Participation of a novel 88-kD protein in the biogenesis of murine class I histocompatibility molecules

Calnexin influences folding of human class I histocompatibility proteins but not their assembly with beta 2-microglobulin

Class I major histocompatibility complex heavy chains bind to calnexin before associating with beta 2-microglobulin (beta 2m) and peptides. Calnexin has been shown to retain in the endoplasmic reticulum those class I heavy chains which have not assembled properly and, thus, to serve as a quality control mechanism. In addition, calnexin may direct the folding of class I subunits or their subsequent assembly. We asked whether calnexin plays a role in the initial folding of HLA-B*0702 heavy chains by assessing disulfide bond formation in vivo. Our results show that class I heavy chains form intrachain disulfide bonds very soon after translation, and that calnexin is bound to both reduced and oxidized forms during this process. When a cell-permeable reducing agent, dithiothreitol, was added to cells, disulfide bond formation in newly synthesized heavy chains was substantially blocked, as was their association with calnexin. The reducing agent appeared to affect calnexin directly, since binding was similarly abolished to a subset of proteins which do not contain internal disulfide bonds. Addition of the glucosidase inhibitor castanospermine to cells, shown previously to disrupt calnexin binding to ligands, slowed formation of disulfide bonds but did not decrease the amount of assembled heavy chain-beta 2m complexes that formed. Our data suggest that calnexin can promote disulfide bond formation in class I heavy chains but does not directly facilitate subsequent binding to beta 2m.

A possible role of ER-60 protease in the degradation of misfolded proteins in the endoplasmic reticulum

Wild-type human lysozyme (hLZM) is secreted when expressed in mouse L cells, whereas misfolded mutant hLZMs are retained and eventually degraded in a pre-Golgi compartment (Omura, F., Otsu, M., Yoshimori, T., Tashiro, Y., and Kikuchi, M. (1992) Eur. J. Biochem. 210, 591-599). These misfolded mutant hLZMs are associated with protein disulfide isomerase (Otsu, M., Omura, F., Yoshimori, T., and Kikuchi, M. (1994) J. Biol. Chem. 269, 6874-6877). From the observation that this degradation is sensitive to cysteine protease inhibitors, such as N-acetyl-leucyl-leucyl-norleucinal and N-acetyl-leucyl-leucyl-methioninal, but not to the serine protease inhibitors, 1-chloro-3-tosylamido-7-amino-2-heptanone and (p-amidinophenyl)methanesulfonyl fluoride, it was suggested that some cysteine proteases are likely responsible for the degradation of abnormal proteins in the endoplasmic reticulum (ER). ER-60 protease (ER-60), an ER resident protein with cysteine protease activity (Urade, R., Nasu, M., Moriyama, T., Wada, K., and Kito, M. (1992) J. Biol. Chem. 267, 15152-15159), was found to associate with misfolded hLZMs, but not with the wild-type protein, in mouse L cells. Furthermore, denatured hLZM is degraded by ER-60 in vitro, whereas native hLZM is not. These results suggest that ER-60 could be a component of the proteolytic machinery for the degradation of misfolded mutant hLZMs in the ER.

Role of the endoplasmic reticulum chaperone calnexin in subunit folding and assembly of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (AChR) is a pentameric complex assembled from four different gene products by mechanisms that are inadequately understood. In this study we investigated the role of the endoplasmic reticulum (ER)-resident molecular chaperone calnexin in AChR subunit folding and assembly. We have shown that calnexin interacts with nascent AChR alpha-subunits (AChR-alpha) in muscle cell cultures and in COS cells transfected with mouse AChR-alpha. In chick muscle cells maximal association of labeled alpha-subunits with calnexin was observed immediately after a 15-min pulse with [35S]methionine/cysteine and subsequently declined with a t1/2 of approximately 20 min. The decrease in association with calnexin was concomitant with the folding of the alpha-subunit to achieve conformational maturation shortly before assembly. Brefeldin A did not inhibit AChR subunit assembly or the dissociation of calnexin from the assembling subunits, confirming that the ER is the site of AChR assembly and that calnexin dissociation is not affected under conditions in which the exit of assembled AChR from the ER is blocked. These results indicate that calnexin participates directly in the molecular events that lead to AChR assembly.

The small envelope glycoprotein (GS) of equine arteritis virus folds into three distinct monomers and a disulfide-linked dimer

The small membrane glycoprotein (GS) of equine arteritis virus (EAV) is a minor virion component but is abundantly expressed in EAV-infected cells. In this study, we have analyzed its membrane topology, folding, oligomerization, and intracellular transport. We show that GS is a class I integral membrane protein with one functional N-glycosylation site. Gel electrophoresis under nonreducing conditions revealed that GS occurs in EAV-infected cells in four monomeric conformations and as disulfide-linked homodimers. The slowest-migrating monomeric form corresponded to the fully reduced GS protein; the three faster-migrating monomeric species are probably generated by the formation of alternative intrachain disulfide bonds between the three luminal cysteines in the molecule. The GS monomers were selectively retained in the endoplasmic reticulum, as judged by their permanent susceptibility to endoglycosidase H, whereas the GS dimers were specifically incorporated into virus particles and became endoglycosidase H resistant and sialylated during passage through the Golgi apparatus.

