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Maturation of Qa-1^b Class I Molecules Requires ß₂-Microglobulin But Is TAP Independent¹

Peter J. Robinson^2,*, Paul J. Travers, Arthur Stackpoole^*, Lorraine Flaherty and Hakim Djaballah^*

^* MRC-CSC, Hammersmith Hospital, and Department of Crystallography, Birbeck College, London, United Kingdom; and Wadsworth Center for Laboratories and Research, Albany, NY 12201

	Abstract

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Two conformationally distinct and stable forms of Qa-1^b, one strongly associated with ß₂-microglobulin (ß₂m) and the other associated with a novel molecule, gp44, were observed during immunochemical studies on the expression of Qa-1^b molecules in mouse spleen cells. Both forms are efficiently processed and expressed at the cell surface. However, a large proportion of Qa-1^b was found to be disulfide linked to gp44 without any detectable ß₂m. In TAP1-deficient mice, both forms undergo carbohydrate processing and are expressed on the cell surface, suggesting that they may traffic using a pathway not requiring a TAP association step. Consistent with this, size exclusion chromatography of newly synthesized class I molecules shows that high molecular mass complexes containing H-2K^k do not contain Qa-1^b. Although Qa-1^b can be stably expressed without ß₂m, there was no maturation of either form in cells from ß₂m-deficient mice where heavy chains were rapidly degraded. These results suggest that Qa-1^b, like most other class I molecules, requires ß₂m for an initial folding step. However, ß₂m is not essential for subsequent processing of Qa-1^b molecules.

	Introduction

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Class I histocompatibility Ags alert the immune system to viruses by displaying peptides derived from virally encoded proteins on the surface of infected cells. Cells expressing class I molecules bearing viral epitopes are efficiently destroyed by host CTL. Newly synthesized class I heavy chains (Hcs) first associate with calnexin, an endoplasmic reticulum (ER)³-resident chaperone, and then bind ß₂-microglobulin (ß₂m) (1, 2). Stabilization of class I/ß₂m heterodimers is usually dependent upon acquisition of a suitable peptide, delivered by the TAP peptide transporter located in the ER (3, 4). In TAP-deficient cells where the peptide supply to class I molecules is severely reduced, many Hcs are unable to form stable structures and are targeted for degradation (5).

Peptides that enter the ER independently of TAPs, such as cleaved leader sequences, can induce assembly of class I molecules in TAP-deficient cells (6). It has also been shown that the expression of some class I molecules is intrinsically TAP independent (7, 8). The class I-like molecules CD1 and TL can both be expressed on the surface of cells lacking a functional TAP transporter, and there is increasing evidence that they may acquire antigenic epitopes in intracellular compartments other than the ER (9, 10).

Mouse Qa-1 is one of a group of class I molecules designated as nonclassical or class Ib histocompatibility Ags, and that elicits strong CTL responses (11). At least two other members of this group, CD1 and H-2 M3, are involved in the presentation of bacterial and mycobacterial Ags to T cells (12, 13). Qa-1^b is expressed in many mouse tissues, associates with ß₂m, and can present peptide Ags to CTL (11). Previous studies have shown that some CTL can recognize Qa-1^b on cells defective in TAP function (14).

Maturation of class I molecules has usually been studied using alloantibodies that recognize correctly folded heterotrimers of Hcs, ß₂m, and peptide. More recently, alternative conformations of class I molecules, including precursor forms as well as cell surface molecules, have been described using other Hc-specific Abs (15). We have produced rabbit antisera against unique peptide sequences present in the cytoplasmic domain of Qa-1^b. These antisera do not cross-react with other mouse class I molecules, and recognize both correctly folded and conformationally altered Qa-1^b molecules. By comparing the reactivity of alloantiserum with peptide-specific Abs, we have identified two forms of Qa-1^b that differ in their association with ß₂m and with other components of the maturation pathway.

	Materials and Methods

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Antibodies

Alloantiserum specific for Qa-1^b was produced by immunization of B6.Tla^a x A strain mice with cells from A.Tla^b. Anti-KSFQ (anti-Qa-1^b polyclonal) was produced by immunizing New Zealand White rabbits with the peptide KSFQKDAMLMF. Hybridoma cells producing monoclonal anti-K^b (34.2.12) and anti-K^k (16.3.1) were obtained from the American Type Culture Collection (Rockville, MD).

