Ct-AMP1

Plant defensins, components of the plant innate immune system, are cationic cysteine-rich antifungal peptides. Evidence from the literature [Thevissen, K., et al. (2003) Peptides 24, 1705-1712] has demonstrated that patches of fungi membrane containing mannosyldiinositolphosphorylceramide and glucosylceramides are selective binding sites for the plant defensins isolated from Dahlia merckii and Raphanus sativus, respectively. Whether plant defensins interact directly or indirectly with fungus intracellular targets is unknown. To identify physical protein-protein interactions, a GAL4-based yeast two-hybrid system was performed using the antifungal plant peptide Pisum sativum defensin 1 (Psd1) as the bait. Target proteins were screened within a Neurospora crassa cDNA library. Nine out of 11 two-hybrid candidates were nuclear proteins. One clone, detected with high frequency per screening, presented sequence similarity to a cyclin-like protein, with F-box and WD-repeat domains, related to the cell cycle control. GST pull-down assay corroborated in vitro this two-hybrid interaction. Fluorescence microscopy analysis of FITC-conjugated Psd1 and DAPI-stained fungal nuclei showed in vivo the colocalization of the plant peptide Psd1 and the nucleus. Analysis of the DNA content of N. crassa conidia using flow cytometry suggested that Psd1 directed cell cycle impairment and caused conidia to undergo endoreduplication. The developing retina of neonatal rats was used as a model to observe the interkinetic nuclear migration during proliferation of an organized tissue from the S toward the M phase of the cell cycle in the presence of Psd1. The results demonstrated that the plant defensin Psd1 regulates interkinetic nuclear migration in retinal neuroblasts.

We used an assay based on the uptake of SYTOX Green, an organic compound that fluoresces upon interaction with nucleic acids and penetrates cells with compromised plasma membranes, to investigate membrane permeabilization in fungi. Membrane permeabilization induced by plant defensins in Neurospora crassa was biphasic, depending on the plant defensin dose. At high defensin levels (10 to 40 microM), strong permeabilization was detected that could be strongly suppressed by cations in the medium. This permeabilization appears to rely on direct peptide-phospholipid interactions. At lower defensin levels (0.1 to 1 microM), a weaker, but more cation-resistant, permeabilization occurred at concentrations that correlated with the inhibition of fungal growth. Rs-AFP2(Y38G), an inactive variant of the plant defensin Rs-AFP2 from Raphanus sativus, failed to induce cation-resistant permeabilization in N. crassa. Dm-AMP1, a plant defensin from Dahlia merckii, induced cation-resistant membrane permeabilization in yeast (Saccharomyces cerevisiae) which correlated with its antifungal activity. However, Dm-AMP1 could not induce cation-resistant permeabilization in the Dm-AMP1-resistant S. cerevisiae mutant DM1, which has a drastically reduced capacity for binding Dm-AMP1. We think that cation-resistant permeabilization is binding site mediated and linked to the primary cause of fungal growth inhibition induced by plant defensins.

Hs-AFP1, an antifungal plant defensin from seed of the plant Heuchera sanguinea, was radioactively labeled using t-butoxycarbonyl-[35S]L-methionine N-hydroxysuccinimidyl ester, resulting in a 35S-labeled peptide with unaltered antifungal activity. [35S]Hs-AFP1 was used to assess binding on living hyphae of the fungus Neurospora crassa. Binding of [35S]Hs-AFP1 was found to be competitive, reversible, and saturable with an apparent Kd of 29 nM and a Bmax of 1.4 pmol/mg protein. [35S]Hs-AFP1 also bound specifically and reversibly to microsomal membranes derived from N. crassa hyphae with a Kd of 27 nM and a Bmax of 102 pmol/mg protein. The similarity in Kd value between binding sites on hyphae and microsomes indicates that Hs-AFP1 binding sites reside on the plasma membrane. Binding of [35S]Hs-AFP1 to both hyphae and microsomal membranes could be competed to some extent by four different structurally related plant defensins but not by various structurally unrelated antimicrobial peptides. In addition, an inactive single amino acid substitution variant of the antifungal plant defensin Rs-AFP2 from Raphanus sativus seed was also unable to displace [35S]Hs-AFP1 from its binding sites, whereas Rs-AFP2 itself was able to compete with [35S]Hs-AFP1.