Identification of an Invariant Chain δ of Murine Class II Antigens and its Relationship to the Invariant Chain γ (Ii)

Abstract

Numerous investigations have permitted to establish the currently accepted model for murine class II antigen organization at the genetic and molecular level(1). Two subregions I-A and I-E were demonstrated to code for immunochemically detectable molecules which were shown to be arranged into two distinct polypeptide chain complexes. Both complexes are comprised of a heterodimer (Aα Aβ and Eα Ae) which is held together by tight noncovalent interactions (2,3). The heavy (α) and especially the light (β) chain of both complexes are structurally polymorphic. A third, invariant polypeptide γ (Ii) was found likewise in noncovalent association with the I-A and I-E dependent molecules (4). Recently Sung et al. demonstrated that Ia antigens situated at the plasma membrane consist of only two chains, α and β, as compared to the three- chain structure α,β,γ, localized primarily in intracellular membranes (5). Earlier we showed that I-A^k dependent light chains occur under physiological conditions in two forms termed β₁,β₂ which are stabilized by internal disulfide bonds (6,7). Here we report on findings suggesting that the molecular heterogeneity of I-A and I-E dependent polypeptide chain complexes is much greater than currently recognized.