Cloning and expression of Qa-1^b/L^d

A genomic clone of H-2L^d carrying the regular H-2 promoter region was kindly provided by Dr. A. Mellor. The Qa-1^b/L^d construct was made by introducing the ₁ and ₂ domains of Qa-1^b into the L^d gene as described previously for the Qa-1^b/D^d construct (16). The construct was cloned into pBR327 and the linearized DNA was used to transfect L cells. Cells were grown in RPMI 1640 (Life Technologies, Grand Island, NY) containing 10% FCS, 50 IU/ml penicillin, 50 IU/ml streptomycin, and 2 mM glutamine (complete medium) at 37°C in 5% CO₂.

Biosynthetic labeling and immunoprecipitation

CBA mouse spleen cells were cultured for 40 to 50 h with 2 µg/ml of Con A in complete medium containing 1 mM 2-ME. For labeling, 2 x 10⁷ cells were washed three times at room temperature in RPMI medium lacking methionine and cysteine and containing no additives. A total of 200 µCi of Pro-Mix, [³⁵S](methionine + cysteine) (Amersham, Little Chalfont, U.K.) was added and cells were incubated for 10 min at 37°C. After labeling, cells were either placed on ice, or chased in complete medium for appropriate times.

Labeled cells were washed once in ice-cold isotonic TBS to remove radioisotopes, and lysed in 1 ml of PBS containing 1% Triton X-100 (Pierce, Chester, U.K.), proteinase inhibitors, and 10 mM iodoacetamide. After 10 min on ice, lysates were centrifuged at 20,000 x g in a Sigma (St. Louis, MO) refrigerated bench microfuge. Cleared lysates were then treated with protein A beads and unimmunized serum (mouse or rabbit) at 4°C overnight. Protein A beads preloaded with specific antiserum were rotated at 4°C with precleared lysates for 2 h, and then harvested at 100 x g. After extensive washing, immunoprecipitates were treated with SDS containing sample buffer at 95°C with or without 10 mM DTT. Samples were then subjected to electrophoresis on 12% polyacrylamide gels, treated with autoradiographic enhancers (Amplify, Amersham, U.K.), dried, and autoradiographed on Kodak BioMax MR film.

Surface labeling with ¹²⁵I

Cells, 2 x 10⁷, were washed thoroughly in cold PBS and suspended in 200 µl of PBS on ice. A total of 5 µl of 1 mg/ml of lactoperoxidase solution was added followed by 5 µl of a dilute hydrogen peroxide solution (30 vol diluted 1:10⁴). Labeling was started by adding 10 µl (1 mCi) of [¹²⁵I]Na solution (Amersham) and removing the tube from the ice. The reaction was continued for 30 min, adding 5 µl of dilute hydrogen peroxide solution at 10-min intervals. Cells were then washed in PBS containing 1 mM sodium iodide and lysed. ¹²⁵I-labeled samples were visualized using Kodak BioMax MS film using an appropriate intensifying screen.

Pulse-chase analysis

Cells labeled with [³⁵S]methionine for 5 min were diluted to 7 ml with warm complete RPMI medium in a 10-ml polypropylene tube, and 1 ml was removed and placed immediately in ice. This chase suspension was maintained at 37°C with occasional mixing, removing samples at 30-min intervals. Lysis and immunoprecipitation were performed as described above, and then immunoprecipitates were treated with Endo (endoglycosidase) H (Boehringer Mannheim, Mannheim, Germany) according to the manufacturer’s instructions before gel electrophoresis.

Two-stage immunoprecipitation

First, immunoprecipitates were treated with 20 µl of 1.0% SDS at 95°C for 5 min with or without 1 mM DTT. Then, 1 ml of PBS containing 1% Triton X-100 (Pierce) was added and the beads removed. The supernatant containing the released Ag was then subjected to a second round of immunoprecipitation using protein A beads preloaded with Ab. Samples were then subjected to gel electrophoresis under reducing conditions.

