The Antifungal Activity of the Penicillium chrysogenum Protein PAF Disrupts Calcium Homeostasis in Neurospora crassa

The antifungal protein PAF fromPenicillium chrysogenum exhibits growth-inhibitory activity against a broad range of filamentous fungi. Evidence from this study suggests that disruption of Ca2+ signaling/homeostasis plays an important role in the mechanistic basis of PAF as a growth inhibitor. Supplementation of the growth medium with high Ca2+ concentrations counteracted PAF toxicity toward PAF-sensitive molds. By using a transgenicNeurospora crassa strain expressing codon-optimized aequorin, PAF was found to cause a significant increase in the resting level of cytosolic free Ca2+ ([Ca2+ ]c ). The Ca2+ signatures in response to stimulation by mechanical perturbation or hypo-osmotic shock were significantly changed in the presence of PAF. BAPTA [bis-(aminophenoxy)-ethane-N ,N ,N ′,N ′-tetraacetic acid], a Ca2+ selective chelator, ameliorated the PAF toxicity in growth inhibition assays and counteracted PAF induced perturbation of Ca2+ homeostasis. These results indicate that extracellular Ca2+ was the major source of these PAF-induced effects. The L-type Ca2+ channel blocker diltiazem disrupted Ca2+ homeostasis in a similar manner to PAF. Diltiazem in combination with PAF acted additively in enhancing growth inhibition and accentuating the change in Ca2+ signatures in response to external stimuli. Notably, both PAF and diltiazem increased the [Ca2+ ]c resting level. However, experiments with an aequorin-expressing Δcch-1 deletion strain ofN. crassa indicated that the L-type Ca2+ channel CCH-1 was not responsible for the observed PAF-induced elevation of the [Ca2+ ]c resting level. This study is the first demonstration of the perturbation of fungal Ca2+ homeostasis by an antifungal protein from a filamentous ascomycete and provides important new insights into the mode of action of PAF.