H-1 NMR study of the solution structure of Ac-AMP2, a sugar binding antimicrobial protein isolated from Amaranthus caudatus

The conformation in water of antimicrobial protein 2 from Amaranthus caudatus (Ac-AMP2) was determined using H-1 NMR, DIANA and restrained molecular modeling. Ac-AMP2 is a 30 amino acid residue, lectin-like protein that specifically binds to chitin, a polymer of beta-1,4-N-acetyl-D-glucosamine. After sequence specific resonance assignments, a total of 198 distance restraints were collected from 2D NOESY buildup spectra at 500 MHz at pH 2, supplemented by a 2D NOESY spectrum at 600 MHz. The location of the three previously unassigned disulfide bridges was determined from preliminary DIANA structures, using a statistical analysis of intercystinyl distances. The solution structure of Ac-AMP2 is presented as a set of 26 DIANA structures, further refined by restrained molecular dynamics using a simulated annealing protocol in the AMBER force field, with a backbone r.m.s.d. for the well defined Glu3-Cys28 segment of 0.69(+/-0.12) Angstrom. The main structural element is an antiparallel beta-sheet from Met13 to Lys23 including a beta(I)-turn over Gln17-Phe18 with a beta bulge at Gly19. Ln addition, a beta'(I) turn over Arg6-Gly7, a beta'(III) turn over Ser11-Gly12 and a helical turn from Gly24 to Cys28 are identified. This structure is very similar to the equivalent regions of the X-ray structure of wheat germ agglutinin and the NMR structure of hevein. (C) 1996 Academic Press Limited