Short peptides assist the folding of free class I heavy chains in solution

Adopting the rapamycin trapping assay to track the trafficking of murine MHC class I alleles, H-2K(b)

Background

In mammalian cells, the quality control (QC) of properly folded proteins is monitored in the early secretory pathway, particularly in the endoplasmic reticulum (ER). Several proteins, including our protein of interest, major histocompatibility complex class I (MHC class I), can bypass the first line of ER-QC and reside in post-ER compartments in an unfolded form. Such forms entail both monomeric and dimeric structures that are devoid of peptides and thus cannot fulfill the immunological function of antigen presentation at the cell surface. MHC class I structures become mature and properly folded once loaded with the appropriate peptides in the framework of the peptide loading complex (PLC). Despite the flood of information on the diverse trafficking behavior of different MHC class I alleles, there is still controversy on the actual trajectory followed by improperly folded murine MHC class I alleles, namely H-2Kb. In this study, we employ an in vitro rapamycin trapping assay, live cell imaging, and a biochemical COPII budding approach to further investigate the trafficking of H-2Kb beyond the level of the ER.

Results

We confirm the egress of H-2Kb in an unfolded form to a post-ER compartment from where they can cycle back to the ER. Deciphering the exact identity of the post-ER compartment by laser scanning microscopy did not only point to the existence of the ERGIC and cis-Golgi compartments as residency areas for unfolded proteins, but also to the involvement of an addional compartment, that lies in close proximity and possesses high resemblance to the aforementioned compartments. Interestingly, we were capable of showing using the same rapamycin trapping assay that H-2Kb can undergo a potential maturation event during their cycling; this is attained upon addition of peptides and trapping of accumulated post-ER molecules at the cell surface.

Conclusions

Our findings deepen the understanding of H-2Kb trafficking outside the ER and pave the way to decipher the role and the trafficking of certain PLC chaperones, such as tapasin, throughout H-2K^b post-ER QC. Finally, we demonstrate the plausible usage of the rapamycin assay to assess the trafficking of defected proteins especially in diseases and under therapeutic studies.

Cellular and molecular requirements for association of the murine cytomegalovirus protein m4/gp34 with major histocompatibility complex class I molecules

The murine cytomegalovirus (MCMV) protein m4/gp34 is unique among known viral genes that target the major histocompatibility complex (MHC) class I pathway of antigen presentation in the following two ways: it is found in association with class I MHC molecules at the cell surface, and it inhibits antigen presentation without reducing cell surface class I levels. The current study was undertaken to define more clearly the structural and cellular requirements for m4/gp34 association with the MHC class I molecule K(b). We first assessed the role of the peptide-loading complex in m4/gp34-K(b) association, using cell lines lacking TAP, tapasin, or beta(2)m. m4/gp34-K(b) complexes formed in the absence of TAP or tapasin, although not as efficiently as in wild-type cells. The expression of full-length and truncation mutants of m4/gp34 in a gutless adenovirus vector revealed that the transmembrane region of m4/gp34 was required for efficient association with the K(b) heavy chain. However, the peptide-loading complex was not absolutely required for the association, since m4/gp34 readily formed complexes with K(b) in detergent lysates. The addition of K(b)-binding peptide to the detergent lysates facilitated but was not essential for the formation of the complexes. The ease of complex formation in detergent lysates contrasted with the small fractions of m4/gp34 and K(b) that form complexes in infected cells, suggesting that the endoplasmic reticulum (ER) environment restricts access of m4/gp34 to K(b). Finally, although m4/gp34-K(b) complexes could form when m4 was carried either by MCMV or by the adenovirus vector, they were only efficiently exported from the ER in MCMV-infected cells, suggesting that MCMV provides additional factors needed for transport of the complexes.

Peptide selection by class I molecules of the major histocompatibility complex

Class I molecules of the major histocompatibility complex (MHC) bind peptides derived from cytoplasmic proteins. Comparison of over 100 such peptides reveals the importance of the carboxy-terminal residue in selective binding. Recent evidence implicates the proteases and transporters of the processing pathway in providing peptides with the correct residues at the carboxyl terminus.

