Structural Comparison of Murine Ia Antigens Determined by the I-A and I-E Subregions

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Abstract
After nonionic detergent solubilization, the Ia molecules determined by the I-A subregion are distributed between a covalently linked “58K” structure composed of one α chain (35,000 daltons) and one β chain (25,000 daltons) separable by treatment with SDS and mercaptoethanol, and a form in which α and β chains are separable by treatment with SDS alone (α, β form). In contrast, the Ia molecules determined by the I-E subregion have α and β chains that are not covalently linked, and can be entirely separated by SDS-PAGE under rigorous (100°C) dissociating conditions without reducing agent. Milder dissociating conditions (37°C) permitted observation of a noncovalently linked 58K form for I-E molecules. Therefore, in their native state, both I-A and I-E molecules probably are composed of associated α and β chains. Iodoacetamide treatment of intact cells before detergent solubilization decreased the fraction of I-A molecules in the covalently linked 58K form. This finding probably means that the covalent linkage is an artifact of solubilization, but because it is restricted to I-A molecules, there still must be an underlying structural difference between I-A and I-E molecules. Selective destruction by iodoacetamide of the antigenic activity of the 58K form of I-A molecules could also account for the loss in 58K form, but only if there is an iodoacetamidesensitive antigenic and/or structural difference between I-A molecules in the 58K and α, β form. We compared these two forms by isoelectric focusing, but found no clear differences between them.