A STRUCTURAL MODEL FOR GROEL-POLYPEPTIDE RECOGNITION

A monomeric peptide fragment of GroEL, consisting of residues 191-376, is a mini-chaperone with a functional chaperoning activity. We have s olved the crystal structure at 1.7 Angstrom resolution of GroEL(191-37 6) with a 17-residue N-terminal tag. The N-terminal tag of one molecul e binds in the active site of a neighboring molecule in the crystal. T his appears to mimic the binding of a peptide substrate molecule. Seve n substrate residues are bound in a relatively extended conformation. Interactions between the substrate and the active site are predominant ly hydrophobic, but there are also four hydrogen bonds between the mai n chain of the substrate and side chains of the active site. Although the preferred conformation of a bound substrate is essentially extende d, the flexibility of the active site may allow it to accommodate the binding of exposed hydrophobic surfaces in general, such as molten glo bule-type structures. GroEL can therefore help unfold proteins by bind ing to a hydrophobic region and exert a binding pressure toward the fu lly unfolded state, thus acting as an ''unfoldase.'' The structure of the mini-chaperone is very similar to that of residues 191-376 in inta ct GroEL, so we can build it into GroEL and reconstruct how a peptide can bind to the tetradecamer. A ring of connected binding sites is not ed that can explain many aspects of substrate binding and activity.