Ia antigens contain two distinct forms of beta chain.

Two new forms of beta chain, beta 1, and beta 2, in I-A immunoprecipitates are described, which differ in their migration values in SDS-PAGE under nonreducing conditions, but which migrate identically in a reduced form. This behavior is very likely due to a different arrangement of intramolecular disulfide bonds which may influence mobility in SDS-PAGE. Peptide map analysis confirmed that beta 1 and beta 2 possess identical primary polypeptide structures. These two forms of beta chain are also expressed on the cell surface and its is suggested that both associate with alpha chains. The structural differences in these complexes may lead to an increase in heterogeneity of Ia antigens which could be of importance for T-cell recognition.