Maturation of Qa-1b class I molecules requires β2-microglobulin but ts TAP independent

Two conformationally distinct and stable forms of Qa-1(b), one strongly associated with beta(2)-microglobulin (beta(2)m) and the other associated with a navel 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 beta(2)m. 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 beta(2)m, there was no maturation of either form in cells from beta(2)m-deficient mice where heavy chains were rapidly degraded. These results suggest that Qa-1(b), like most other class I molecules, requires beta(2)m for an initial folding step. However, beta(2)m is not essential for subsequent processing of Qa-1(b) molecules.