Recovering antibody secretion using a hapten ligand as a chemical chaperone

Engineered antibodies have come to the forefront as research reagents and clinical therapeutics. However, reduced stability or expression levels pose a major problem with many engineered antibodies. As a model for understanding functional consequences of variable region mutation, we have studied the assembly and trafficking of anti-phenylphosphocholine antibodies. Previously, we identified severe secretion defects because of mutations in the heavy chain second complementarity determining region, which is involved in antigen binding. Here we demonstrate that immunoglobulin secretion is increased up to 27-fold by incubating stably transfected PCG1-1 cells with cognate hapten p-nitrophenylphosphocholine. Secretion was unaffected by nonbinding analogs. Radiotracer and metabolic labeling experiments demonstrated specific cellular uptake of p-nitrophenylphosphocholine and increased intracellular heavy and light chain assembly. Brefeldin A inhibited hapten-mediated immunoglobulin secretion but not assembly, indicating that assembly occurs early within the biosynthetic pathway. Recovery of secretion correlated with antigen binding capacity, suggesting that the rescue mechanism involves stabilization of heavy and light chain variable domains. This model system provides the first demonstration that cognate ligands can increase intracellular assembly of functional anti-hapten antibody within mammalian cells and suggests that small molecules of appropriate specificity and affinity acting as chemical chaperones may find application for increasing or regulating immunoglobulin expression.

A soluble major histocompatibility complex class I peptide-binding platform undergoes a conformational change in response to peptide epitopes

Class I major histocompatibility complexes (MHC) are heterotrimeric structures comprising heavy chains (HC), beta2-microglobulin (beta2-m), and short antigenic peptides of 8-10 amino acids. These components assemble in the endoplasmic reticulum and are released to the cell surface only when a peptide of the appropriate length and sequence is incorporated into the structure. The binding of beta2-m and peptide to HC is cooperative, and there is indirect evidence that the formation of a stable heterotrimer from an unstable HC:beta2-m heterodimer involves a peptide-induced conformational change in the HC. Such a conformational change could ensure both a strong interaction between the three components and also signal the release of stably assembled class I MHC molecules from the endoplasmic reticulum. A peptide-induced conformational change in HC has been demonstrated in cell lysates lacking beta2-m to which synthetic peptides were added. Many features of this conformational change suggest that it may be physiologically relevant. In an attempt to study the peptide-induced conformational change in detail we have expressed a soluble, truncated form of the mouse H-2Db HC that contains only the peptide binding domains of the class I molecule. We have shown that this peptide-binding "platform" is relatively stable in physiological buffers and undergoes a conformational change that is detectable with antibodies, in response to synthetic peptides. We also show that the structural features of peptides that induce this conformational change in the platform are the same as those required to observe the conformational change in full-length HC. In this respect, therefore, the HC alpha1 and alpha2 domains, which together form the peptide binding site of class I MHC, are able to act independently of the rest of the molecule.

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.

The rational design of TAP inhibitors using peptide substrate modifications and peptidomimetics

The major histocompatibility complex (MHC)-encoded transporter associated with antigen processing (TAP) translocates peptides from the cytosol into the lumen of the endoplasmic reticulum. This step precedes the binding of peptides to MHC class I molecules and is essential for cell surface expression of the MHC class I/peptide complex. TAP has a broad sequence specificity and a preference for peptides of around 9 amino acids. To synthesize inhibitors for TAP, we studied various alterations of the peptide substrate. The results indicate that TAP is stereospecific and that peptide bonds engineered into isosteric structures can improve translocation of the peptide. Furthermore, TAP is able to translocate peptides with large side chains that correspond to a peptide of approximately 21 amino acids in extended conformation. Peptides with longer side chains compete for the peptide binding site of TAP but fail to be translocated. Therefore, they represent the first rationally designed inhibitors of TAP.

