Measuring interactions of MHC class I molecules using surface plasmon resonance

To examine the molecular interactions between major histocompatibility complex (MHC)-encoded molecules and peptides, monoclonal antibodies (mAbs), or T cell receptors, we have developed model systems employing genetically engineered soluble MHC class I molecules (MHC-I), synthetic peptides, purified mAbs, and engineered solubilizable T cell receptors. Direct binding assays based on immobilization of one of the interacting components to the dextran modified gold biosensor surface of a surface plasmon resonance (SPR) detector have been developed for each of these systems. The peptide binding site of the MHC-I molecule can be sterically mapped by evaluation of a set of peptides immobilized through the thiol group of cysteine substitutions at each peptide position. Kinetic binding studies indicate that the MHC-I/peptide interaction is characterized by a low to moderate apparent kass (approximately 5000-60000 M-1 s-1) and very small kdis (approximately 10(-4)-10(-6) s-1) consistent with the biological requirement for a long cell surface residence time to permit engagement with T cell receptors. Several mAb directed against different MHC-I epitopes were examined, and kinetic parameters of their interaction with MHC molecules were determined. These showed characteristic moderate association rate constants and moderate dissociation rate constants (kass approximately 10(4)-10(6) M-1 s-1 and kdis approximately 10(-2)-10(-4) s-1), characteristic of many antibody/protein antigen interactions. The interaction of an anti-idiotypic anti-TCR mAb with its purified cognate TCR was of moderate affinity and revealed kinetic binding similar to that of the anti-MHC mAbs. The previously determined interaction of a purified T cell receptor with its MHC-I/peptide ligand is characterized by kinetic constants more similar to those of the antibody/antigen interaction than of the MHC-I/peptide interaction, but is remarkable for rapid dissociation rates (apparent kdis approximately 10(-2) s-1). Such binding studies of reactions involving the MHC-I molecules offer insight into the mechanisms responsible for the initial specific events required for the stimulation of T cells.

A family of murine NK cell receptors specific for target cell MHC class I molecules

The Ly-49A molecule is an NK cell receptor specific for MHC class I molecules on target cells. When Ly-49A engages H-2Dd, Ly-49A+ NK cells become globally incapable of killing their targets in vitro. This interaction also occurs in vivo. Ly-49A belongs to a family of highly related molecules, including Ly-49C (5E6 antigen) and LGL-1 that also determine NK cell specificity. In the NK gene complex, the Ly-49 family is genetically linked to genes encoding NKR-P1 and CD69 that are structurally related and capable of activating NK cells. Finally, Ly-49 may be related to human molecules that are selectively expressed on NK cells and influence NK cell specificity. These findings highlight the emerging significance of the Ly-49 family in NK cell activity.

Molecular analysis of the same HIV peptide functionally binding to both a class I and a class II MHC molecule

Although several peptides have been found to bind to both class I and class II molecules, the basis for this binding of the same peptide to two classes of MHC molecules has not been compared previously. We have analyzed one such peptide, P18 from the V3 loop of HIV-1 gp160, which we have previously shown to be recognized by CD8+ CTL with the class I molecule H-2Dd, and by CD4+ Th cells with the class II molecule I-Ad. With the use of truncated and substituted peptides, we found that the minimal core peptides are very similar, that the residues required for class I binding precisely fit the recently identified consensus motif for peptides binding to Dd (XGPX[R/K/H]XXX(X) [L/I/F]), and that at least three of the same residues are involved in binding to class II I-Ad. In addition, several of the same residues are involved in TCR interaction when the peptide is presented by class I and class II molecules. Modeling shows results to be consistent with the crystal structure of a peptide-class II MHC complex. Thus, the recognition of this versatile peptide by CD4+ Th cells with class II MHC molecules and by CD8+ cytotoxic T cells with class I MHC molecules is remarkably similar in both the core peptide used and the role of different residues in the ternary complex.

