Differential Antifungal and Calcium Channel-Blocking Activity among Structurally Related Plant Defensins  

Abstract

Plant defensins are a family of small Cys-rich antifungal proteins that play important roles in plant defense against invading fungi. Structures of several plant defensins share a Cys-stabilized a/b-motif. Structural determinants in plant defensins that govern their antifungal activity and the mechanisms by which they inhibit fungal growth remain unclear. Alfalfa (Medicago sativa) seed defensin, MsDef1, strongly inhibits the growth of Fusarium graminearum in vitro, and its antifungal activity is markedly reduced in the presence of Ca 21. By contrast, MtDef2 from Medicago truncatula, which shares 65% amino acid sequence identity with MsDef1, lacks antifungal activity against F. graminearum. Characterization of the in vitro antifungal activity of the chimeras containing portions of the MsDef1 and MtDef2 proteins shows that the major determinants of antifungal activity reside in the carboxy-terminal region (amino acids 31-45) of MsDef1. We further define the active site by demonstrating that the Arg at position 38 of MsDef1 is critical for its antifungal activity. Furthermore, we have found for the first time, to our knowledge, that MsDef1 blocks the mammalian L-type Ca 21 channel in a manner akin to a virally encoded and structurally unrelated antifungal toxin KP4 from Ustilago maydis, whereas structurally similar MtDef2 and the radish (Raphanus sativus) seed defensin Rs-AFP2 fail to block the L-type Ca 21 channel. From these results, we speculate that the two unrelated antifungal proteins, KP4 and MsDef1, have evolutionarily converged upon the same molecular target, whereas the two structurally related antifungal plant defensins, MtDef2 and Rs-AFP2, have diverged to attack different targets in fungi.

Involvement of the Pepper Antimicrobial Protein CaAMP1 Gene in Broad Spectrum Disease Resistance

“…The accumulation of PR proteins represents a major quantitative change in protein composition that occurs during the hypersensitive response (HR), leading to the synthesis of low molecular weight compounds, proteins, and peptides with antimicrobial activity (Fritig et al, 1998). Many plant proteins are known to possess antimicrobial activity in vitro (Spelbrink et al, 2004;Oh et al, 2005;Prost et al, 2005), and they are characterized by low molecular weight, globular structures with disulfide bonds and domains rich in Cys (Garcia-Olmedo et al, 1998;Spelbrink et al, 2004). To date, several proteins with antibacterial and/ or antifungal properties have been isolated and characterized from different plant species (Selitrennikoff, 2001).…”

Antimicrobial peptides

“…83 Moreover, MsDef1 has been characterized to block the mammalian L-type Ca 2+ channel in a manner akin to structurally unrelated antifungal toxin KP4 from Ustilago maydis. 84 It is, as well, documented that bamboo defensins-like AMPs, PpAMP1 and PpAMP2, bind to chitin in microbial cell walls, 85 and Aspergillus giganteus defensin, AFP, inhibits the chitin biosynthesis in susceptible fungi. 86 Consistently, chitin synthase mutants of Fusarium oxysporum and Aspergillus oryzae are less susceptible to AFP.…”

Calcium sulphate soil treatments augment the survival of phosphite-sprayed Banksia leptophylla infected with Phytophthora cinnamomi

“…See Erwin & Ribeiro (1996) Inhibition of a Ras G-protein in Aspergillus fumigatus reduced growth and virulence (Fortwendle et al, 2005) In vitro mycelia growth of Aphanomyces was optimal between 0.5 -5 mM Ca 2+ . No apparent relationship between reduction in growth rate and disease reduction in P sojae 200mg/L (2 mM) CaCl2.2H2O inhibited growth of P. palmivora (Singh, 1975) Plant defensins block Ca 2+ channels and inhibited growth of pathogen (Spelbrink et al, 2004). …”