Biochemical characterization of an invariant polypeptide associated with Ia antigens in human and mouse.

Ii, a 31,000 mol. wt polypeptide chain associated with murine and human Ia antigens was investigated for its labeling pattern, carbohydrate content and structural polymorphism. Two-dimensional gel electrophoretic analysis of tunicamycin treated cells from mouse and human lymphocytes shows that Ii contains two N-linked carbohydrate chains. Ii is a methionine rich polypeptide. Tryptic and chymotryptic two dimensional peptide maps of Ii chain associated with I-A and I-E subregion products are identical. This absence of polymorphism holds true when Ii chain is isolated from different mouse haplotypes. Human Ii chains from different HLA-DR types appear also invariant by peptide map analysis. By molecular weight, carbohydrate content, charge and tryptic and chymotryptic maps criteria, Ii of mouse and human are strikingly homologous.