The interaction of beta 2-microglobulin (beta 2m) with mouse class I major histocompatibility antigens and its ability to support peptide binding. A comparison of human and mouse beta 2m

The function of major histocompatibility complex (MHC) class I molecules is to sample peptides derived from intracellular proteins and to present these peptides to CD8+ cytotoxic T lymphocytes. In this paper, biochemical assays addressing MHC class I binding of both peptide and beta 2-microglobulin (beta 2m) have been used to examine the assembly of the trimolecular MHC class I/beta 2m/peptide complex. Recombinant human beta 2m and mouse beta 2ma have been generated to compare the binding of the two beta 2m to mouse class I. It is frequently assumed that human beta 2m binds to mouse class I heavy chain with a much higher affinity than mouse beta 2m itself. We find that human beta 2m only binds to mouse class I heavy chain with slightly (about 3-fold) higher affinity than mouse beta 2m. In addition, we compared the effect of the two beta 2m upon peptide binding to mouse class I. The ability of human beta 2m to support peptide binding correlated well with its ability to saturate mouse class I heavy chains. Surprisingly, mouse beta 2m only facilitated peptide binding when mouse beta 2m was used in excess (about 20-fold) of what was needed to saturate the class I heavy chains. The inefficiency of mouse beta 2m to support peptide binding could not be attributed to a reduced affinity of mouse beta 2m/MHC class I complexes for peptides or to a reduction in the fraction of mouse beta 2m/MHC class I molecules participating in peptide binding. We have previously shown that only a minor fraction of class I molecules are involved in peptide binding, whereas most of class I molecules are involved in beta 2m binding. We propose that mouse beta 2m interacts with the minor peptide binding (i.e. the "empty") fraction with a lower affinity than human beta 2m does, whereas mouse and human beta 2m interact with the major peptide-occupied fraction with almost similar affinities. This would explain why mouse beta 2m is less efficient than human beta 2m in generating the peptide binding moiety, and identifies the empty MHC class I heavy chain as the molecule that binds human beta 2m preferentially.

Glucose trimming and reglucosylation determine glycoprotein association with calnexin in the endoplasmic reticulum

To determine the role of N-linked oligosaccharides in the folding of glycoproteins, we analyzed the processing of in vitro translated influenza hemagglutinin (HA) in dog pancreas microsomes. We found that binding to calnexin, a membrane-bound molecular chaperone, was specific to molecules that possessed monoglucosylated core glycans. In the microsomes, these were generated either by glucose removal from the original triglucosylated core oligosaccharide by glucosidases I and II or by reglucosylation of already unglucosylated high mannose glycans. Release of fully folded HA from calnexin required the removal of the remaining glucose by glucosidase II. The results provided an explanation for trimming and reglucosylation activities in the endoplasmic reticulum and established a direct correlation between glycosylation and folding.

Autocrine motility factor receptor is a marker for a distinct membranous tubular organelle

Autocrine motility factor (AMF) is secreted by tumor cells and is capable of stimulating the motility of the secreting cells. In addition to being expressed on the cell surface, its receptor, AMF-R, is found within a Triton X-100 extractable intracellular tubular compartment. AMF-R tubules can be distinguished by double immunofluorescence microscopy from endosomes labeled with the transferrin receptor, lysosomes labeled with LAMP-2, and the Golgi apparatus labeled with beta-COP. AMF-R can also be separated from a LAMP-2 containing lysosomal fraction by differential centrifugation of MDCK cells and is found within a 100,000 g membrane pellet. By electron microscopic immunocytochemistry, AMF-R is localized predominantly to smooth vesicular and tubular membranous organelles as well as to a lesser extent to the plasma membrane and rough endoplasmic reticulum. AMF-R tubules have a variable diameter of 50-250 nm and can acquire an elaborate branched morphology. By immunofluorescence microscopy, AMF-R tubules are clearly distinguished from the calnexin labeled rough endoplasmic reticulum and AMF-R tubule expression is stable to extended cycloheximide treatment. The AMF-R tubule is therefore not a biosynthetic subcompartment of the endoplasmic reticulum. The tubular morphology of the AMF-R tubule is modulated by both the actin and microtubule cytoskeletons. In a similar fashion to that described previously for the tubular lysosome and endoplasmic reticulum, the linear extension and peripheral cellular orientation of the AMF-R tubule are dependent on the integrity of the microtubule cytoskeleton. The AMF-R tubule may thus form part of a family of microtubule-associated tubular organelles.

Calreticulin functions as a molecular chaperone in the biosynthesis of myeloperoxidase

Myeloperoxidase (MPO), a lysosomal heme protein found exclusively in neutrophils and monocytes, is necessary for efficient oxygen-dependent microbicidal activity. Acquisition of heme by the heme-free MPO precursor apopro-MPO appears to be a prerequisite for its subsequent proteolytic processing and advancement along the biosynthetic pathway to mature MPO. We present data indicating that calreticulin (CRT), a high capacity calcium-binding protein residing in the lumen of the endoplasmic reticulum of a wide variety of cells, interacts specifically with fully glycosylated apopro-MPO. Biosynthetically radiolabeled CRT (60 kDa) and apopro-MPO (90 kDa) were coprecipitated from PLB 985 cells by monospecific antiserum against CRT when the immunoprecipitations were performed either under nondenaturing conditions or following reversible crosslinking. Nonglycosylated MPO precursors synthesized in the presence of tunicamycin did not interact with CRT. The CRT-apopro-MPO interaction was restricted to an early phase of MPO biosynthesis, and CRT did not interact with the later appearing, heme-containing species of MPO, i.e. pro-MPO or the heavy subunit of mature MPO. These data show that CRT participates in the post-translational processing of MPO, perhaps by maintaining apopro-MPO in a conformation competent to accommodate insertion of the heme group. In this general way, CRT shares certain functional properties with the structurally homologous transmembrane calcium-binding endoplasmic reticulum protein calnexin. Both interact with glycosylated biosynthetic precursors of proteins selectively expressed in specialized cells.

