Solution structure by NMR of circulin A: A macrocyclic knotted peptide having anti-HIV activity

The three-dimensional solution structure of circulin A, a 30 residue poly p eptide from the African plant Chassalia parvifolia, has been determined usi ng two-dimensional H-1-NMR spectroscopy. Circulin A was originally identifi ed based upon its inhibition of the cytopathic effects and replication of t he human immunodeficiency virus. Structural restraints consisting of 369 in terproton distances inferred from nuclear Overhauser effects, and 21 backbo ne dihedral and nine chi(1) angle restraints from spin-spin coupling consta nts were used as input for simulated annealing calculations and energy mini misation in the program X-PLOR. The final set of 12 structures had mean pai rwise rms differences over the whole molecule of 0.91 Angstrom for the back bone atom, and 1.68 Angstrom for all heavy atoms. For the well-defined regi on encompassing residues 2-12 and 18-27, the corresponding values were 0.71 and 1.66 Angstrom, respectively. Circulin A adopts a compact structure con sisting of beta-turns and a distorted segment of triple-stranded beta-sheet . Fluorescence spectroscopy provided additional evidence for a solvent-expo sed Trp residue. The molecule is stabilised by three disulfide bonds, two o f which form an embedded loop completed by the backbone fragments connectin g the cysteine residues. A third disulfide bond threads through the centre of this loop to form a "cystine-knot" motif. This motif is present in a ran ge of other biologically active proteins, including omega-contoxin GVIA and Cucurbita maxima trypsin inhibitor. Circulin A belongs to a novel class of macrocyclic peptides which have been isolated from plants in the Rubiaceae family. The global fold of circulin A is similar to kalata B1, the only me mber of this class for which a structure has previously been determined. (C ) 1999 Academic Press.