Molecular analysis of the same HIV peptide functionally binding to both a class I and a class II MHC molecule

Although several peptides have been found to bind to both class I and class II molecules, the basis for this binding of the same peptide to two classes of MHC molecules has not been compared previously. We have analyzed one such peptide, P18 from the V3 loop of HIV-1 gp160, which we have previously shown to be recognized by CD8+ CTL with the class I molecule H-2Dd, and by CD4+ Th cells with the class II molecule I-Ad. With the use of truncated and substituted peptides, we found that the minimal core peptides are very similar, that the residues required for class I binding precisely fit the recently identified consensus motif for peptides binding to Dd (XGPX[R/K/H]XXX(X) [L/I/F]), and that at least three of the same residues are involved in binding to class II I-Ad. In addition, several of the same residues are involved in TCR interaction when the peptide is presented by class I and class II molecules. Modeling shows results to be consistent with the crystal structure of a peptide-class II MHC complex. Thus, the recognition of this versatile peptide by CD4+ Th cells with class II MHC molecules and by CD8+ cytotoxic T cells with class I MHC molecules is remarkably similar in both the core peptide used and the role of different residues in the ternary complex.

The HLA-B14 peptide binding site can accommodate peptides with different combinations of anchor residues

Most peptides that bind to a particular major histocompatibility complex class I molecule share amino acid residues important for binding at one or two positions. Sequence analyses of peptides bound to HLA-B14 revealed at least four candidates for these so-called anchor residues: Arg at P2, Tyr at P3, Arg at P5, and Leu at P9. Combinations of any three of these amino acids sufficed for binding to HLA-B14 in vitro. Using this information, we identified an antigenic peptide critical for cytotoxic T lymphocyte recognition of virus-infected cells. Molecular models of HLA-B14 peptide complexes were constructed to investigate how the potential anchor residues might function. By using binding data to calculate the contribution to binding of each amino acid at anchor positions and predicting the stability of all possible nonapeptide complexes that could be formed from antigenic proteins, we estimate that three known antigenic nonapeptides are in the highest affinity cohort of peptides. Thus, even when multiple combinations of anchor residues contribute to binding, antigenic peptides are routinely identifiable.

Minimal requirements for peptide mediated activation of CD8+ CTL

A physical chemical model of T cell stimulation by class I-peptide complexes was developed and used to analyse in vitro studies of gamma-interferon release as a function of the number of peptide and MHC molecules. The analysis provided reasonable estimates of well identified parameters, including equilibrium constants and the minimum number of T cell receptor-class I-peptide ternary complexes on a presenting cell required to activate T cells. The latter number was estimated as 3-5 per T cell. This is in distinct contrast to estimates in the literature of the number of peptide-MHC complexes required for activity, which is necessarily larger. The analysis also predicted that activity is potentiated by interaction between class I molecules, even if one member of the pair is not bound by antigen. The analytical approach used in this paper may be applicable to other activation systems.

Functional cell surface expression by a recombinant single-chain class I major histocompatibility complex molecule with a cis-active beta 2-microglobulin domain

As a preliminary step towards the use of cell surface single-chain class I major histocompatibility complex (MHC) molecules as T cell immunogens, we have engineered a recombinant gene encoding a full-length cell surface single-chain version of the H-2Dd class I MHC molecule (SC beta Ddm) which has beta 2-microglobulin (beta 2m) covalently linked to the amino terminus of a full-length H-2Dd heavy chain via a peptide spacer. The single-chain protein is correctly folded and stably expressed on the surface of transfected L cells. It can present an antigenic peptide to an H-2Dd-restricted antigen-specific T cell hybridoma. When expressed in peptide-transport-deficient cells, SC beta Ddm can be stabilized and pulsed for antigen presentation by incubation with extracellular peptide at 27 degrees or 37 degrees C, allowing the preparation of cells with single-chain molecules that are loaded with a single chosen antigenic peptide. SC beta Ddm can be stably expressed in beta 2m-negative cells, showing that the single-chain molecule uses its own beta 2m domain to achieve correct folding and surface expression. Furthermore, the beta 2m domain of SC beta Ddm, unlike transfected free beta 2m, does not rescue surface expression of endogenous class I MHC in the beta 2m-negative cells. This strict cis activity of the beta 2m domain of SC beta Ddm makes possible the investigation of class I MHC function in cells, and potentially in animals, that express but a single type of class I MHC molecule.

