP2 was found to associate with folded K(d) molecules following their endocytosis and to increase the amount of endocytosed K(d). In addition, increased expression of APLP2 was shown to decrease K(d) surface expression and thermostability. Correspondingly, K(d) thermostability and surface expression were increased by down-regulation of APLP2 expression. Overall, these data suggest that APLP2 modulates the stability and endocytosis of K(d) molecules.

Specificity of amyloid precursor-like protein 2 interactions with MHC class I molecules

The ubiquitously expressed amyloid precursor-like protein 2 (APLP2) has been previously found to regulate cell surface expression of the major histocompatibility complex (MHC) class I molecule K(d) and bind strongly to K(d). In the study reported here, we demonstrated that APLP2 binds, in varied degrees, to several other mouse MHC class I allotypes and that the ability of APLP2 to affect cell surface expression of an MHC class I molecule is not limited to K(d). L(d), like K(d), was found associated with APLP2 in the Golgi, but K(d) was also associated with APLP2 within intracellular vesicular structures. We also investigated the effect of beta(2)m on APLP2/MHC interaction and found that human beta(2)m transfection increased the association of APLP2 with mouse MHC class I molecules, likely by affecting H2 class I heavy chain conformation. APLP2 was demonstrated to bind specifically to the conformation of L(d) having folded outer domains, consistent with our previous results with K(d) and indicating APLP2 interacts with the alpha1alpha2 region on each of these H2 class I molecules. Furthermore, we observed that binding to APLP2 involved the MHC alpha3/transmembrane/cytoplasmic region, suggesting that conserved as well as polymorphic regions of the H2 class I molecule may participate in interaction with APLP2. In summary, we demonstrated that APLP2's binding, co-localization pattern, and functional impact vary among H2 class I molecules and that APLP2/MHC association is influenced by multiple domains of the MHC class I heavy chain and by beta(2)m's effects on the conformation of the heavy chain.

Redox regulation of peptide receptivity of major histocompatibility complex class I molecules by ERp57 and tapasin

The function of the oxidoreductase ERp57 in the major histocompatibility complex (MHC) class I peptide-loading complex has remained elusive. Here we show that in the absence of tapasin, the alpha2 disulfide bond in the MHC class I peptide-binding groove was rapidly reduced. Covalent sequestration of ERp57 by tapasin was needed to protect the alpha2 disulfide bond against reduction and thus to maintain the binding groove in a peptide-receptive state. Allelic variations in MHC class I tapasin dependency reflected their susceptibility to reduction of the alpha2 disulfide bond. In the absence of sequestration, ERp57 acted directly on the alpha2 disulfide bond. Our work provides insight into how the immune system customizes 'quality control' in the endoplasmic reticulum to fit the needs of antigen presentation.

Ubiquitination of serine, threonine, or lysine residues on the cytoplasmic tail can induce ERAD of MHC-I by viral E3 ligase mK3

The mechanism by which substrates for endoplasmic reticulum–associated degradation are retrotranslocated to the cytosol remains largely unknown, although ubiquitination is known to play a key role. The mouse γ-herpesvirus protein mK3 is a viral RING-CH–type E3 ligase that specifically targets nascent major histocompatibility complex I heavy chain (HC) for degradation, thus blocking the immune detection of virus-infected cells. To address the question of how HC is retrotranslocated and what role mK3 ligase plays in this action, we investigated ubiquitin conjugation sites on HC using mutagenesis and biochemistry approaches. In total, our data demonstrate that mK3-mediated ubiquitination can occur via serine, threonine, or lysine residues on the HC tail, each of which is sufficient to induce the rapid degradation of HC. Given that mK3 has numerous cellular and viral homologues, it will be of considerable interest to determine the pervasiveness of this novel mechanism of ubiquitination.

The DMalpha and DMbeta chain cooperate in the oxidation and folding of HLA-DM

HLA-DM (DM) is a heterodimeric MHC molecule that catalyzes the peptide loading of classical MHC class II molecules in the endosomal/lysosomal compartments of APCs. Although the function of DM is well-established, little is known about how DMalpha and beta-chains fold, oxidize, and form a complex in the endoplasmic reticulum (ER). In this study, we show that glycosylation promotes, but is not essential for, DMalphabeta ER exit. However, glycosylation of DMalpha N15 is required for oxidation of the alpha-chain. The DMalpha and beta-chains direct each others fate: single DMalpha chains cannot fully oxidize without DMbeta, while DMbeta forms disulfide-linked homodimers without DMalpha. Correct oxidation and subsequent ER egress depend on the unique DMbeta C25 and C35 residues. This suggests that the C25-C35 disulfide bond in the peptide-binding domain overcomes the need for stabilizing peptides required by other MHC molecules.

