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-na tive state; however, the details of the interaction between SecB and i ts ligands are unknown. As a step in elucidation of the molecular mech anism of binding, we have mapped the region of a physiologic ligand (g alactose-binding protein) that is in contact with SecB. The binding fr ame comprises similar to 160 amino-acyl residues and is located in the central portion of the primary sequence. Comparison to the binding fr ame within maltose-binding protein, which is similarly long and positi oned around the center of that polypeptide, reveals no similarity in s equence or in folding motif. The results are consistent with the propo sal that the selectivity in binding exhibited by SecB is based on the simultaneous occupancy of multiple binding sites, each of which demons trates low specificity, by flexible stretches of polypeptide that are only accessible in nonnative proteins.