Fractionation of metabolically labeled lysates by size exclusion chromatography

CBA spleen cell blasts were metabolically labeled for 20 min as described above and lysed in PBS containing proteinase inhibitors and 1% digitonin. Lysates were centrifuged at 20,000 x g for 15 min and precleared with unimmunized serum. Precleared lysates were loaded onto a Superose 6 size exclusion gel filtration column (Pharmacia, St. Albans, U.K.) equilibrated with PBS containing 1% digitonin and run at a flow rate of 0.25 ml/min. The column was calibrated using gel filtration protein standards (Bio-Rad, Hemel Hempstead, U.K.), thyroglobulin (670 kDa), bovine gamma globulin (158 kDa), and chicken OVA (44 kDa). One-milliliter fractions were collected and each fraction subjected to immunoprecipitation with protein A beads preloaded with specific antiserum/Ab at 4°C for 4 h. After three washes in digitonin-containing PBS buffer, samples were analyzed by gel electrophoresis under reducing conditions.

	Results

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Two conformations of Qa-1^b

To visualize Qa-1^b molecules, Con A-activated CBA mouse spleen cells (H-2^k, Qa-1^b) were either metabolically labeled with [³⁵S]methionine or surface labeled with ¹²⁵I. Cell lysates were treated either with an anti-Qa-1^b alloantiserum or with a rabbit antiserum prepared against a Qa-1^b cytoplasmic peptide motif (anti-KSFQ) and protein A beads. Immunoprecipitates were subjected to gel electrophoresis under reducing conditions and the results shown in Figure 1, A and B. Both reagents detect Qa-1^b Hcs, but in very different amounts. This was not due to differences in Ab titer, since Ab was always present in excess. Interestingly, both reagents coprecipitated equal amounts of ß₂m. Similar results were obtained with metabolically labeled (Fig. 1A) and surface-labeled (Fig. 1B) cells. When lysates were extensively precleared with the alloantiserum, all of the ß₂m-associated Hcs were removed (Fig. 1C). However, Hcs devoid of ß₂m were still detectable using the anti-KSFQ Ab (Fig. 1C). To investigate whether anti-KSFQ Ab might cross-react with proteins other than Qa-1^b, it was tested on lysates of [³⁵S]methionine-labeled B10.BR (H-2^k, Qa-1^a) spleen cells. No 40- to 50-kDa material was immunoprecipitated from these lysates (data not shown), indicating that all of the labeled 40- to 50-kDa material detected with anti-KSFQ was associated with Qa-1^b. These results suggested that CBA lysates contain two serologically distinguishable forms of Qa-1^b Hcs. One is a stable Qa-1^b/ß₂m heterodimer recognized both by the alloantiserum and by anti-KSFQ. There are also Qa-1^b Hcs present that contain little or no ß₂m and are recognized only by anti-KSFQ. It also shows that a significant proportion of the Qa-1^b does not form a stable association with ß₂m.

View larger version (37K): [in this window] [in a new window]   FIGURE 1. Alloantiserum and anti-KSFQ define different forms of Qa-1b. A, [35S]methionine-labeled CBA lysates immunoprecipitated using alloantiserum or anti-KSFQ; B, 125I surface-labeled lysates treated similarly; and C, [35S]methionine-labeled lysates precleared three times sequentially with alloantiserum, then treated with anti-KSFQ. Hc denotes class I heavy chain.

To examine the intracellular trafficking of Qa-1^b molecules, CBA spleen cell blasts were labeled for 10 min with [³⁵S]methionine and chased for various times in medium containing excess unlabeled methionine. Precleared lysates were treated with anti-KSFQ and maturation of Qa-1^b monitored by treatment with the glycosidase Endo H to monitor the conversion of immature, high-mannose, carbohydrates to complex forms. As shown in Figure 2A, Endo H-sensitive Qa-1^b molecules have a half-life of about 60 min before becoming resistant. The half-life of immature carbohydrates on H-2K^k molecules in the same cells was less than 30 min (Fig. 2B). Samples from the pulse-chase analysis were also investigated by electrophoresis under nonreducing conditions. Without reduction of the samples, high m.w. forms of Qa-1^b were found immediately after the labeling pulse, and were increased in quantity during the chase period (Fig. 2C). Potential artifacts due to oxidation occurring in the lysate were minimized by including iodoacetamide in the lysis buffer. No high m.w. forms of H-2K^k molecules were found in the same lysate (data not shown). These results show that some Qa-1^b molecules form disulfide-linked complexes early in the maturation pathway.

