A POTENT ANTIMICROBIAL PROTEIN FROM ONION SEEDS SHOWING SEQUENCE HOMOLOGY TO PLANT LIPID TRANSFER PROTEINS

An antimicrobial protein of about 10 kD, called Ace-AMP1, was isolated from onion (Allium cepa L.) seeds. Based on the near-complete amino a cid sequence of this protein, oligonucleotides were designed for polym erase chain reaction-based cloning of the corresponding cDNA. The matu re protein is homologous to plant nonspecific lipid transfer proteins (nsLTPs), but it shares only 76% of the residues that are conserved am ong all known plant nsLTPs and is unusually rich in arginine. Ace-AMP1 inhibits all 12 tested plant pathogenic fungi at concentrations below 10 mu g mL(-1). Its antifungal activity is either not at all or is we akly affected by the presence of different cations at concentrations a pproximating physiological ionic strength conditions. Ace-AMP1 is also active on two Gram-positive bacteria but is apparently not toxic for Cram-negative bacteria and cultured human cells. In contrast to nsLTPs such as those isolated from radish or maize seeds, Ace-AMP1 was unabl e to transfer phospholipids from liposomes to mitochondria. On the oth er hand, lipid transfer proteins from wheat and maize seeds showed lit tle or no antimicrobial activity, whereas the radish lipid transfer pr otein displayed antifungal activity only in media with low cation conc entrations. The relevance of these findings with regard to the functio n of nsLTPs is discussed.