Expression of LA45 reactive beta 2-microglobulin free HLA class I alpha-chains on activated T-cells is regulated by internalization, constitutive and protein kinase C inducible release

HLA Class I molecules on activated T cells are expressed as mAb W6/32 reactive heterodimers associated with beta 2-microglobulin (beta 2-m) and also as mAb LA45 reactive beta 2-m free HLA Class I alpha-chains. However, the regulation of free alpha-chain expression remained enigmatic. Here we show, that the amount of cell surface expressed free heavy chains is influenced by two distinct mechanisms. Firstly, a proportion of expressed molecules are cleaved and give rise to a soluble pool of HLA Class I molecules. We provide evidence that, besides the previously described constitutive release of free alpha chains, a second phorbol ester inducible release mechanism involving activation of protein kinase C (PKC) does exist. We demonstrate that both the constitutive and the enhanced release of LA45 reactive HLA Class I alpha-chains are the consequence of a cell membrane bound proteolytic activity with the characteristics of a 1, 10 phenanthroline sensitive metalloprotease. Secondly, we report that a distinct fraction of mAb tagged free alpha-chains is internalized via an n-ethylmaleimide sensitive pathway. Together, this data suggests that the expression of free alpha-chains is regulated by pathways governing release and internalization.

Expression of beta 2-microglobulin-free HLA class I alpha-chains on activated T cells requires internalization of HLA class I heterodimers

HLA class I molecules on activated T cells are expressed as heterodimers associated with beta 2-microglobulin (beta 2-m) and also beta 2-m-free HLA class I alpha-chains. Mechanisms leading to the expression of the activation associated beta 2-m-free HLA class I alpha-chains are poorly defined, however. Upon enzymatical removal of HLA class I alpha-chains on activated T cells, re-expression is observed within minutes upon reculture, reaching half-maximal levels within 1 hr. This process is independent of de novo protein synthesis and of export of newly synthesized proteins. Inhibition of the formation of coated pits by potassium depletion of cells abrogated the re-expression of HLA class I alpha-chains, suggesting that recycling events of HLA class I heterodimers via endosomal compartments are required for the generation of monoclonal antibody LA45-reactive alpha-chains. Furthermore, the rate of alpha-chain generation seems to be governed by the amount of cell surface-expressed HLA class I heterodimers. Taken together these findings suggest that beta 2-m-free HLA class I alpha-chains are generated during the process of class I heterodimer recycling.

Species-specific differences in chaperone interaction of human and mouse major histocompatibility complex class I molecules

Previous studies have shown that immature mouse class I molecules transiently associate with a resident endoplasmic reticulum protein of 88 kD that has been proposed to act as a chaperone for class I assembly. Subsequently, this protein was demonstrated to be identical to calnexin and to associate with immature forms of the T cell receptor complex, immunoglobulin, and human class I HLA heavy chains. In this paper we define further the interaction of human class I HLA heavy chains with chaperone proteins and find key differences with the complexes observed in the mouse system. First, calnexin and immunoglobulin binding protein (BiP) both associate with immature HLA class I heavy chains. The two chaperones are not found within the same molecular complex, suggesting that calnexin and BiP do not interact simultaneously with the same HLA class I heavy chain. Second, only free HLA class I heavy chains, and not beta 2-microglobulin (beta 2m)-associated heavy chains are found associated with the chaperones. Indeed, addition of free beta 2m in vitro induces dissociation of chaperone-class I HLA heavy chain complexes. The kinetics for dissociation of the class I HLA heavy chain-chaperone complexes and for formation of the class I HLA heavy chain-beta 2m complex display a reciprocity that suggests the interactions with chaperone and beta 2m are mutually exclusive. Mouse class I heavy chains expressed in human cells exhibit the mouse pattern of interaction with human chaperones and human beta 2m and not the human pattern, showing the difference in behavior is purely a function of the class I heavy chain sequence.

