Antibacterial peptides from plants: what they are and how they probably work

Abstract

Plant antibacterial peptides have been isolated from a wide variety of species. They consist of several protein groups with different features, such as the overall charge of the molecule, the content of disulphide bonds, and structural stability under environmental stress. Although the three-dimensional structures of several classes of plant peptides are well determined, the mechanism of action of some of these molecules is still not well defined. However, further studies may provide new evidences for their function on bacterial cell wall. Therefore, this paper focuses on plant peptides that show activity against plant-pathogenic and human-pathogenic bacteria. Furthermore, we describe the folding of several peptides and similarities among their three-dimensional structures. Some hypotheses for their mechanisms of action and attack on the bacterial membrane surface are also proposed.

Figure 1

(a) Alignment of antibacterial peptides with defined three-dimensional structure: circulin A (1BH4); circulin B (2ERI); kalata B2 (1PT4); VrD1 (1BK8); VrD2 (2GL1); Ac-AMP2 (1MMC); brazzein (1BRZ). (b) Alignment of several antibacterial peptides from plant sources with sequences available at the Protein Data Bank. Hevein-like: Ac-AMP1 (AAB22103.1); knottin peptides: Mc-AMP1 (081338.1), Pa-AMP1 (P81418.1), Mj-AMP1 (P25403.4), Mj-AMP2 (P25404.2); snakins: snakin1 (AAD01518.1), snakin2 (ABL74292.1); defensins and thionins: pp-Thionin (P07504.1), Pth-St1 (AAB31351.1), and Cp-thionin (P84920.1); cyclotide: cyclopsychotride A (P56872.2); other peptides: MBP-1 (AAB23306.1). Asterisks show conserved cysteine residues. Sequences in bold represent peptides with cyclic three-dimensional conformations, indicating that they do not have N- and C-termini. Therefore, the alignment of cyclotides was based through comparison with the N- and C-termini of the other peptide groups.

Figure 2

Three-dimensional structure of cyclotides (a) circulin A (PDB: 1BH4); (b) circulin B (PDB: 2ERI); (c) kalata B2 (PDB: 1PT4); defensins (d) VrD1 (PDB: 1TI5); (e) VrD2 (PDB: 2GL1); (f) Ah-AMP1 (PDB: 1BK8); (g) brazzein (1BRZ); and hevein-like (h) Ac-AMP2 (1MMC).

Figure 3

Scheme of the action mechanism for antibacterial peptides. (a) Barrel-stave model; (i) peptides in monomer or oligomer form come close to the membrane target; (ii) positively-charged residues from the peptides interact with the head group of the phospholipids from the membrane; (iii) at a threshold concentration of peptides, the pores are formed. In toroidal model, the major difference is the type of pore formed, where lipids and peptides are overlapped. (b) Carpet model; (i) peptides in monomer or oligomers come close to the membrane target; (ii) hydrophilic regions of peptides are exposed to solvent and hydrophobic regions to membrane; (iii) at threshold concentration of peptides, the permeability of the membrane increases, facilitating pore formation; (iv) membrane disintegration. Adapted from Shai, 2002.