View larger version (53K): [in this window] [in a new window]   FIGURE 2. Pulse-chase analysis of Qa-1b and H-2Kk molecules in CBA cells. A, Cells were labeled for 10 min with [35S]methionine, chased for the times indicated, and immunoprecipitated with anti-KSFQ. B, Lysates from A were immunoprecipitated with anti-H-2Kk. In A and B, samples were treated with Endo H before SDS-PAGE. Hc denotes class I heavy chain. EHR Hc denotes running position of Endo H-resistant Hcs. C, Samples from A were run unreduced. **, Denotes high molecular mass aggregates.

CBA spleen cells were surface-labeled and lysates were treated with either the alloantiserum or with anti-KSFQ. To reduce the complexity of the gel patterns, N-linked carbohydrate side chains were removed by treatment of immunoprecipitates with endoglycosidase F (Endo F). Endo F treatment of labeled material isolated using the alloantiserum reduced the size of Qa-1^b Hcs to a single band of 35 kDa, consistent with removal of the two N-linked carbohydrate chains. However, Endo F treatment of anti-KSFQ immunoprecipitates revealed the 35-kDa Hc band and an additional component of 37 kDa (Fig. 3A). The 37-kDa component was not eliminated by extended treatment with excess Endo F and is not, therefore, a consequence of incomplete carbohydrate removal. To determine whether the 37-kDa polypeptide was synthesized internally, cells were labeled with [³⁵S]methionine for 1 h and a similar immunoprecipitation conducted. Very little 37-kDa material was detectable after a 1-h label, but if cells were labeled continuously for 5 h with [³⁵S]methionine, 37-kDa material was clearly detectable using anti-KSFQ (Fig. 3B). This indicates that the 37-kDa Qa-1^b-associated polypeptide is indeed synthesized by the cells but either has a slow turnover rate or a large intracellular pool size. Since the fully glycosylated cell surface form of the 37-kDa polypeptide is approximately 44 kDa, it has been called gp44.

View larger version (31K): [in this window] [in a new window]   FIGURE 3. Coisolation of Qa-1b with gp44 using conformation-independent Abs. A, Immunoprecipitates of 125I-labeled Qa-1b molecules treated with Endo F. B, CBA cells were [35S]methionine labeled for 1 h (lane 2) or 5 h (lanes 1 and 3) and immunoprecipitated with anti-KSFQ. Immunoprecipitates in lanes 2 and 3 were treated with Endo F. C, Immunoprecipitates of 125I-labeled Qa-1b/Ld molecules not treated with Endo F. Hc denotes class I heavy chain. Hc(EF) denotes Endo F-treated Hc. gp44(EF) denotes Endo F-treated gp44.

To investigate further the nature of the association between Qa-1^b and gp44, a two-stage immunoprecipitation experiment was performed. Lysates of surface-labeled cells were treated with anti-KSFQ and immunoprecipitates washed as before. Labeled polypeptides were then released from the immunosorbent with a hot SDS solution, fresh lysis buffer added, and the eluted material treated a second time with anti-KSFQ on protein A beads. Finally, the samples were treated with Endo F and subjected to gel electrophoresis. As shown in Figure 4, when immunosorbents from the first immunoprecipitation were treated with SDS alone, Qa-1^b and gp44 remain associated in the second round (Fig. 4, lane 1). If, however, the reducing agent DTT is included in the first SDS retrieval step, gp44 does not reappear in the second immunoprecipitate (Fig. 4, lane 2). This indicates that Qa-1^b and gp44 are disulfide linked. It also shows that anti-KSFQ does not recognize gp44 directly but does so only by virtue of its association with Qa-1^b.

View larger version (86K): [in this window] [in a new window]   FIGURE 4. Qa-1b and gp44 are covalently linked. 125I-labeled CBA lysates were immunoprecipitated with alloantiserum (lane 3) or anti-KSFQ (lane 4). Alternatively, anti-KSFQ immunoprecipitates were treated with either SDS alone (lane 1) or SDS and DTT (lane 2) and subjected to a second anti-KSFQ immunoprecipitation step as described in Materials and Methods. All samples were Endo F treated. Hc denotes class I heavy chain.