Determinant selection of major histocompatibility complex class I-restricted antigenic peptides is explained by class I-peptide affinity and is strongly influenced by nondominant anchor residues

The contribution of major histocompatibility complex (MHC) class I-peptide affinity to immunodominance of particular peptide antigens (Ags) in the class I-restricted cytotoxic T lymphocyte (CTL) response is not clearly established. Therefore, we have compared the H-2Kb-restricted binding and presentation of the immunodominant ovalbumin (OVA)257-264 (SIINFEKL) determinant to that of a subdominant OVA determinant OVA55-62 (KVVRFDKL). Immunodominance of OVA257-264 was not attributable to the specific T cell repertoire but correlated instead with more efficient Ag presentation. This enhanced Ag presentation could be accounted for by the higher affinity of Kb/OVA257-264 compared with Kb/OVA55-62 despite the presence of a conserved Kb-binding motif in both peptides. Kinetic binding studies using purified soluble H-2Kb molecules (Kbs) and biosensor techniques indicated that the Kon for association of OVA257-264-C6 and Kbs at 25 degrees C was integral of 10-fold faster (5.9 x 10(3) M-1 s-1 versus 6.5 x 10(2) M-1 s-1), and the Koff approximately twofold slower (9.1 x 10(-6) s-1 versus 1.6 x 10(-5) s-1), than the rate constants for interaction of OVA55-62-C6 and Kbs. The association of these peptides with Kb was significantly influenced by multiple residues at presumed nonanchor sites within the peptide sequence. The contribution of each peptide residue to Kb-binding was dependent upon the sequence context and the summed contributions were not additive. Thus the affinity of MHC class I-peptide binding is a critical factor controlling presentation of peptide Ag and immunodominance in the class I-restricted CTL response.

Host MHC class I molecules modulate in vivo expression of a NK cell receptor

Target cell expression of certain MHC class I molecules correlates with resistance to lysis by NK cells. To explain this correlation, one hypothesis states that NK cells may possess two types of receptors; one may activate NK cells whereas another, specific for target cell MHC class I molecules, may inhibit natural killing by transducing negative signals. The cell surface molecule, Ly-49, is expressed on an NK cell subpopulation (15% to 20%) in spleens from C57BL/6 (H-2b) mice. Previously, we showed that lysis by Ly-49+ IL-2-activated NK cells was globally inhibited when targets expressed either H-2Dd or an H-2k-class I molecule, consistent with the hypothesis that Ly-49 is an inhibitory NK cell receptor that engages these MHC class I molecules. We now have determined the influence of specific host MHC class I molecules on Ly-49 expression. In two-color flow cytometric examination of splenic cells, Ly-49+ NK1.1+ cells were undetectable in MHC-congenic strains expressing Dd or Dk, in C57BL/6 mice transgenic for membrane-bound Dd, and in B10.D2dm1 mice. These data establish that Dd itself is sufficient for this effect and suggest that Ly-49 engages Dd-alpha 1/alpha 2 domains. Cross-linking of Ly-49 with membrane-bound Dd may be required because Ly-49+ NK1.1+ cells were readily detectable in C57BL/6 strains transgenic for soluble forms of Dd. To examine whether this effect could be the result of down-regulation of Ly-49 expression or negative selection of Ly-49+ cells, we determined Ly-49 expression on highly purified, freshly isolated NK cell populations (> 90% NK1.1+ CD3- cells). These experiments demonstrated that Ly-49+ cells were present in normal numbers but that Ly-49 expression was markedly decreased in congenic mice expressing H-2Dd or Dk, and in the strain transgenic for membrane-bound H-2Dd. Thus, expression of a putative MHC class I-specific NK cell receptor is modulated by its apparent interaction with alpha 1/alpha 2 domains of host MHC class I molecules.