The molecular chaperone calnexin binds Glc1Man9GlcNAc2 oligosaccharide as an initial step in recognizing unfolded glycoproteins

Calnexin is a molecular chaperone that resides in the membrane of the endoplasmic reticulum. Most proteins that calnexin binds are N-glycosylated, and treatment of cells with tunicamycin or inhibitors of initial glucose trimming steps interferes with calnexin binding. To test if calnexin is a lectin that binds early oligosaccharide processing intermediates, a recombinant soluble calnexin was created. Incubation of soluble calnexin with a mixture of Glc0-3Man9GlcNAc2 oligosaccharides resulted in specific binding of the Glc1Man9GlcNAc2 species. Furthermore, Glc1Man5-7GlcNAc2 oligosaccharides bound relatively poorly, suggesting that, in addition to a requirement for the single terminal glucose residue, at least one of the terminal mannose residues was important for binding. To assess the involvement of oligosaccharide-protein interactions in complexes of calnexin and newly synthesized glycoproteins, alpha 1-antitrypsin or the heavy chain of the class I histocompatibility molecule were purified as complexes with calnexin and digested with endoglycosidase H. All oligosaccharides on either glycoprotein were accessible to this probe and could be removed without disrupting the association with calnexin. Furthermore, the addition of 1 M alpha-methyl glucoside or alpha-methyl mannoside had no effect on complex stability. These findings suggest that once complexes between calnexin and glycoproteins are formed, oligosaccharide binding does not contribute significantly to the overall interaction. However, it is likely that the binding of Glc1Man9GlcNAc2 oligosaccharides is a crucial event during the initial recognition of newly synthesized glycoproteins by calnexin.

The Schizosaccharomyces pombe homologue of the chaperone calnexin is essential for viability

We have cloned a Schizosaccharomyces pombe gene, here designated cnx1, encoding the homologue of the endoplasmic reticulum molecular chaperone calnexin. Disruption of the cnx1 gene was lethal, demonstrating that it has an essential cellular function. Transcription of cnx1 mRNA is initiated at multiple sites, and it can be induced by various stress treatments that lead to the accumulation of unfolded and/or misfolded proteins in the endoplasmic reticulum. The encoded Cnx1p protein more closely resembles its plant and animal calnexin homologues than that of Saccharomyces cerevisiae. Cnx1p is acidic and migrates aberrantly on SDS-polyacrylamide gel electrophoresis, similar to its mammalian counterparts. Cnx1p contains the hallmark KPEDWD motifs that are found in all members of the calnexin/calreticulin family of proteins. Using an in vitro translation-processing system, we have shown that Cnx1p has the characteristic type I topology of calnexin proteins. Unlike its higher eukaryotic homologues, Cnx1p has a site for N-glycosylation that was modified in an in vitro translation-processing assay.

Regulation of cell surface beta 1 integrin levels during keratinocyte terminal differentiation

Integrins of the beta 1 family play a central role in controlling adhesion and terminal differentiation within the epidermis. When human epidermal keratinocytes undergo terminal differentiation, intracellular transport of newly synthesized integrins is inhibited, and mature receptors are lost from the cell surface. We have examined the mechanisms underlying these processes, using an experimental model in which keratinocytes are placed in suspension to induce terminal differentiation. The block in intracellular transport was keratinocyte- and integrin-specific since it was not observed when fibroblasts were placed in suspension and did not affect E-cadherin synthesis in suspended keratinocytes. Newly synthesized beta 1 integrins associated with an endoplasmic reticulum resident protein, calnexin; the association was prolonged when keratinocytes were placed in suspension, suggesting a role for calnexin in the inhibition of transport. After 24 h, the level of beta 1 integrin mRNA declines in suspended keratinocytes, reflecting inhibition of gene transcription, but in fibroblasts, the level remained constant. Transport of integrins could be blocked in both adherent keratinocytes and fibroblasts by inhibiting total protein synthesis, raising the possibility that transport is coupled to de novo integrin synthesis. The fate of receptors on the surface of keratinocytes was followed by confocal immunofluorescence microscopy, immunoelectron microscopy, and biochemical analysis: with the onset of terminal differentiation, endocytosed receptors were transported to the lysosomes. These experiments reveal novel mechanisms by which integrin levels can be controlled. Together with our earlier evidence for transcriptional regulation and affinity modulation of integrins, they highlight the complexity of the mechanisms which ensure that the onset of terminal differentiation is linked to detachment of keratinocytes from the underlying basement membrane.