It has been previously reported that T cells can recognize Qa-1^b in a TAP-dependent or a TAP-independent way (14). This implies that Qa-1^b molecules can reach the cell surface in the absence of TAP molecules. To test this, activated spleen cells from TAP1-deficient mice (H-2^b, Qa-1^b) were surface labeled and lysates prepared as before. Using anti-KSFQ, Qa-1^b was easily detectable by immunoprecipitation (Fig. 5A, lane 2). Surface forms were also detected using the alloantiserum (data not shown). In contrast, only low levels of H-2K^b were found in the same lysates (Fig. 5A, lane 4). Pulse-chase analysis showed that a significant proportion of Qa-1^b molecules are processed to Endo H-resistant forms in TAP1-deficient cells, indicating that intracellular transport of Qa-1^b occurs in the absence of TAP function (Fig. 5B).

View larger version (63K): [in this window] [in a new window]   FIGURE 5. Expression of Qa-1b in TAP1-deficient mice. A, Surface expression of Qa-1b and H-2Kb in 125I-labeled B10 and TAP1-deficient mice. B, Pulse-chase analysis of Qa-1b in TAP1-deficient mice. Lysates were immunoprecipitated with anti-KSFQ and samples were Endo H treated. Hc denotes class I heavy chain. EHR Hc denotes running position of Endo H-resistant Hcs.

It has been shown recently that class I molecules associate transiently in the ER with a protein complex consisting of the molecular chaperons calnexin and calreticulin, another molecule called tapasin, and the TAP1/TAP2 heterodimer, as a prelude to peptide loading (17). To investigate a possible physical association between Qa-1^b and the TAP machinery, lysates of metabolically labeled CBA blasts were fractionated by size-exclusion chromatography in digitonin-containing buffer to preserve weak molecular associations. Fractions were then treated sequentially with anti-KSFQ and 16.3.1 to detect Qa-1^b and H-2K^k, respectively. The results are shown in Figure 6. Using this methodology, we were able to detect class I molecules associated with different molecular mass ranges. Some H-2K^k was found in complexes with molecular masses above the 670-kDa marker, possibly representing molecules associated with the TAP machinery (Fig. 6A). H-2K^k molecules were also found in complexes with molecular masses below the 158-kDa standard marker, probably representing assembled heterotrimeric complexes of Hc, ß₂m, and peptide. In contrast, Qa-1^b was found in complexes with molecular masses in the range of 100- to 300-kDa only, and not in the high molecular mass complexes that contain H-2K^k (Fig. 6B). The presence of material in the 300- to 400-kDa range in Figure 6B indicates some variability in the composition of Qa-1^b-containing complexes. In a separate experiment, anti-TAP1 immunoprecipitates, in digitonin-containing buffer, of metabolically labeled CBA lysates were eluted and reprecipitated with anti-KSFQ. No Qa-1^b molecules were found (data not shown), consistent with their failure to strongly associate with TAPs.

View larger version (75K): [in this window] [in a new window]   FIGURE 6. Gel filtration analysis of Qa-1b- and H-2Kk-containing complexes. Fractions 7 to 18 were immunoprecipitated using anti-H-2Kk (A) or anti-KSFQ (B) as described in Materials and Methods. Running positions of size markers are shown above each panel. Hc denotes class I heavy chain.

Our results show that ß₂m is not associated with a large proportion of Qa-1^b Hcs in CBA cells. This suggested that Qa-1^b expression might be independent of ß₂m. We therefore examined the maturation and cell surface expression of Qa-1^b in ß₂m-deficient mice by pulse-chase analysis. We were unable to detect any Endo H-resistant forms of Qa-1^b in ß₂m-deficient mice, and Endo H-sensitive Hcs disappear during the chase period (Fig. 7). Furthermore, no cell surface Qa-1^b was detectable by surface labeling (data not shown). These results show that, without ß₂m, Qa-1^b is rapidly degraded. They also show that, once the Hcs have undergone their initial folding step, ß₂m is no longer essential for subsequent maturation and stable cell surface expression.

