Guillen-Chable Francisco A and Estrada Georgina
Structural Motifs in Class I and Class II Plant Defensins for Phospholipid Interactions:Intriguing Role of Ligand Binding and Modes of Action
Defensins are cationic antimicrobial peptides widespread in nature. Plants, as sessile organisms exposed to fungi and other pathogens have developed mechanisms to deal with infection. Plant defensins are considered infection responsive molecules, nevertheless, several abiotic factors are elicitors and a set of modes of action are reported. Phospholipid binding profiles are characteristic of both, class I and class II defensins. Comparing the amino acid composition of class I, MtDEF4 and class II, NaD1, structural differences or motifs of primary structure for phospholipid ligand are analyzed, as a result a Gly residue of the γ–core loop is proposed as feature of class I phospholipid binding defensins in this review. Because the two groups of defensins are present in Solanaceae plants, the synergy or overlaping function is a possibility. The N-ter and the γ–core loop define the 3D ligand binding site of phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) in NaD1 class II defensin, while the basic residues in the γ–core loop of the class I representative defensin MtDEF4; are essential for cell entry. Defensins of Capsicum highlight structural constrains for distinctive lipid binding profiles of class I and class II defensins. Primary and 3D structural motifs, particularly residues in positions 4 and 42 and the widely studied γ−core are involved in the interaction site for protein-PIP2/PA binding, while the hydrophobic surfaces must interact with additional components of membrane or cell wall constituents. The 3D binding pocket for phospholipid binding shows that class II members target head-group-phosphates of the PIP2 in the inositol ring, as demonstrated for NaD1, thus charged residues are involved in phosphate ligand coordination also in MtDEF4. Phylogenetic relationships with Medicago truncatula defensins denote the conserved class I peptide motifs in Solanaceae and Leguminoseae. This review summarizes the state of the art related to modes of action of sphigolipid vs. phospholipid binding plant defensin and unveil particular motifs of mature peptide class I and class II defensins in Solanaceae that have shown distinctive binding to phosphoinositides and phosphatidic acid.
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