HIGH-RESOLUTION SOLUTION STRUCTURE OF SIAMYCIN-II - NOVEL AMPHIPATHICCHARACTER OF A 21-RESIDUE PEPTIDE THAT INHIBITS HIV FUSION

The 21-amino acid peptides siamycin II (BMY-29303) and siamycin I (BMY -29304), derived from Streptomyces strains AA3891 and AA6532, respecti vely, have been found to inhibit HIV-1 fusion and viral replication in cell culture. The primary sequence of siamycin II is CLGIGSCNDFAGCGYA IVCFW. Siamycin I differs by only one amino acid; it has a valine resi due at position 4. In both peptides, disulfide bonds link Cys(1) with Cys(13) and Cys(7) with Cys(19), and the side chain of Asp(9) forms an amide bond with the N-terminus. Siamycin II, when dissolved in a 50:5 0 mixture of DMSO and H2O, yields NOESY spectra with exceptional numbe rs of cross peaks for a peptide of this size. We have used 335 NOE dis tance constraints and 13 dihedral angle constraints to generate an ens emble of 30 siamycin II structures; these have average backbone atom a nd all heavy atom rmsd values to the mean coordinates of 0.24 and 0.52 Angstrom, respectively, The peptide displays an unusual wedge-shaped structure, with one face being predominantly hydrophobic and the other being predominantly hydrophilic. Chemical shift and NOE data show tha t the siamycin I structure is essentially identical to siamycin II. Th ese peptides may act by preventing oligomerization of the HIV transmem brane glycoprotein gp41, or by interfering with interactions between g p41 and the envelope glycoprotein gp120, the cell membrane or membrane -bound proteins [Frechet, D. et al. (1994) Biochemistry, 33, 42-50]. T he amphipathic nature of siamycin II and siamycin I suggests that a po lar (or apolar) site on the target protein may be masked by the apolar (or polar) face of the peptide upon peptide/protein complexation.