Host MHC class I molecules modulate in vivo expression of a NK cell receptor

Target cell expression of certain MHC class I molecules correlates with resistance to lysis by NK cells. To explain this correlation, one hypothesis states that NK cells may possess two types of receptors; one may activate NK cells whereas another, specific for target cell MHC class I molecules, may inhibit natural killing by transducing negative signals. The cell surface molecule, Ly-49, is expressed on an NK cell subpopulation (15% to 20%) in spleens from C57BL/6 (H-2b) mice. Previously, we showed that lysis by Ly-49+ IL-2-activated NK cells was globally inhibited when targets expressed either H-2Dd or an H-2k-class I molecule, consistent with the hypothesis that Ly-49 is an inhibitory NK cell receptor that engages these MHC class I molecules. We now have determined the influence of specific host MHC class I molecules on Ly-49 expression. In two-color flow cytometric examination of splenic cells, Ly-49+ NK1.1+ cells were undetectable in MHC-congenic strains expressing Dd or Dk, in C57BL/6 mice transgenic for membrane-bound Dd, and in B10.D2dm1 mice. These data establish that Dd itself is sufficient for this effect and suggest that Ly-49 engages Dd-alpha 1/alpha 2 domains. Cross-linking of Ly-49 with membrane-bound Dd may be required because Ly-49+ NK1.1+ cells were readily detectable in C57BL/6 strains transgenic for soluble forms of Dd. To examine whether this effect could be the result of down-regulation of Ly-49 expression or negative selection of Ly-49+ cells, we determined Ly-49 expression on highly purified, freshly isolated NK cell populations (> 90% NK1.1+ CD3- cells). These experiments demonstrated that Ly-49+ cells were present in normal numbers but that Ly-49 expression was markedly decreased in congenic mice expressing H-2Dd or Dk, and in the strain transgenic for membrane-bound H-2Dd. Thus, expression of a putative MHC class I-specific NK cell receptor is modulated by its apparent interaction with alpha 1/alpha 2 domains of host MHC class I molecules.

T cell receptor-MHC class I peptide interactions: affinity, kinetics, and specificity

The critical discriminatory event in the activation of T lymphocytes bearing alpha beta T cell receptors (TCRs) is their interaction with a molecular complex consisting of a peptide bound to a major histocompatibility complex (MHC)-encoded class I or class II molecule on the surface of an antigen-presenting cell. The kinetics of binding were measured of a purified TCR to molecular complexes of a purified soluble analog of the murine MHC class I molecule H-2Ld (sH-2Ld) and a synthetic octamer peptide p2CL in a direct, real-time assay based on surface plasmon resonance. The kinetic dissociation rate of the MHC-peptide complex from the TCR was rapid (2.6 x 10(-2) second-1, corresponding to a half-time for dissociation of approximately 27 seconds), and the kinetic association rate was 2.1 x 10(5) M-1 second-1. The equilibrium constant for dissociation was approximately 10(-7) M. These values indicate that TCRs must interact with a multivalent array of MHC-peptide complexes to trigger T cell signaling.

H-2Dd exploits a four residue peptide binding motif

We have characterized the amino acid sequences of over 20 endogenous peptides bound by a soluble analog of H-2Dd, H-2Dds. Synthetic analogs corresponding to self, viral, tumor, or motif peptides were then tested for their ability to bind to H-2Dd by serologic epitope induction assays using both purified soluble protein and cell surface H-2Dd. The dominant primary sequence motif included glycine at position 2, proline at position 3, and a hydrophobic COOH terminus: leucine, isoleucine, or phenylalanine at position 9 or 10. Ancillary support for high affinity binding was contributed by a positively charged residue at position 5. Three-dimensional computer models of H-2Dds/peptide complexes, based on the crystallographic structure of the human HLA-B27/peptide complex, showed that the basic residue at position 5 was in position to form a salt bridge with aspartic acid at position 156, a polymorphic residue of the H-2Dd heavy (H) chain. Analysis of 28 such models, including 17 based on nonamer self-peptides, revealed considerable variation in the structure of the major histocompatibility complex (MHC) surrounding peptide residue 1, depending on the size and charge of the side chain. Interactions between the side chains of peptide residues 5 and 7, and 6 and 8 commonly occurred. Those peptide positions with limited sequence variability and least solvent accessibility may satisfy structural requirements for high affinity binding of the peptide to the MHC class I H chain, whereas the highly variable positions of the peptide (such as positions 4, 6, and 8) may contribute more to the T cell epitopes.