Peptide influences the folding and intracellular transport of free major histocompatibility complex class I heavy chains

Class I major histocompatibility complex molecules require both beta 2-microglobulin (beta 2m) and peptide for efficient intracellular transport. With the exception of H-2Db and Ld, class I heavy chains have not been detectable at the surface of cells lacking beta 2m. We show that properly conformed class I heavy chains can be detected in a terminally glycosylated form indicative of cell surface expression in H-2b, H-2d, and H-2s beta 2m-/- concanavalin A (Con A)-stimulated splenocytes incubated at reduced temperature. Furthermore, we demonstrate the presence of Kb molecules at the surface of beta 2m-/- cells cultured at 37 degrees C. The mode of assembly of class I molecules encompasses two major pathways: binding of peptide to preformed "empty" heterodimers, and binding of peptide to free heavy chains, followed by recruitment of beta 2m. In support of the existence of the latter pathway, we provide evidence for a role of peptide in intracellular transport of free class I heavy chains, through analysis of Con A-stimulated splenocytes from transporter associated with antigen processing 1 (TAP1)-/-, beta 2m-/-, and double-mutant TAP1/beta 2m-/- mice.

The Merck Frosst Award Lecture 1994/La conference Merck Frosst 1994. Calnexin: a molecular chaperone with a taste for carbohydrate

Calnexin is an integral membrane protein of the endoplasmic reticulum (ER) that binds transiently to a wide array of newly synthesized membrane and secretory proteins. It also exhibits prolonged binding to misfolded or incompletely folded proteins. Recent studies have demonstrated that calnexin functions as a molecular chaperone to facilitate the folding and assembly of proteins in the ER. It is also a component of the quality control system that prevents proteins from progressing along the secretory pathway until they have acquired proper tertiary or quaternary structure. Most proteins that are translocated into the ER are glycosylated at Asn residues, and calnexin's interactions are almost exclusively restricted to proteins that possess this posttranslational modification. The preference for glycoproteins resides in calnexin's ability to function as a lectin with specificity for the Glc1Man9GlcNAc2 oligosaccharide, an early intermediate in the processing of Asn-linked oligosaccharides. Calnexin also has the capacity to bind to polypeptide segments of unfolded glycoproteins. Available evidence suggests that calnexin utilizes its lectin property during initial capture of a newly synthesized glycoprotein and that subsequent association (and chaperone function) is mediated through polypeptide interactions. Unlike other molecular chaperones that are soluble proteins, calnexin is an intrinsic component of the ER membrane. Its unique ability to capture unfolded glycoproteins through their large oligosaccharide moieties may have evolved as a means to overcome accessibility problems imposed by being constrained within a lipid bilayer.

Molecular requirements for the interaction of class II major histocompatibility complex molecules and invariant chain with calnexin

Molecular chaperones are believed to retain misfolded and incompletely assembled oligomeric proteins in the endoplasmic reticulum (ER). Here, we have further analyzed the association of one such chaperone, calnexin, with human major histocompatibility complex class II alpha and beta subunits and the invariant chain. Calnexin associates with transport-competent invariant chain trimers (p33 or p41), as well as ER-retained trimers (p35/33 or p43/41), suggesting that ER retention of the latter is not because of calnexin association. Neither the replacement of the transmembrane segment of the DR beta subunit with a glycosyl phosphatidylinositol anchor nor deglycosylation of any of these proteins with tunicamycin or endoglycosidase H treatment abolished calnexin association. Using a cell-permeabilization system, we were unable to observe association of newly synthesized glycopeptides with calnexin, arguing that calnexin may not act like a simple lectin for association with glycoproteins. The results indicate that neither transmembrane regions nor N-linked glycans are exclusively responsible for calnexin association. Based on our data and the observations of others, we suggest that these features may have varying significance for different glycoproteins in determining their interaction with calnexin.

HLA-DR beta chains enter into an aggregated complex containing GRP-78/BiP prior to their degradation by the pre-Golgi degradative pathway

HLA class II molecules are membrane proteins which are assembled in the endoplasmic reticulum shortly after synthesis of the alpha and beta and invariant chain (Ii) monomers. DR beta chains, in the absence of DR alpha, are rapidly and completely degraded by the pre-Golgi degradative pathway. Here we have examined those factors which target DR beta chains for degradation in a DR alpha deficient cell line, 9.22.3. The DR beta monomers in 9.22.3 were initially incorporated into a proteinaceous complex containing BiP. With time, the DR beta complexes were further aggregated. In wild type cells, which can assemble DR alpha-beta dimers, the secondary phase of aggregation of DR beta was not seen. Additional evidence that aggregation of DR beta in 9.22.3 cells was progressive was that a more mature form of DR beta was found exclusively in the largest DR beta complexes. Furthermore, the most highly aggregated DR beta chains were degraded more rapidly than bulk DR beta chains. These data suggest that DR beta aggregates are intermediates in the pre-Golgi pathway of DR beta degradation. They further suggest that formation of large DR beta aggregates is a proximal event to DR beta degradation. We conclude that DR beta chains are targeted for degradation as a consequence of a change of state, coincident with their aggregation into slow forming, high molecular weight complexes.

