Solution Structure of PAFP-S: A New Knottin-Type Antifungal Peptide from the Seeds of Phytolacca americana,

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Abstract
The three-dimensional solution structure of PAFP-S, an antifungal peptide extracted from the seeds of Phytolacca americana, was determined using 1H NMR spectroscopy. This cationic peptide contains 38 amino acid residues. Its structure was determined from 302 distance restraints and 36 dihedral restraints derived from NOEs and coupling constants. The peptide has six cysteines involved in three disulfide bonds. The previously unassigned parings have now been determined from NMR data. The solution structure of PAFP-S is presented as a set of 20 structures using ab initio dynamic simulated annealing, with an average RMS deviation of 1.68 Å for the backbone heavy atoms and 2.19 Å for all heavy atoms, respectively. For the well-defined triple-stranded β-sheet involving residues 8−10, 23−27, and 32−36, the corresponding values were 0.39 and 1.25 Å. The global fold involves a cystine-knotted three-stranded antiparallel β-sheet (residues 8−10, 23−27, 32−36), a flexible loop (residues 14−19), and four β-reverse turns (residues 4−8, 11−14, 19−22, 28−32). This structure features all the characteristics of the knottin fold. It is the first structural model of an antifungal peptide that adopts a knottin-type structure. PAFP-S has an extended hydrophobic surface comprised of residues Tyr23, Phe25, Ile27, Tyr32, and Val34. The side chains of these residues are well-defined in the NMR structure. Several hydrophilic and positively charged residues (Arg9, Arg38, and Lys36) surround the hydrophobic surface, giving PAFP-S an amphiphilic character which would be the main structural basis of its biological function.