Multiple pathways are involved in the extracellular processing of MHC class I-restricted peptides

T cell stimulation by certain class I-restricted antigenic peptides, such as the HIV 1 gp160-derived peptide, P18, requires peptide processing by angiotensin-1 converting enzyme (ACE) in FCS. We observed that longer versions of P18 and the murine cytomegalovirus pp89-derived core peptide, pMCMV, which could stimulate T cell hybridomas in FCS, were not as sensitive to the ACE inhibitor captopril as P18. Using cell-free soluble murine class I MHC molecules and protease inhibitors, we found that there are pathways of differing efficiency that use enzymes other than ACE for the proteolytic processing of peptides in serum. The kinetics of the generation of T cell stimulatory activity among P18 variant peptides in serum differed with peptide length, and with the nature of amino and COOH-terminal extensions. Such processing occurs in human plasma as well as in FCS. The understanding of this processing, its kinetics, and its inhibitors can lead to better design of peptide-based therapies, including vaccines.

Multiple pathways are involved in the extracellular processing of MHC class I-restricted peptides

T cell stimulation by certain class I-restricted antigenic peptides, such as the HIV 1 gp160-derived peptide, P18, requires peptide processing by angiotensin-1 converting enzyme (ACE) in FCS. We observed that longer versions of P18 and the murine cytomegalovirus pp89-derived core peptide, pMCMV, which could stimulate T cell hybridomas in FCS, were not as sensitive to the ACE inhibitor captopril as P18. Using cell-free soluble murine class I MHC molecules and protease inhibitors, we found that there are pathways of differing efficiency that use enzymes other than ACE for the proteolytic processing of peptides in serum. The kinetics of the generation of T cell stimulatory activity among P18 variant peptides in serum differed with peptide length, and with the nature of amino and COOH-terminal extensions. Such processing occurs in human plasma as well as in FCS. The understanding of this processing, its kinetics, and its inhibitors can lead to better design of peptide-based therapies, including vaccines.

Role of conserved regions of class I MHC molecules in the activation of CD8+ cytotoxic T lymphocytes by peptide and purified cell-free class I molecules

To analyze the molecular interactions involved in CD8+ cytotoxic T lymphocyte (CTL) recognition quantitatively, we developed a cell-free antigen presenting system. Genetically engineered soluble H-2Dd molecules coated on plastic microtiter plates could present HIV envelope peptide to an antigen-specific CTL clone, inducing it to produce IFN-gamma in the absence of accessory cells and their accessory or co-stimulatory molecules. The peptide-MHC complexes were functionally stable for over 24 h. The magnitude of T cell activation was dependent on the concentrations of both class I MHC molecule and the peptide, but was more sensitive to the concentration of the MHC molecule than to that of peptide. This result suggests that one MHC molecule can play more than one role in activating the CTL. One such role is the interaction between CD8 and a conserved region of class I MHC, as suggested by the finding that holding the total MHC concentration constant with an irrelevant class I MHC molecule (H-2Kb engineered to have the same alpha 3 domain as H-2Dd) made the T cell response less sensitive to the change in concentration of the relevant MHC molecule (H-2Dd). The irrelevant class I MHC molecule (H-2Kb), unable to present this peptide by itself, augmented the T cell response at lower concentrations of peptide. These results suggest that the conserved alpha 3 domain of the class I MHC heavy chain as well as polymorphic regions play an important role in T cell activation and that T cell interaction with MHC molecules not presenting peptide can still augment the response.