Molecular chaperones in antigen presentation

As is the case with most proteins of the secretory pathway, the biogenesis of MHC class I and class II molecules occurs in association with molecular chaperones. Considerable progress has been made in identifying the chaperones involved and recent studies on two of these, calnexin and invariant chain, have shown that they influence multiple processes including protein stability, folding, assembly and intercellular retention.

Effect of TAP on the generation and intracellular trafficking of peptide-receptive major histocompatibility complex class I molecules

Using a fluorescein-conjugated antigenic peptide, peptide-receptive H-2Kb MHC class I molecules were found throughout the secretory pathways of RMA cells and peptide transporter (TAP)-deficient derivative cells (RMA/S). RMA/S cells displayed higher levels of intracellular peptide-receptive molecules, while, surprisingly, RMA cells expressed 3- to 5-fold more cell surface peptide-receptive molecules. Metabolic radiolabeling of Kb-associated oligosaccharides with [1-3H]galactose demonstrated that despite a large difference in the fraction of Kb molecules in native conformation in detergent extracts, Kb transport rates from the trans-Golgi complex to the surfaces of RMA and RMA/S cells were similar. Thus, although considerable numbers of class I alpha chains reach the RMA/S cell surface, they are a less productive source of peptide-receptive molecules than class I molecules synthesized by TAP-expressing RMA cells, suggesting paradoxically that TAP functions to increase the amount of peptide-receptive molecules at the cell surface.

Saccharomyces cerevisiae CNE1 encodes an endoplasmic reticulum (ER) membrane protein with sequence similarity to calnexin and calreticulin and functions as a constituent of the ER quality control apparatus

We have used a polymerase chain reaction strategy to identify in the yeast Saccharomyces cerevisiae genes of the mammalian calnexin/calreticulin family, and we have identified and isolated a single gene, CNE1. The protein predicted from the CNE1 DNA sequence shares some of the motifs with calnexin and calreticulin, and it is 24% identical and 31% similar at the amino acid level with mammalian calnexin. On the basis of its solubility in detergents and its lack of extraction from membranes by 2.5 M urea, high salt, and sodium carbonate at pH 11.5, we have established that Cne1p is an integral membrane protein. However, unlike calnexins, the predicted carboxyl-terminal membrane-spanning domain of Cne1p terminates directly. Furthermore, based on its changed mobility from 76 to 60 kDa after endoglycosidase H digestion Cne1p was shown to be N-glycosylated. Localization of the Cne1p protein by differential and analytical subcellular fractionation as well as by confocal immunofluorescence microscopy showed that it was exclusively located in the endoplasmic reticulum (ER), despite the lack of known ER retention motifs. Although six Ca(2+)-binding proteins were detected in the ER fractions, they were all soluble proteins, and Ca2+ binding activity has not been detected for Cne1p. Disruption of the CNE1 gene did not lead to inviable cells or to gross effects on the levels of secreted proteins such as alpha-pheromone or acid phosphatase. However, in CNE1 disrupted cells, there was an increase of cell-surface expression of an ER retained temperature-sensitive mutant of the alpha-pheromone receptor, ste2-3p, and also an increase in the secretion of heterologously expressed mammalian alpha 1-antitrypsin. Hence, Cne1p appears to function as a constituent of the S. cerevisiae ER protein quality control apparatus.

Delivery of protein antigen to the major histocompatibility complex class I-restricted antigen presentation pathway

Major histocompatibility complex (MHC) class I-restricted antigen presentation normally requires a protein antigen to be synthesized in the cytosol of the antigen presenting cell (APC). Exogenous protein antigen could gain access to the class I presentation pathway if the protein is introduced into the cytosolic compartment of the APC. Approaches which release the protein antigen from endocytic vesicles have been employed to deliver protein antigen for the recognition by class I-restricted cytotoxic T lymphocytes (CTL). These include osmotic shock, electroporation, cationic and pH-sensitive liposomes. An alternative approach is to deliver a gene that encodes the protein antigen. In this case, the APC is transfected with a gene which synthesizes the "exogenous protein" in the cytosol. Delivery of protein antigen targeted for CTL induction in vivo follows a different strategy and generally requires an antigen carrier of lipidic/membranous nature, such as liposomes, immunostimulating complexes, and/or lipid conjugates. Macrophages that are responsible for scavenging the antigen play an important role in CTL induction. An optimal CTL inductive vaccine must contain other immuno-modulatory activities in addition to its activity in delivering antigen to the class I pathway. Attempts to attenuate viral infection and to improve anti-tumor immunity have been successful by delivering the exogenous antigen entrapped in liposomes. These animal model studies should be of great value in the development of potential vaccine formulation.

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.

Endogenous antigen presentation by MHC class II molecules

T cell recognition of antigen requires that a complex form between peptides derived from the protein antigen and cell surface glycoproteins encoded by genes within the major histocompatibility complex (MHC). MHC class II molecules present both extracellular (exogenous) and internally synthesized (endogenous) antigens to the CD4 T cells subset of lymphocytes. The mechanisms of endogenous antigen presentation are the subject of this review. Isolation and amino acid sequencing of peptides bound to the class II molecule indicate that a very high proportion (70-90%) of the total peptides presented by the class II molecule are in fact derived from the pool of proteins that are synthetized within the antigen-presenting cell (APC). This type of sequence information as well as the study of model antigens has indicated that proteins expressed in a diversity of intracellular sites, including the cell surface, endoplasmic reticulum and cytosol can gain access to the class II molecule, albeit with different efficiencies. The main questions that remain to be answered are the intracellular trafficking patterns that allow colocalization of internally synthesized antigens with the class II molecule, the site(s) within the cell where peptide:class II molecule complex formation can take place and whether presentation of 'foreign' as well as 'self' antigens takes place by mechanisms that vary from one cell type to another or that vary with the metabolic state of the APC. If such variability exists, is would imply that the array of peptides displayed by class II molecules at the cell surface has similar variability, a possibility that would impact on self tolerance and autoimmunity.

