Mapping of the Binding Frame for the Chaperone SecB within a Natural Ligand, Galactose-binding Protein (∗)

The chaperone SecB selectively binds polypeptides that are in a non-native state; however, the details of the interaction between SecB and its ligands are unknown. As a step in elucidation of the molecular mechanism of binding, we have mapped the region of a physiologic ligand (galactose-binding protein) that is in contact with SecB. The binding frame comprises ˜160 aminoacyl residues and is located in the central portion of the primary sequence. Comparison to the binding frame within maltose-binding protein, which is similarly long and positioned around the center of that polypeptide, reveals no similarity in sequence or in folding motif. The results are consistent with the proposal that the selectivity in binding exhibited by SecB is based on the simultaneous occupancy of multiple binding sites, each of which demonstrates low specificity, by flexible stretches of polypeptide that are only accessible in non-native proteins.