Class II-associated invariant chain peptide-independent binding of invariant chain to class II MHC molecules

The class II-associated invariant chain peptide (CLIP) region of invariant chain (li) is believed to play a critical role in the assembly and transpor t of MHC class II alpha beta Ii complexes through its interaction with the class II peptide-binding site. The role of the CLIP sequence was investigat ed by using mutant Ii molecules with altered affinity for the DR1 peptide-b inding site. Both high- and low-affinity mutants were observed to efficient ly assemble with DR1 and mediate transport to endosomal compartments in COS cell transfectants. Using N- and C-terminal truncations, a region adjacent to CLIP within Ii(103-118) was identified that can complement loss of affi nity for the peptide-binding site in mediating efficient assembly of alpha beta Ii. A C-terminal fragment completely lacking the CLIP region, Ii(103-2 16), was observed binding stably to class II molecules in immunoprecipitati on studies and experiments with purified proteins. The Ii(103-118) region w as required for this binding, which occurs through interactions outside of the alpha beta peptide-binding groove. We conclude that strong interactions involving Ii(103-118) and other regions of Ii cooperate in the assembly of functional alpha beta Ii under conditions where CLIP has little or no affi nity for the class II peptide-binding site. Our results support the hypothe sis that the CLIP sequence has evolved to avoid high-stability interactions with the peptide-binding sites of MHC class II molecules rather than as a promiscuous binder with moderate affinity for an class II molecules.