I-Ak exclusively binds extended HEL(48-61) peptides through
one single register. (A) A Lys52 P1 anchor inhibited the
binding of HEL(48-61), blocking register 1. Prevention of register 1
binding with Lys52 substitutions inhibited the binding of the extended
HEL(48-61) peptides containing three additional putative P1 anchors.
(B) The register 1-specific antibody, Aw3.18, was used
to immunoprecipitate I-Ak complexes individually formed
with radiolabeled HEL(40-61), HEL(44-61), or HEL(48-61) peptides.
Despite the presence of four potential binding registers, the Aw3.18
antibody equally immunoprecipitated I-Ak complexes with the
extended HEL peptides. Consequently, I-Ak bound the
extended HEL(40-61) and HEL(44-61) peptides through the same single
register as HEL(48-61).