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.

Folding of VSV G protein: sequential interaction with BiP and calnexin

The endoplasmic reticulum (ER) contains molecular chaperones that facilitate the folding of proteins in mammalian cells. Biosynthetic labeling was used to study the interactions of two chaperones, BiP and calnexin, with vesicular stomatitis virus (VSV) glycoprotein (G protein). Coimmunoprecipitation of G protein with the chaperones showed that BiP bound maximally to early folding intermediates of G protein, whereas calnexin bound after a short lag to more folded molecules. Castanospermine, an inhibitor of ER glucosidases, blocked the binding of proteins to calnexin and inhibited G protein folding. Interaction with calnexin was necessary for efficient folding of G protein and for retention of partially folded forms.

A polymorphic residue in the amino terminal alpha 1 hemi-domain of the mouse Ld class I molecule affects its assembly and surface expression

In comparison to Dd and most other mouse major histocompatibility complex class I molecules, the Ld molecule is poorly expressed on the cell surface, has a lower affinity for beta 2-microglobulin and is trafficked more slowly to the cell surface. Previous studies using Ld-Dd exon-shuffled constructs and the chimeric Ddm1 molecules suggested that the Ld alpha 1 domain was responsible for this phenotype. Two constructs, one containing an Ld-Dd hemi-exon-shuffled alpha 1 exon and the other containing a Dd-Ld hemi-exon-shuffled alpha 1 exon, were inserted into either Ld or Dd to replace the intact alpha 1 exon. These constructs were transfected into mouse L cells. Flow cytometric analyses of the resulting transfectants indicate that the Dd-Ld alpha 1/Ld molecules, similar to the Dd alpha 1/Dd alpha 2/Ld molecules, were expressed at a higher level on the cell surface than either the Ld-Dd alpha 1/Ld molecules or intact Ld molecules. Analyses of the molecules in lysates suggested that a higher proportion of the Dd-Ld alpha 1/Ld molecules, like the Dd alpha 1/Dd alpha 2/Ld molecules, as compared to the Ld-Dd alpha 1/Ld and intact Ld molecules were assembled as detected by alpha 2 domain-reactive monoclonal antibodies. Pulse-chase and lysate stability studies suggested that the lower steady state levels of assembled Ld-Dd alpha 1 molecules resulted from a slower assembly rate rather than instability. Collectively, these studies suggest that residues in the amino terminal half of the Ld alpha 1 domain are responsible for its inefficient assembly, probably leading to its low cell surface expression. To determine which polymorphic residues in the amino terminal alpha 1 hemi-domain might influence this phenotype, several Ld point mutants, in which a Dd amino terminal alpha 1 hemi-domain residue was substituted into the corresponding position of Ld, were analysed. These analyses suggested that, while the residue at position 9 has only a slight effect on beta 2-microglobulin association, it has a striking effect on assembly and cell surface expression.

67-kD elastin-binding protein is a protective "companion" of extracellular insoluble elastin and intracellular tropoelastin

The 67-kD elastin-binding protein (EBP) mediates cell adhesion to elastin and elastin fiber assembly, and it is similar, if not identical, to the 67-kD enzymatically inactive, alternatively spliced beta-galactosidase. The latter contains an elastin binding domain (S-GAL) homologous both to the aorta EBP and to NH2-terminal sequences of serine proteinases (Hinek, A., M. Rabinovitch, F. W. Keeley, and J. Callahan. 1993. J. Clin. Invest. 91:1198-1205). We now confirm the functional importance of this homology by showing that elastolytic activity of a representative serine elastase, porcine pancreatic elastase, was prevented by an antibody (anti-S-GAL) and by competing with purified EBP or S-GAL peptide. Immunohistochemistry of adult aorta indicates that the EBP exists as a permanent component of mature elastic fibers. This observation, together with the in vitro studies, suggests that the EBP could protect insoluble elastin from extracellular proteolysis and contribute to the extraordinary stability of this protein. Double immunolabeling of fetal lamb aorta with anti-S-GAL and antitropoelastin antibodies demonstrated, under light and electron microscopy, intracellular colocalization of the proteins in smooth muscle cells (SMC). Incubation of SMC with galactosugars to dissociate tropoelastin from EBP caused intracellular aggregation of tropoelastin. A tropoelastin/EBP complex was extracted from SMC lysates by coimmunoprecipitation and cross-linking, and its functional significance was addressed by showing that its dissociation by galactosugars caused degradation of tropoelastin by endogenous serine proteinase(s). This suggests that the EBP may also serve as a "companion" to intracellular tropoelastin, protecting this highly hydrophobic protein from self-aggregation and proteolytic degradation.