Redox Regulation Facilitates Optimal Peptide Selection by MHC Class I during Antigen Processing

Activated CD8(+) T cells discriminate infected and tumor cells from normal self by recognizing MHC class I-bound peptides on the surface of antigen-presenting cells. The mechanism by which MHC class I molecules select optimal peptides against a background of prevailing suboptimal peptides and in a considerably proteolytic ER environment remained unknown. Here, we identify protein disulfide isomerase (PDI), an enzyme critical to the formation of correct disulfide bonds in proteins, as a component of the peptide-loading complex. We show that PDI stabilizes a peptide-receptive site by regulating the oxidation state of the disulfide bond in the MHC peptide-binding groove, a function that is essential for selecting optimal peptides. Furthermore, we demonstrate that human cytomegalovirus US3 protein inhibits CD8(+) T cell recognition by mediating PDI degradation, verifying the functional relevance of PDI-catalyzed peptide editing in controlling intracellular pathogens. These results establish a link between thiol-based redox regulation and antigen processing.

Impaired assembly of the major histocompatibility complex class I peptide-loading complex in mice deficient in the oxidoreductase ERp57

The thiol-oxidoreductase ERp57 is an integral component of the peptide-loading complex of the major histocompatibility complex (MHC) class I pathway, but its function is unknown. To investigate its function in antigen presentation, we generated ERp57-deficient mice. Death in utero caused by ubiquitous ERp57 deletion was prevented by specific deletion in the B cell compartment. We demonstrate that ERp57 was central for recruitment of MHC class I molecules into the loading complex. In ERp57-deficient cells, we found short-lived interaction of MHC class I molecules with the loading complex. Thus, in the steady state, very few MHC class I molecules were present in the loading complex. Surface H-2K(b)-peptide expression and stability were reduced, and presentation of a model antigen was decreased. Our results indicate that ERp57 does not influence the redox state of MHC class I molecules but is an essential structural component required for stable assembly of the peptide-loading complex.

Recognition of open conformers of classical MHC by chaperones and monoclonal antibodies

There is considerable evidence that the conformation and stability of class I and class II major histocompatibility complex (MHC) proteins is dependent upon high-affinity peptide ligation, but structural data for an empty MHC protein unfortunately is lacking. However, several monoclonal antibodies (mAbs) that specifically detect open MHC conformers have been characterized, and they provide insights into the changes associated with peptide loading and unloading. Here, the structural changes make the argument that certain of these open conformer-specific mAbs recognize analogous MHC segments as the molecular chaperones tapasin and DM. MHC residues located in regions flanking the peptide-terminal anchoring pockets have been implicated in both chaperone and monoclonal antibody binding. Indeed, we propose these regions serve as peptide-binding hinges that are uniquely accessible in open MHC.

Accessory molecules in the assembly of major histocompatibility complex class I/peptide complexes: how essential are they for CD8+ T-cell immune responses?

Assembly of major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum is a highly coordinated process that results in abundant class I/peptide complexes at the cell surface for recognition by CD8(+) T cells and natural killer cells. During the assembly process, a number of chaperones and accessory molecules, such as transporter associated with antigen processing, tapasin, ER60, and calreticulin, assist newly synthesized class I molecules to facilitate loading of antigenic peptides and to optimize the repertoire of surface class I/peptide complexes. This review focuses on the relative importance of these accessory molecules for CD8(+) T-cell responses in vivo and discusses reasons that may help explain why some CD8(+) T-cell responses develop normally in mice deficient in components of class I assembly, despite impaired antigen presentation.

Tapasin and other chaperones: models of the MHC class I loading complex

MHC (major histocompatibility complex) class I molecules bind intracellular virus-derived peptides in the endoplasmic reticulum (ER) and present them at the cell surface to cytotoxic T lymphocytes. Peptide-free class I molecules at the cell surface, however, could lead to aberrant T cell killing. Therefore, cells ensure that class I molecules bind high-affinity ligand peptides in the ER, and restrict the export of empty class I molecules to the Golgi apparatus. For both of these safeguard mechanisms, the MHC class I loading complex (which consists of the peptide transporter TAP, the chaperones tapasin and calreticulin, and the protein disulfide isomerase ERp57) plays a central role. This article reviews the actions of accessory proteins in the biogenesis of class I molecules, specifically the functions of the loading complex in high-affinity peptide binding and localization of class I molecules, and the known connections between these two regulatory mechanisms. It introduces new models for the mode of action of tapasin, the role of the class I loading complex in peptide editing, and the intracellular localization of class I molecules.