An unstable beta 2-microglobulin: major histocompatibility complex class I heavy chain intermediate dissociates from calnexin and then is stabilized by binding peptide

Proper assembly of the class I heavy chain (HC), beta 2-microglobulin (beta 2m), and peptide must occur in the endoplasmic reticulum (ER) in order for MHC class I molecules to be expressed on the cell surface. Newly synthesized class I HC bind calnexin, an ER resident chaperone. These calnexin-associated class I HC appeared to lack the stable association with beta 2m in peptide transporter-deficient T2 cells since beta 2m-unassociated class I HC-specific HC10 antibody, but not beta 2m-associated class I HC-specific W6/32 antibody, coimmunoprecipitated calnexin. To determine the precursor-product relationship of the pool of HC that bind peptide, class I-restricted peptides were added to lysates of T2 cells in vitro. These peptides stabilized preexisting beta 2m-associated HC complexes (beta 2m+:HC:pep-), but had no significant effect on the preexisting pool of calnexin-associated HC that lack beta 2m. Release of HC from calnexin appeared to be controlled by the binding of beta 2m, since beta 2m-deficient FO-1 cells showed a prolonged association of class I HC with calnexin, while beta 2m-transfected FO-1 cells displayed a more rapid dissociation of class I HC from calnexin. Consistent with this result, the dissociation of class I HC from calnexin did not appear to be dependent on peptide binding since the dissociation rates were similar in peptide transporter-deficient T2 cells and in wild-type T1 cells. From these observations, we speculate that in the stepwise assembly of class I molecules, calnexin may mediate dimerization of class I HC with beta 2m, and that the unstable beta 2m+:HC:pep- complexes, after dissociation from calnexin, subsequently bind peptide to complete the assembly.

Interaction of MHC class I molecules with the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) delivers cytosolic peptides into the endoplasmic reticulum (ER) where they bind to nascent class 1 histocompatibility molecules. Class 1-peptide complexes are then displayed at the cell surface for recognition by cytotoxic T lymphocytes. Immunoprecipitation of either TAP or class 1 molecules revealed an association between the transporter and diverse class 1 products. TAP bound preferentially to heterodimers of the class 1 heavy chain and beta 2-microglobulin, and the complex subsequently dissociated in parallel with transport of class 1 molecules from the ER to the Golgi apparatus. The TAP-class 1 complexes could also be dissociated in vitro by the addition of class 1-binding peptides. The association of class 1 molecules with TAP likely promotes efficient capture of peptides before their exposure to the lumen of the ER.

Folding and assembly of major histocompatibility complex class I heterodimers in the endoplasmic reticulum of intact cells precedes the binding of peptide

Major histocompatibility complex (MHC) class I molecules are heterotrimers consisting of a polymorphic H chain, beta 2-microglobulin (beta 2m) and peptide. Peptides are thought to associate early during biosynthesis but the order of assembly of class I molecules from their component subunits in intact cells is not settled. We have studied the assembly of MHC class I molecules in intact cells with or without peptide transporters. MHC class I H chain/beta 2m heterodimers can be efficiently recovered only 4 min after translation and are preceded by a folding intermediate. Approximately 2 min after their formation, the class I heterodimers are loaded with peptides resulting in stable class I heterotrimers. In these in vivo studies, no evidence was obtained that peptide binding to the H chain preceded the association with beta 2m. In contrast, nonassembled class I H chains could be recovered immediately after translation, but this pool did not participate in the formation of class I molecules.

Dissociation of beta 2-microglobulin is responsible for selective reduction of HLA class I antigenicity following acid treatment of cells

Beta 2-Microglobulin (beta 2 m) dissociated from surface HLA class I complex following exposure of cells to low pH and was detected in supernatant by radioimmunoprecipitation with specific monoclonal antibodies (mAbs). As the concn of beta 2m in supernatant increased, the binding of mAbs, specific for HLA class I heavy chains associated with beta 2m, to the cell surface declined. Binding of mAb specific for free HLA class I heavy chain to the cell surface increased after acid treatment. Reassociation with exogenous beta 2m confirmed increase in the number of free HLA class I heavy chains on surface of the cells after their exposure to low pH and also at least partially restored the reactivity with mAbs specific to HLA class I heavy chains associated with beta 2m. Dissociation of beta 2m from CD1 complex following acid treatment was also accompanied with the changes in antigenicity of cell surface CD1 molecules.