THE SOLUTION STRUCTURE OF THE SYNTHETIC CIRCULAR PEPTIDE CGVSRQGKPYC NMR-STUDIES OF THE FOLDING OF A SYNTHETIC MODEL FOR THE DNA-BINDING LOOP OF THE SSDNA-BINDING PROTEIN ENCODED BY GENE-V OF PHAGE M13

The cyclic disulfide peptide CGVSRQGKPYC was prepared to obtain a cons trained analogue of residues 17-27 of the DNA-binding loop of the gene -V-encoded ssDNA-binding protein of filamentous bacteriophage M13. Ami no acid sequences very similar to that of this beta-loop have been fou nd in various phage-encoded ssDNA-binding proteins, and it has been pr oposed that such a loop may occur as a common motif in this class of p roteins. The conformation, in aqueous solution, of the synthetic gene- V-protein binding-loop analogue has been investigated by means of two- dimensional-H-1-NMR techniques. Subsequent structure calculations show that the molecule forms a beta-loop that includes a turn formed by th ree residues. This structure. very unusually for a cyclic disulfide pe ptide, is highly similar to that of the analogous part of the binding loop of the native protein. Comparison with experiments on other cycli c disulfide peptides indicates that the formation, of the beta-sheet ( beta-hairpin) secondary structure is essentially governed by the amino acid composition of the 11-residue sequence. The disulfide bridge in the 11-residue sequence is essential for conformational stability, as indicated by the finding that the open peptide analogue that encompass es residues Ser17-Ser27 does not adapt a detectable secondary structur e in water. The bridge replaces the role of the loop formed by residue s 49-58 in the protein, which act as a scaffold to hold the N-terminal and C-terminal ends of the DNA-binding loop together.