Quality control in the secretory pathway: retention of a misfolded viral membrane glycoprotein involves cycling between the ER, intermediate compartment, and Golgi apparatus

Proteins synthesized in the ER are generally transported to the Golgi complex and beyond only when they have reached a fully folded and assembled conformation. To analyze how the selective retention of misfolded proteins works, we monitored the long-term fate of a membrane glycoprotein with a temperature-dependent folding defect, the G protein of tsO45 vesicular stomatitis virus. We used indirect immunofluorescence, immunoelectron microscopy, and a novel Nycodenz gradient centrifugation procedure for separating the ER, the intermediate compartment, and the Golgi complex. We also employed the folding and recycling inhibitors dithiothreitol and AIF4-, and coimmunoprecipitation with calnexin antibodies. The results showed that the misfolded G protein is not retained in the ER alone; it can move to the intermediate compartment and to the cis-Golgi network but is then recycled back to the ER. In the ER it is associated with calnexin and BiP/GRP78. Of these two chaperones, only BiP/GRP78 seems to accompany it through the recycling circuit. Thus, the retention of this misfolded glycoprotein is the result of multiple mechanisms including calnexin binding in the ER and selective retrieval from the intermediate compartment and the cis-Golgi network.

Calnexin retains unassembled major histocompatibility complex class I free heavy chains in the endoplasmic reticulum

The assembly of major histocompatibility complex (MHC) class I molecules involves the association of heavy (H) chain with beta 2-microglobulin (beta 2m) and peptide. Unassembled class I H chains do not exit the endoplasmic reticulum (ER) and this is exemplified by the beta 2m-deficient human melanoma FO-1 where free class I H chains are unable to complete assembly. In pulse chase experiments involving FO-1 cells, unassembled free class I H chains were shown to be stably associated with calnexin (IP90/p88), a 90-kD integral membrane molecular chaperone of the ER. To establish a role for calnexin in mediating this retention, we transfected FO-1 cells with a cytoplasmic tail deletion mutant of calnexin. Since the cytoplasmic tail contains the ER retention motif, these mutant calnexin molecules leave the ER and progress to the cell surface. In these stable transfectants of FO-1, free class I H chains also exited the ER and trafficked to the cell surface with calnexin, thus establishing a role for calnexin in the quality control of MHC class I assembly through mediating the ER retention of incompletely assembled class I H chains.

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.

Human cytomegalovirus-infected cells have unstable assembly of major histocompatibility complex class I complexes and are resistant to lysis by cytotoxic T lymphocytes

Viruses which cause persistence in the naturally infected host are predicted to have evolved immune evasion mechanisms. Human cytomegalovirus (HCMV) causes significant morbidity and mortality in immunocompromised patients yet persists without clinical manifestations in seropositive individuals who have normal immune function. We report that HCMV infection in vitro impairs major histocompatibility complex class I (MHC-I) assembly accompanied by resistance to killing by cytotoxic CD8+ T lymphocytes. Pulse-chase metabolic labelling experiments show that MHC-I complexes continue to be assembled by both uninfected and HCMV-infected cells. However, MHC-I molecules are unstable in HCMV-infected cells and are rapidly broken down. Endoglycosidase H treatment of immunoprecipitates indicates that the breakdown of MHC-I complexes in HCMV-infected cells occurs primarily in a pre-Golgi compartment. Interference with normal MHC-I assembly and expression, if relevant in vivo, may have implications for the restriction of the diversity of the CD8+ cytotoxic T lymphocyte repertoire directed against HCMV antigens and may be an important mechanism of viral persistence.

How N-linked oligosaccharides affect glycoprotein folding in the endoplasmic reticulum

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Calnexin: a membrane-bound chaperone of the endoplasmic reticulum

Calnexin is a new type of molecular chaperone that interacts with many nascent membrane and soluble proteins of the secretory pathway. Calnexin is unrelated to molecular chaperones of the Hsp60, Hsp70 and Hsp90 families, and is further distinguished from them in that it is an integral membrane protein. One of its demonstrated functions is the retention of incorrectly or incompletely folded proteins, suggesting that calnexin is a component of the quality control system of the endoplasmic reticulum.

Role of N-linked oligosaccharide recognition, glucose trimming, and calnexin in glycoprotein folding and quality control

Using a pulse-chase approach combined with immunoprecipitation, we showed that newly synthesized influenza virus hemagglutinin (HA) and vesicular stomatitis virus G protein associate transiently during their folding with calnexin, a membrane-bound endoplasmic reticulum (ER) chaperone. Inhibitors of N-linked glycosylation (tunicamycin) and glucosidases I and II (castanospermine and 1-deoxynojirimycin) prevented the association, whereas inhibitors of ER alpha-mannosidases did not. Our results indicated that binding of these viral glycoproteins to calnexin correlated closely with the composition of their N-linked oligosaccharide side chains. Proteins with monoglucosylated oligosaccharides were the most likely binding species. On the basis of our data and existing information concerning the role of monoglucosylated oligosaccharides on glycoproteins, we propose that the ER contains a unique folding and quality control machinery in which calnexin acts as a chaperone that binds proteins with partially glucose-trimmed carbohydrate side chains. In this model glucosidases I and II serve as signal modifiers and UDP-glucose:glycoprotein glucosyltransferase, as a folding sensor.

