Structure of Ia antigens: identification of dimeric complexes formed by the invariant chain.

A 2-dimensional gel technique was used for the identification of disulfide-linked polypeptide complexes formed by Ia chains and their corresponding subunits. Several dimers containing the invariant (Ii) chain were found. The most prominent one consisted of 2 Ii chains. In addition, Ii molecules were found to be covalently linked to 3 unknown components of 41,000, 27,000, and 10,000 to 15,000 m.w. The dimeric form of Ii exists also after alkylation of free SH-groups with iodoacetamide, which suggests that formation of the Ii dimer is not the result of experimental conditions. Furthermore, noncovalently associated dimers formed by processed alpha- and beta-chains as well as by their precursors alpha p and beta p could be detected. No mixed associated chains composed of precursors and processor molecules were observed, which indicates that processing and insertion into the membrane take place only after dimerization.