Because the stress protein GRP94 can augment presentation of peptides to T cells, it is important to define how it, as well as all other HSP90 family members, binds peptides. Having previously shown that the N-terminal half of GRP94 can account for the peptide binding activity of the full-length protein, we now locate this binding site by testing predictions of a molecular docking model. The best predicted site was on the opposite face of the β sheet from the pan-HSP90 radicicol-binding pocket, in close proximity to a deep hydrophobic pocket. The peptide and radicicol-binding sites are distinct, as shown by the ability of a radicicol-refractive mutant to bind peptide. When the fluorophore acrylodan is attached to Cys(117)within the hydrophobic pocket, its fluorescence is reduced upon peptide binding, consistent with proximity of the two ligands. Substitution of His(125), which contacts the bound peptide, compromises peptide-binding activity. We conclude that peptide binds to the concave face of the β sheet of the N-terminal domain, where binding is regulated during the action cycle of the chaperone. \ud \u
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