Chaperones and folding of MHC class I molecules in the endoplasmic reticulum

In this review we discuss the influence of chaperones on the general phenomena of folding as well as on the specific folding of an individual protein, MHC class I. MHC class I maturation is a highly sophisticated process in which the folding machinery of the endoplasmic reticulum (ER) is heavily involved. Understanding the MHC class I maturation per se is important since peptides loaded onto MHC class I molecules are the base for antigen presentation generating immune responses against virus, intracellular bacteria as well as tumours. This review discusses the early stages of MHC class I maturation regarding BiP and calnexin association, and differences in MHC class I heavy chain (HC) interaction with calnexin and calreticulin are highlighted. Late stage MHC class I maturation with focus on the dedicated chaperone tapasin is also discussed.

Purification of correctly oxidized MHC class I heavy-chain molecules under denaturing conditions: a novel strategy exploiting disulfide assisted protein folding

The aim of this study has been to develop a strategy for purifying correctly oxidized denatured major histocompability complex class I (MHC-I) heavy-chain molecules, which on dilution, fold efficiently and become functional. Expression of heavy-chain molecules in bacteria results in the formation of insoluble cellular inclusion bodies, which must be solubilized under denaturing conditions. Their subsequent purification and refolding is complicated by the fact that (1). correct folding can only take place in combined presence of beta(2)-microglobulin and a binding peptide; and (2). optimal in vitro conditions for disulfide bond formation ( approximately pH 8) and peptide binding ( approximately pH 6.6) are far from complementary. Here we present a two-step strategy, which relies on uncoupling the events of disulfide bond formation and peptide binding. In the first phase, heavy-chain molecules with correct disulfide bonding are formed under non-reducing denaturing conditions and separated from scrambled disulfide bond forms by hydrophobic interaction chromatography. In the second step, rapid refolding of the oxidized heavy chains is afforded by disulfide bond-assisted folding in the presence of beta(2)-microglobulin and a specific peptide. Under conditions optimized for peptide binding, refolding and simultaneous peptide binding of the correctly oxidized heavy chain was much more efficient than that of the fully reduced molecule.

Accessory proteins and the assembly of human class I MHC molecules: a molecular and structural perspective

The cell-surface presentation of antigenic peptides by class I major histocompatibility complex (MHC) molecules to CD8+ T-cell receptors is part of an immune surveillance mechanism aimed at detecting foreign antigens. This process is initiated in the endoplasmic reticulum (ER) with the folding and assembly of class I MHC molecules which are then transported to the cell surface via the secretory pathway. In recent years, several accessory proteins have been identified as key components of the class I maturation process in the ER. These proteins include the lectin chaperones calnexin (CNX) and calreticulin (CRT), the thiol-dependent oxidoreductase ERp57, the transporter associated with antigen processing (TAP), and the protein tapasin. This review presents the most recent advances made in characterizing the biochemical and structural properties of these proteins, and discusses how this knowledge advances our current understanding of the molecular events underlying the folding and assembly of human class I MHC molecules in the ER.

Tapasin-the keystone of the loading complex optimizing peptide binding by MHC class I molecules in the endoplasmic reticulum

MHC class I molecules are loaded with peptides that mostly originate from the degradation of cytosolic protein antigens and that are translocated across the endoplasmic reticulum (ER) membrane by the transporter associated with antigen processing (TAP). The ER-resident molecule tapasin (Tpn) is uniquely dedicated to tether class I molecules jointly with the chaperone calreticulin (Crt) and the oxidoreductase ERp57 to TAP. As learned from the study of a Tpn-deficient cell line and from mice harboring a disrupted Tpn gene, the transient association of class I molecules with Tpn and TAP is critically important for the stabilization of class I molecules and the optimization of the peptide cargo presented to cytotoxic T cells. The different functions of molecular domains of Tpn and the highly coordinated formation of the TAP-associated peptide loading complex will also be discussed in this review.