View larger version (101K): [in this window] [in a new window]   FIGURE 7. Pulse-chase analysis of Qa-1b in cells from ß2m-deficient mice. [35S]methionine-labeled cells were chased for the indicated times and immunoprecipitated with anti-KSFQ. Samples were Endo H treated. Hc denotes class I heavy chain. EHR Hc denotes running position of Endo H-resistant Hcs.

	Discussion

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An interesting and special feature of Qa-1^b is its ability to bind the nonamer peptide, AMAPRTLLL, derived from the leader sequences of H-2D and H-2L class I molecules, in preference to other peptides (11). Recently, peptide elution studies show AMAPRTLLL indeed to be the major peptide bound, with very few others detected (16, 22). In this study, we have shown that, as expected, association of Qa-1^b with ß₂m is poor in CBA (H-2^k) mice that lack AMAPRTLLL. We were, however, surprised to find that, despite poor ß₂m association, Qa-1^b molecules do not undergo rapid proteolysis, but rather traffic normally and are stably expressed on the cell surface. These cell surface Qa-1^b molecules, which lack ß₂m and did not react with our conformation-dependent alloantiserum, were detected with the conformation-independent peptide-specific antiserum (anti-KSFQ). This suggests that, if AMAPRTLLL is not present, Qa-1^b adopts an alternative stable conformation.

It is widely accepted that trafficking and maturation of class I Hcs involves the recruitment of several accessory molecules. One of these is calnexin, an ER resident lectin-like chaperone, to which nascent Hcs bind cotranslationally and remain bound while they undergo correct folding upon acquisition of ß₂m. On subsequent binding to the TAP machinery and on binding peptides they are then released and bound for the cell surface. It should be noted that recent studies have shown that, in calnexin-deficient cells, rates of class I Hc maturation were not affected, suggesting that other ER molecular chaperons can perform the same job (23, 24). One possible explanation for the weak association between Qa-1^b and ß₂m observed here is that Hcs are capable of folding correctly independently of ß₂m. This is not clearly the case, since Qa-1^b Hcs are rapidly degraded in ß₂m-deficient mice, and cell surface labeling experiments showed them to be absent. Therefore, Qa-1^b molecules appear to share the same ß₂m-dependent initial folding steps as other class I molecules, but subsequent ß₂m association appears not to be essential for their trafficking and maturation. An exception to the ß₂m-dependence rule is H-2D^b, which is expressed on the surface of cells from ß₂m-deficient cells, although at low levels (18).

Previous studies have shown that, in cells defective in TAP expression and function, the rate of intracellular transport of class I molecules is reduced, possibly because calnexin remains associated for longer with Hc-ß₂m heterodimers (25). Our pulse-chase experiments show that Qa-1^b becomes Endo H resistant 60 to 90 min after synthesis, much slower than H-2K^k molecules, which become resistant within 30 min. Similar rates of maturation and cell surface expression of Qa-1^b were observed in both normal and TAP1-deficient mice. Cell surface expression of most class I molecules in TAP-deficient cells is generally low, since Hc-ß₂m dimers without peptide are unstable. However, Qa-1^b molecules are abundant and stable on the cell surface in TAP1-deficient mice. In this respect, Qa-1^b resembles the class Ib molecules TL and CD1 (7, 8). Earlier work has shown that CTL recognition of Qa-1^b is not dependent on TAPs (14, 26). Paradoxically, many Qa-1-specific CTL recognize their targets in a TAP-dependent manner, possibly reflecting a requirement for the transporter in delivering peptide epitopes into the lumen of the ER. Our results, therefore, clearly distinguish the dual roles of TAP molecules in the assembly of class I molecules and in the delivery of peptides.

