Immunological significances of invariant chain from the aspect of its structural homology with the cystatin family

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Abstract
The primary structure of p31 of invariant chain (Ii‐chain) shows about 50% homology with those of the cystatin family which are endogenous cysteine protease inhibitors. The binding domains between Ii‐chain and HLA‐DR‐7 were estimated from the structural homology between cystatin and Ii‐chain and also between cathepsins and DR‐7, respectively. The QL64–71 and GS76–88 of Ii‐Chain were estimated to be the binding domains with GG45–51, and VS57–63 of HLA‐DR7, respectively. The purified human Ii‐chain from spleen is capable of forming four molecular forms from monomer to tetramer by redox‐potential dependent disulfide bond formation. The Ii‐chain inhibits cathepsin L and H competitively as a dimer and the Ki value for cathepsin L was 4.1 × 10−8 M, but cathepsin B was not inhibited at all. The Ii‐chain showed mainly a dimer (60 kDa) under the assay condition of cathepsins with cysteine and was not degraded by these cathepsins. The Ii‐chain may play an important role in the regulation of antigenic peptide presentation to MHC class II.