Polymorphism at position 97 in MHC class I molecules affects peptide specificity, cell surface stability, and affinity for beta2-microglobulin

The two mouse MHC class I alleles, L(d) and L(q), share complete amino acid sequence identity except in the alpha2 domain, where they differ at six positions. Despite their similarity, L(q) has a stronger association with beta2-microglobulin (beta2m), is expressed at higher levels on the cell surface, demonstrates an increased cell surface half-life, and has fewer open forms on the cell surface than L(d). To determine the basis for their phenotypic differences, L(d) molecules containing chimeric L(d)-L(q) alpha2 domains were characterized, and these analyses implicated residue 97 (L(d)Trp and L(q)Arg) as the polymorphic site responsible for the disparity in beta2m association between the two alleles. Single substitution analysis at this site (L(d)W97R and L(q)R97W) confirmed this. Furthermore, the L(d)W97R mutant molecule has a longer cell surface half-life than either L(q) or L(d), and fewer open forms of L(d)W97R are observed on the cell surface. In addition, both L(d)W97R and L(q) possess decreased binding affinity for the L(d)-restricted tum(-) P91A(14-22) peptide compared with L(d). Collectively, these results and the known location of Trp(97) in the peptide binding cleft of L(d) strongly suggest that the substitution of Arg for Trp(97) in L(d) alters the peptide binding cleft, increasing its affinity for endogenous peptides, which results in greater cell surface stability and better retention of beta2m. Furthermore, these results imply that Trp(97) plays an important role in the ability of L(d) to efficiently participate in alternative MHC class I Ag presentation pathways.

HLA class I polymorphism has a dual impact on ligand binding and chaperone interaction

This article will describe coordinated analyses of how amino acid substitutions in the HLA class I antigen binding groove modify chaperone interaction and peptide ligand presentation. By parallel testing of ligand presentation and chaperone interaction with a series of natural HLA-B subtypes, this study has discovered that position 116 of the HLA-B15 class I heavy chain is pivotal in both peptide selection and control of interaction between the assembly complex and the class I heavy chain. Correlated with these qualitative differences in peptide selection and chaperone association are quantitative differences in the expression levels of the HLA molecules at the cell surface. These parallel studies, therefore, demonstrate that particular HLA class I polymorphisms can simultaneously influence ligand presentation and interaction with intracellular chaperones.

Calreticulin binds to the alpha1 domain of MHC class I independently of tapasin

Prior to binding to antigenic peptide, the major histocompatibility complex (MHC) heavy chain associates with an assembly complex of proteins that includes calreticulin, tapasin, and the transporter associated with antigen processing (TAP). Our data show that calreticulin can bind weakly to Ld without tapasin's assistance, and that deglycosylation of the alpha1 domain results in a primary loss of binding to calreticulin rather than tapasin. We have also shown that high amounts of wild-type tapasin are still unable to associate with MHC class I in the absence of the MHC class I/calreticulin interaction, confirming the central role of calreticulin in the formation of the MHC class I assembly complex.

Disulfide bond isomerization and the assembly of MHC class I-peptide complexes

The presence of a disulfide bond inside the peptide binding groove of MHC class I molecules and of the thiol oxidoreductase ERp57 in the class I loading complex suggests that disulfide bond isomerization may play a role in peptide loading. Here we show that ERp57 and tapasin are disulfide linked inside the loading complex. Mutagenesis of cysteine 95 in tapasin not only abolishes formation of the ERp57-tapasin bond but also prevents complete oxidation of the class I heavy chain in the loading complex. The resulting MHC class I-beta2m heterodimers are poorly loaded with high-affinity peptides in the ER but nevertheless escape to the cell surface where they are unstable. These findings suggest a role for disulfide bond isomerization in tapasin-mediated peptide loading.

Loss of a glycine in the alpha2 domain affects MHC peptide binding but not chaperone binding

Prior to the binding of peptide in the endoplasmic reticulum (ER), the major histocompatibility complex (MHC) class I heavy chain associates with an assembly complex that includes the transporter associated with antigen processing (TAP). The proximity of a part of the MHC class I alpha2 domain alpha-helix to areas previously shown to influence assembly complex binding suggests that this region might also be involved in chaperone association. Position 151, found in this part of the alpha2 domain alpha-helix, has a side chain that points up, away from direct contact with peptide, and is occupied by a glycine in all murine MHC class I heavy chains. We found that substitution of this glycine in H-2L(d) with a histidine substantially increased the proportion of peptide-free forms, although TAP binding was not abrogated. Thus, interaction of the heavy chain with peptides, but not with the assembly complex, is influenced by this glycine.

