The Human “Invariant” Chain: Structural Characterization

Abstract

Initial structural studies of Ia antigens isolated from murine and human cell populations have led to the description of a bimolecular complex of 27000–29000 daltons (β chain) and 32000–35000 daltons (α chain). Two dimensional gel electrophoresis has revealed in both species a more complex pattern with the presence of a third major polypeptide; the invariant chain : Ii (1,2). This chain is a 31000 daltons and very basic glycoprotein which display very unique features. Highly glycosylated Ii is not expre- sed at the cell surface. Its peptide composition reveals an unusually high number of methionine residues. Although its expression parrallels the expression of the α and β chains it is not known if Ii is encoded within the Major Histo-compatibility Complex. We present in this paper additional information on the structure and expression of the human Ii chain.