Direct detection of major histocompatibility complex class I binding to antigenic peptides using surface plasmon resonance. Peptide immobilization and characterization of binding specificity

We have developed model systems in which the binding of purified, genetically engineered, soluble analogues of major histocompatibility complex (MHC) class I molecules to immobilized antigenic peptides can be monitored in real time using surface plasmon resonance (SPR). Synthetic analogues of several peptides known to bind different mouse and human MHC class I molecules were prepared with cysteine residues substituted at appropriate positions. The analogue peptides were immobilized via the bifunctional reagent N-gamma-maleimidobutyryloxy-succinimide to amino groups generated on the dextran-modified gold surface of a biosensor flow cell. Using this approach, each position in the sequence of an H-2Ld-specific viral peptide, pMCMV (YPHFMPTNL), was used for coupling, and the resulting surfaces were tested for binding of the soluble analogue of H-2Ld, H-2Lds. In accord with our previously described H-2Ld/pMCMV three-dimensional structural model, only those residues of the peptide that remain exposed following binding (positions 4-8) can be replaced by cysteine and used for coupling. Stable binding of soluble MHC class I molecules, H-2Lds, H-2Dds, H-2Kbs, and HLA-A2s to their respective immobilized cognate peptides was detected by SPR. Specificity of the peptide/MHC interaction was characterized both by direct binding using immobilized peptides and by competition with peptides in solution, and in general was consistent with known immunological reactivity. Some peptides bound not only their cognate MHC molecule, but others at lower apparent affinity. Measurement of real time binding of MHC class I molecules to peptides immobilized through specific side chains suggests the application of a similar approach to the study of the interaction of peptides with a wide variety of peptide-binding macromolecules.

MHC class I/peptide interactions: binding specificity and kinetics

Recent developments in the preparation of soluble analogues of the major histocompatibility complex (MHC) class I molecules as well as in the application of real time biosensor technology have permitted the direct analysis of the binding of MHC class I molecules to antigenic peptides. Using synthetic peptide analogues with cysteine substitutions at appropriate positions, peptides can be immobilized on a dextran-modified gold biosensor surface with a specific spatial orientation. A full set of such substituted peptides (known as 'pepsicles', as they are peptides on a stick) representing antigenic or self peptides can be used in the functional mapping of the MHC class I peptide binding site. Scans of sets of peptide analogues reveal that some amino acid side chains of the peptide are critical to stable binding to the MHC molecule, while others are not. This is consistent with functional experiments using substituted peptides and three-dimensional molecular models of MHC/peptide complexes. Detailed analysis of the kinetic dissociation rates (kd) of the MHC molecules from the specifically coupled solid phase peptides reveals that the stability of the complex is a function of the particular peptide, its coupling position, and the MHC molecule. Measured kd values for antigenic peptide/class I interactions at 25 degrees C are in the range of ca 10(-4)-10(-6)/s. Biosensor methodology for the analysis of the binding of MHC class I molecules to solid-phase peptides using real time surface plasmon resonance offers a rational approach to the general analysis of protein/peptide interactions.

The biochemistry and cell biology of antigen processing and presentation

T lymphocytes with alpha beta receptors recognize antigen in association with the polymorphic products of the class I and class II loci of the major histocompatibility complex (MHC). This presentation of antigen results from the intracellular generation of protein fragments, and the binding and transport to the cell surface of these peptides in stable association with the MHC class I and class II molecules. Each class of MHC molecule appears specialized for capture of peptides present in a particular intracellular compartment. We describe here the structural basis of peptide-MHC molecule interaction, the differences in biochemical behavior that focus the two classes of MHC molecules on peptides of distinct size and location, and the cell biology of MHC molecule transport, peptide generation, and intracellular movement. The importance of conformational changes accompanying peptide binding that affect subunit stability of MHC molecules, and the relationship between these changes and the handling of proteins by intracellular chaperones, are emphasized as key features in the operation of the class I and class II presentation pathways.