A region of tapasin that affects L(d) binding and assembly

Tapasin has been shown to stabilize TAP and to link TAP to the MHC class I H chain. Evidence also has been presented that tapasin influences the loading of peptides onto MHC class I. To explore the relationship between the ability of tapasin to bind to TAP and the MHC class I H chain and the ability of tapasin to facilitate class I assembly, we have created novel tapasin mutants and expressed them in 721.220-L(d) cells. One mutant has a deletion of nine amino acid residues (tapasin Delta334-342), and the other has amino acid substitutions at positions 334 and 335. In this report we describe the ability of these mutants to interact with L(d) and their effects on L(d) surface expression. We found that tapasin Delta334-342 was unable to bind to the L(d) H chain, and yet it facilitated L(d) assembly and expression. Tapasin Delta334-342 was able to bind and stabilize TAP, suggesting that TAP stabilization may be important to the assembly of L(d). Tapasin mutant H334F/H335Y, unlike tapasin Delta334-342, bound to L(d). Expression of tapasin H334F/H335Y in 721.220-L(d) reduced the proportion of cell surface open forms of L(d) and retarded the migration of L(d) from the endoplasmic reticulum. In total, our results indicate that the 334-342 region of tapasin influences L(d) assembly and transport.

Class I MHC molecules: assembly and antigen presentation

Several years ago, the only factor known to be necessary for the assembly and surface expression of class I MHC was beta 2m; even for beta 2m, it was unclear at what point in class I maturation its role was played. Recent experiments that employed attachment of an endoplasmic reticulum (ER) retention signal to beta 2m have shown that the point of time at which beta 2m is required is while the class I heavy chain is in the ER. Later association between beta 2m and class I is not vital in order for properly folded class I to be expressed at the cell surface. After crystallization of the first class I MHC molecule, it was realized that not only is antigen presented by class I, but that antigen is presented in the form of a peptide that stabilizes the class I structure and allows its transit to the cell surface. Class I allelic differences influence interactions with both peptide and beta 2m, with likely consequences for the ability of the class I heavy chains to present antigen through alternative pathways. Furthermore, it is now also clear that formation of appropriate disulfide bonds in the class I heavy chain is needed before class I can bind peptide antigen securely, a process that may be assisted by an ER chaperone. Many different proteins that are resident in the ER, such as calnexin, transporter associated with antigen processing (TAP), calreticulin, and tapasin, have been found to be integral to class I assembly. TAP, tapasin, and calreticulin bind preferentially to the open form of class I, which can be distinguished with the use of a monoclonal antibody specific for this form. Calreticulin and calnexin contrast in their interactions with class I, despite other similarities between these two chaperones. Overall, class I MHC assembly is now understood to involve the interplay of multiple intra- and intermolecular events in a defined chronological order which ensure continual reporting of cellular contents to cytotoxic T lymphocytes.

Definition and transfer of a serological epitope specific for peptide-empty forms of MHC class I

Nascent class I molecules have been hypothesized to undergo a conformational change when they bind peptide based on the observation that most available antibodies only detect peptide-loaded class I. Furthermore recent evidence suggests that this peptide-facilitated conformational change induces the release of class I from association with transporter associated with antigen processing (TAP)/tapasin and other endoplasmic reticulum proteins facilitating class I assembly. To learn more about the structure of peptide-empty class I, we have studied mAb 64-3-7 that is specific for peptide-empty forms of L(d). We show here that mAb 64-3-7 detects a linear stretch of amino acids including principally residues 48Q and 50P. Furthermore, we demonstrate that the 64-3-7 epitope can be transferred to other class I molecules with limited mutagenesis. Interestingly, in the folded class I molecule residues 48 and 50 are on a loop connecting a beta strand (under the bound peptide) with the alpha(1) helix (rising above the ligand binding site). Thus it is attractive to propose that this loop is a hinge region. Importantly, the three-dimensional structure of this loop is strikingly conserved among class I molecules. Thus our findings suggest that all class I molecules undergo a similar conformational change in the loop around residues 48 and 50 when they associate with peptide.

An Extensive Region of an MHC Class I α2 Domain Loop Influences Interaction with the Assembly Complex

Presentation of antigenic peptides to CTLs at the cell surface first requires assembly of MHC class I with peptide and β2-microglobulin in the endoplasmic reticulum. This process involves an assembly complex of several proteins, including TAP, tapasin, and calreticulin, all of which associate specifically with the β2-microglobulin-assembled, open form of the class I heavy chain. To better comprehend at a molecular level the regulation of class I assembly, we have assessed the influence of multiple individual amino acid substitutions in the MHC class I α2 domain on interaction with TAP, tapasin, and calreticulin. In this report, we present evidence indicating that many residues surrounding position 134 in H-2Ld influence interaction with assembly complex components. Most mutations decreased association, but one (LdK131D) strongly increased it. The Ld mutants, with the exception of LdK131D, exhibited characteristics suggesting suboptimal intracellular peptide loading, similar to the phenotype of Ld expressed in a tapasin-deficient cell line. Notably, K131D was less peptide inducible than wild-type Ld, which is consistent with its unusually strong association with the endoplasmic reticulum assembly complex.