Regulation of MHC class I transport by the molecular chaperone, calnexin (p88, IP90)

Assembled class I histocompatibility molecules, consisting of heavy chain, beta 2-microglobulin, and peptide ligand, are transported rapidly to the cell surface. In contrast, the intracellular transport of free heavy chains or peptide-deficient heavy chain-beta 2-microglobulin heterodimers is impaired. A 90-kilodalton membrane-bound chaperone of the endoplasmic reticulum (ER), termed calnexin, associates quantitatively with newly synthesized class I heavy chains, but the functions of calnexin in this interaction are unknown. Class I subunits were expressed alone or in combination with calnexin in Drosophila melanogaster cells. Calnexin retarded the intracellular transport of both peptide-deficient heavy chain-beta 2-microglobulin heterodimers and free heavy chains. Calnexin also impeded the rapid intracellular degradation of free heavy chains. The ability of calnexin to protect and retain class I assembly intermediates is likely to contribute to the efficient intracellular formation of class I-peptide complexes.

Retention of unassembled components of integral membrane proteins by calnexin

Quality control mechanisms prevent the cell surface expression of incompletely assembled multisubunit receptors such as the T cell receptor (TCR). The molecular chaperone function of calnexin (IP90, p88), a 90-kilodalton protein that resides in the endoplasmic reticulum (ER), in the retention of representative chains of the TCR-CD3 complex in the ER was tested. Truncation mutants of calnexin, when transiently expressed in COS cells, were exported from the ER and either accumulated in the Golgi or progressed to the cell surface. CD3 epsilon chains cotransfected with the forms of calnexin that were not retained in the ER exited the ER and colocalized with calnexin. Since engineered calnexin determined the intracellular localization of the proteins associated with it, it is concluded that calnexin interacts with incompletely assembled TCR components and retains them in the ER.

Mechanism of B cell antigen receptor function: transmembrane signaling and triggering of apoptosis

The antigen receptor of B lymphocytes (BCR) plays important roles in virtually every stage in the development, inactivation, or activation of B cells. The BCR is a complex of membrane immunoglobulin (mIg) and a heterodimer of two transmembrane polypeptides called Ig-alpha and Ig-beta. Site directed mutation of the mu immunoglobulin heavy chain has demonstrated that the mu transmembrane domain plays a key role in the assembly of mIgM with Ig-alpha/Ig-beta. In addition, there is a strong correlation between the ability of various mutant mIgM molecules to associate with Ig-alpha/Ig-beta and their ability to induce signal transduction reactions such as protein tyrosine phosphorylation and phosphoinositide breakdown. The cytoplasmic domains of Ig-alpha and Ig-beta share a region of limited homology with each other and with components of the T cell antigen receptor and of the Fc receptor. The presence of regions of the cytoplasmic domains of Ig-alpha or Ig-beta including this conserved amino acid sequence motif is sufficient to confer signaling function on chimeric transmembrane proteins. Both Ig-alpha and Ig-beta chimeras are capable of inducing all of the BCR signaling events tested. Based on these and related observations, we propose that the motifs act to initiate the BCR signaling reactions by binding and activating tyrosine kinases. Among the important events mediated by BCR signaling is induced expression of a series of genes referred to as early response genes. In B cells these include transcription factors and at least one component that regulates signaling events. One of these genes, c-myc, appears to play an important role in mediating apoptosis in B cells stimulated via the BCR complex.

Model for the in vivo assembly of nascent Ld class I molecules and for the expression of unfolded Ld molecules at the cell surface

To characterize the process of class I assembly and maturation, we have studied the Ld molecule of the mouse. Previous studies have shown that a significant proportion of intracellular and surface Ld molecules can be detected in an alternative conformation designated Ldalt1. Nascent Ldalt molecules are non-peptide ligand associated and are weakly associated with beta 2-microglobulin (beta 2m). Unexpectedly, when monoclonal antibodies were added directly to the lysis buffer, significant amounts of Ldalt/beta 2m heterodimer were detected, suggesting that beta 2m association is not necessarily sufficient to induce Ld conformation. By contrast, addition of peptide to cell lysates rapidly induced the folding of beta 2m-associated Ldalt to conformed Ld. Furthermore, the time course and dynamics of this conversion correlated precisely with peptide binding to Ld. The precursor-product relationship of Ldalt and conformed Ld was also visualized in vivo by pulse-chase analysis of BALB/c splenocytes. To investigate the factors that regulate intracellular transport of class I molecules, expression of Ld was studied in the peptide transport-deficient cell line, RMA.S-Ld, and in beta 2m-/- splenocytes. In contrast to wild-type cell lines, both Ldalt and conformed Ld are poorly expressed at the cell surface of RMA.S-Ld and beta 2m-/- splenocytes. Therefore, surface expression of Ldalt is dependent upon the concomitant expression of conformed Ld molecules. To determine whether surface Ldalt molecules can result from melting of conformed Ld molecules, surface Ld molecules were loaded with several different known Ld peptide ligands. Complexes of Ld with different ligands were found to have dramatically disparate surface half-lives. Importantly, the Ld peptide complexes that turned over the most rapidly resulted in the most gain in surface Ldalt, implying that peptide dissociation can induce the accumulation of nonconformed Ld heavy chains at the cell surface.