THE STRUCTURE OF AN INTERMEDIATE IN CLASS-II MHC MATURATION - CLIP BOUND TO HLA-DR3

A complex between HLA-DR3 and a fragment of invariant chain called CLI P was isolated from a human cell line defective in antigen presentatio n and its X-ray crystal structure determined. Previous data indicate t hat this complex is an intermediate in class II histocompatibility mat uration, occurring between invariant chain-DR3 and antigenic peptide-D R3 complexes. The structure shows that the CLIP fragment binds to DR3 in a way almost identical to that in which antigenic peptides bind cla ss II histocompatibility glycoproteins. The structure is the substrate for the loading of antigenic peptides by an exchange process catalyse d by DM.