Endogenous peptides of a soluble major histocompatibility complex class I molecule, H-2Lds: sequence motif, quantitative binding, and molecular modeling of the complex

To gain insight into the rules that govern the binding of endogenous and viral peptides to a given major histocompatibility complex (MHC) class I molecule, we characterized the amino acid sequences of a set of self peptides bound by a soluble analogue of murine H-2Ld, H-2Lds. We tested corresponding synthetic peptides quantitatively for binding in several different assays, and built three-dimensional computer models of eight peptide/H-2Lds complexes, based on the crystallographic structure of the human HLA-B27/peptide complex. Comparison of primary and tertiary structures of bound self and antigenic peptides revealed that residues 2 and 9 were not only restricted in sequence and tolerant of conservative substitutions, but were spatially constrained in the three-dimensional models. The degree of sequence variability of specific residues in MHC-restricted peptides reflected the lack of structural constraint on those amino acids. Thus, amino acid residues that define a peptide motif represent side chains required or preferred for a close fit with the MHC class I heavy chain.

A recombinant, soluble, single-chain class I major histocompatibility complex molecule with biological activity

Heterodimeric class I major histocompatibility complex molecules, which consist of a 45-kDa heavy-chain and a 12-kDa beta 2-microglobulin (beta 2m) light chain, bind endogenously synthesized peptides for presentation to antigen-specific T cells. We have synthesized a gene encoding a single-chain, soluble class I molecule derived from mouse H-2Dd, in which the carboxyl terminus of beta 2m is linked via a peptide spacer to the amino terminus of the heavy chain. The chimeric protein is secreted efficiently from transfected L cells, is thermostable, and when loaded with an appropriate antigenic peptide, stimulates an H-2Dd-restricted antigen-specific T-cell hybridoma. Thus, functional binding of peptide does not require the complete dissociation of beta 2m, implying that a heavy chain/peptide complex is not an obligate intermediate in the assembly of the heavy-chain/beta 2m/peptide heterotrimer. Single-chain major histocompatibility complex molecules uniformly loaded with peptide have potential uses for structural studies, toxin or fluor conjugates, and vaccines.

Independent and synergistic effects of disulfide bond formation, beta 2-microglobulin, and peptides on class I MHC folding and assembly in an in vitro translation system

We have examined the post-translational processing, intrachain disulfide bond formation, folding, and assembly of MHC class I H chains with beta 2-microglobulin after coupled in vitro translation of homogeneous mRNA and transport of nascent chains into canine microsomal vesicles. The formation of native alpha 3 domain conformation was dependent on conditions that optimized intrachain disulfide bond formation, and efficient folding of the alpha 1 alpha 2 domain required exposure to antigenic peptide. beta 2-microglobulin and peptide acted synergistically in forming native alpha 1 alpha 2 domain structure, and a small proportion of molecules with native alpha 1 alpha 2, but non-native alpha 3 structure were detected, indicating that alpha 3 domain folding is not an absolute prerequisite for the formation of native alpha 1 alpha 2 domain structure.

Independent and synergistic effects of disulfide bond formation, beta 2-microglobulin, and peptides on class I MHC folding and assembly in an in vitro translation system

We have examined the post-translational processing, intrachain disulfide bond formation, folding, and assembly of MHC class I H chains with beta 2-microglobulin after coupled in vitro translation of homogeneous mRNA and transport of nascent chains into canine microsomal vesicles. The formation of native alpha 3 domain conformation was dependent on conditions that optimized intrachain disulfide bond formation, and efficient folding of the alpha 1 alpha 2 domain required exposure to antigenic peptide. beta 2-microglobulin and peptide acted synergistically in forming native alpha 1 alpha 2 domain structure, and a small proportion of molecules with native alpha 1 alpha 2, but non-native alpha 3 structure were detected, indicating that alpha 3 domain folding is not an absolute prerequisite for the formation of native alpha 1 alpha 2 domain structure.