SOLUTION STRUCTURE OF ACE-AMP1, A POTENT ANTIMICROBIAL PROTEIN EXTRACTED FROM ONION SEEDS - STRUCTURAL ANALOGIES WITH PLANT NONSPECIFIC LIPID TRANSFER PROTEINS

The three-dimensional solution structure of Ace-AMP1, an antifungal pr otein extracted from onion seeds, was determined using H-1 NMR spectro scopy and molecular modeling, This cationic protein contains 93 amino acid residues and four disulfide bridges, Its structure was determined from 1260 NOE-derived distance restraints and 173 dihedral restraints derived from NOEs and (3)J(C2HNH) coupling constants. The global fold involves four helical segments connected by three loops and a C-termi nal tail without regular secondary structures, except for a 3(10)-heli x turn and a beta-turn. The most striking feature is the absence of an y continuous cavity running through the whole molecule as found in rec ently determined structures of nonspecific transfer proteins extracted from wheal and maize seeds, although their global folds are very simi lar. Consistent with the absence of a cavity in the core of Ace-AMP1, it was found that this protein, in contrast to ns-LTPs, does not bind fluorescently labeled phospholipids in solution. On the other hand, Ac e-AMP1 is able to interact with phospholipid membranes as shown by the release of carboxyfluorescein from the lumen of artificial liposomes and by the induction of alterations in fluorescence polarization of fl uorescently labeled phospholipids embedded in artificial liposomes.