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 (Ii) is believed to play a critical role in the assembly and transport of MHC class II alphabetaIi complexes through its interaction with the class II peptide-binding site. The role of the CLIP sequence was investigated by using mutant Ii molecules with altered affinity for the DR1 peptide-binding site. Both high- and low-affinity mutants were observed to efficiently 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 affinity for the peptide-binding site in mediating efficient assembly of alphabetaIi. A C-terminal fragment completely lacking the CLIP region, Ii(103-216), was observed binding stably to class II molecules in immunoprecipitation studies and experiments with purified proteins. The Ii(103-118) region was required for this binding, which occurs through interactions outside of the alphabeta peptide-binding groove. We conclude that strong interactions involving Ii(103-118) and other regions of Ii cooperate in the assembly of functional alphabetaIi under conditions where CLIP has little or no affinity for the class II peptide-binding site. Our results support the hypothesis 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 all class II molecules.
Thayer
W P
WP
Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
Ignatowicz
L
L
Weber
D A
DA
Jensen
P E
PE
D000595:Amino Acid Sequence; D000818:Animals; D000944:Antigens, Differentiation, B-Lymphocyte / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D001693:Biological Transport, Active; D019556:COS Cells; D017931:DNA Primers / Q000235:genetics; D015798:HLA-DR1 Antigen / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D051379:Mice; D009154:Mutation; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012995:Solubility