Calreticulin and Calnexin Interact with Different Protein and Glycan Determinants During the Assembly of MHC Class I

Before peptide binding, a variety of endoplasmic reticulum (ER) proteins are associated with class I including calnexin, TAP, calreticulin, and tapasin. Although the selective functions of any one of these ER proteins have been difficult to define, individually or in combination they perform two general chaperone functions for class I. They promote assembly of the class I heterotrimeric molecule (heavy (H) chain, β2m, and peptide) and they retain incompletely assembled complexes in the ER. In this study, we present evidence that calreticulin clearly differs from calnexin in how it associates with class I. Regarding the structural basis of the association, the oligosaccharide moiety in the α1 domain and the amino acid residue at position 227 in the α3 domain were both found to be critical for the interaction of class I with calreticulin. Interestingly, calreticulin displayed sensitivity to class I peptide binding even in TAP-deficient human or mouse cells. Thus, calreticulin is clearly more specific than calnexin in the structures and conformation of the class I molecule with which it can interact.

Structural characterization of a soluble and partially folded class I major histocompatibility heavy chain/beta 2m heterodimer

Class I major histocompatibility (MHC) heavy chain (HC) must fold and assemble with beta 2 microglobulin (beta 2m) prior to binding peptides in the endoplasmic reticulum (ER). Each of these events is mediated by association with chaperones and other proteins and is an essential requirement for the maturation and normal cell surface expression of stable class I MHC-peptide complexes. Here we describe the biochemical and structural characterization of a soluble HC (B*0702)/beta 2m heterodimer, apparently free of peptide. Results suggest that the peptide binding domains (alpha 1 and alpha 2) of this folding intermediate are unstable and possess many of the properties ascribed to the molten globule state. The partially folded state of the HC/beta 2m heterodimer is consistent with the suggestion that it is stabilized by chaperones and other proteins in the ER. This soluble intermediate may be useful for studying protein-assisted folding and peptide binding of class I MHC molecules.

Antigen processing and presentation by the class I major histocompatibility complex

Major histocompatibility complex (MHC) class I molecules bind peptides derived from cellular proteins and display them for surveillance by the immune system. These peptide-binding molecules are composed of a heavy chain, containing an antigen-binding groove, which is tightly associated with a light chain (beta 2-microglobulin). The majority of presented peptides are generated by degradation of proteins in the cytoplasm, in many cases by a large multicatalytic proteolytic particle, the proteasome. Two beta-subunits of the proteasome, LMP2 and LMP7, are inducible by interferon-gamma and alter the catalytic activities of this particle, enhancing the presentation of at least some antigens. After production of the peptide in the cytosol, it is transported across the endoplasmic reticulum (ER) membrane in an ATP-dependent manner by TAP (transporter associated with antigen presentation), a member of the ATP-binding cassette family of transport proteins. There are minor pathways for generating presented peptides directly in the ER, and some evidence suggests that peptides may be further trimmed in this location. The class I heavy chain and beta 2-microglobulin are cotranslationally translocated into the endoplasmic reticulum where their assembly may be facilitated by the sequential association of the heavy chain with chaperone proteins BiP and calnexin. The class I molecule then associates with the lumenal face of TAP where it is retained, presumably awaiting a peptide. After the class I molecule binds a peptide, it is released for exocytosis to the cell surface where cytotoxic T lymphocytes examine it for peptides derived from foreign proteins.

Conformational changes induced in the MHC class I molecule by peptide and beta 2-microglobulin

Assembly of the class I MHC molecule is inextricably linked to the antigen presentation function of the class I molecule. Association of the class I MHC molecule with beta 2-microglobulin (beta 2m) is a prerequisite for association with the heterodimeric protein TAP, and once peptide is acquired, the class I molecule folds and begins its sojourn to the cell surface. To maintain its folded conformation, class I MHC requires peptide but not beta 2m, and the sequence of the peptide bound exercises a subtle influence on the structure of the class I molecule that is likely to be a factor in T cell receptor discrimination of MHC/peptide complexes.