It is not unusual for free class I Hcs to form either homodimers or associate with additional proteins. Indeed, this may be a way to stabilize free Hcs that would otherwise be targeted for degradation. Wolf and Cook reported that Qa-1^b Hcs associate with free H-2L^d Hc and with another unidentified molecule, Qsm, and in doing so exclude ß₂m (27). In their experiments, no additional molecules were detected in H-2^k strains. In contrast, our results demonstrate another polypeptide, gp44, which associates with Qa-1^b in H-2^k and H-2^b strains. Using biosynthetic labeling, gp44 labels poorly, and is therefore difficult to detect. Also, if carbohydrates are not removed, gp44 was difficult to distinguish from Qa-1^b on SDS-PAGE due to the closeness of their polypeptide chain sizes. It was, in contrast, easy to distinguish Qa-1^b/L^d Hc from gp44 since the extra carbohydrate on the chimeric class I Hc reduces substantially its SDS-PAGE mobility (Fig. 3C). This experiment also demonstrates that the ₃ domain of Qa-1^b is unimportant for gp44 association.

Evidence that gp44 is a novel molecule comes from the following observations. First, gp44 associates with Qa-1^b in mice deficient in the class II-associated invariant chain (data not shown). Second, other known accessory molecules in the TAP pathway, e.g., calreticulin (28), ectocalreticulin (28), and tapasin (29, 30) have higher molecular masses: 52 kDa, 62 kDa, and 47.5 kDa, respectively. And third, preliminary N-terminal sequence analysis of gp44 appears to exclude other known class I Hcs (H. Djaballah and P. J. Robinson, unpublished observations). It is not yet clear whether gp44 is membrane bound; further characterization is under way.

All class I molecules have four conserved cysteine residues that form two intrachain disulfide bonds, Cys₁₀₁-Cys₁₆₄ and Cys₂₀₃-Cys₂₅₉. The additional cysteine residue at position 114 of Qa-1^b is located on one of the ß strands at the base of the peptide-binding cleft, making it solvent accessible and able to form a disulfide bridge with free cysteine residues on gp44 (Fig. 8). A cysteine residue at this position of class I Hcs is unusual. Although some mouse class I molecules have a free cysteine residue at position 121, this is less likely to form disulfide bridges because of poor solvent accessibility. However, this may account for the formation of disulfide-linked homodimers observed previously (31). Cysteine residues are also present in the cytoplasmic domains of several class I Hcs, including Qa-1^b. However, the reducing environment of the cytoplasm effectively rules out formation of disulfide bridges in vivo involving cytoplasmic cysteine residues. In addition, we find no evidence for dimers of Qa-1^b, but detect larger complexes that form within minutes of Hc synthesis and contain gp44. One might predict that formation of such complexes may not allow a good contact between the Hcs and the TAP molecular complex. Our gel filtration studies showing that Qa-1^b and H-2K^k are present in different molecular mass fractions is consistent with this hypothesis.

View larger version (43K): [in this window] [in a new window]   FIGURE 8. Molecular images of the peptide-combining site of Qa-1b highlighting the position of Cys114. The main part of the figure shows a cartoon view of the 2 domain of the model of Qa-1b (this region is indicated by the gray shading in the inset, where the molecule is seen from the top). The positions of the serine residues at positions 143 and 147, which are well conserved as threonine and tryptophan in other class I molecules, are shown, as is the conserved disulfide bond between Cys101 and Cys164. A free cysteine residue is present at position 114, on the second ß strand of the 2 domain and points up into the peptide binding groove. The figure was produced using MOLSCRIPT (32).

View larger version (15K): [in this window] [in a new window]   FIGURE 9. Proposed scheme for Qa-1b maturation and possible origins of the two cell surface forms.

	Acknowledgments

 
We thank Drs. O. Smithies and M. Merkenschlager for ß₂m-deficient mice; Drs. D. Kioussis, O. Williams, and S. Tonegawa for TAP1-deficient mice; Drs. C. Benoist and D. Mathis for Ii-deficient mice; and John Trowsdale for anti-TAP1 Abs. We also thank Drs. Jim Kaufman and Danny Altman for critically reading the manuscript.

	Footnotes

 
¹ H.D. is supported by the Leukaemia Research Fund.

² Address correspondence and reprint requests to Dr. Peter J. Robinson, Transplantation Biology Group, MRC-CSC, Imperial College School of Medicine, Hammersmith Hospital, Du Cane Road, London W12 ONN, U.K. E-mail address:

³ Abbreviations used in this paper: ER, endoplasmic reticulum; Hc, heavy chain; ß₂m, ß₂-microglobulin; Endo, endoglycosidase.

Received for publication September 29, 1997. Accepted for publication December 5, 1997.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

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