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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

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.

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