Characterization of the Antifungal Protein Secreted by the MouldAspergillus giganteus

Abstract

An antifungal polypeptide (AFP) of 51 amino acid residues, secreted by the mouldAspergillus giganteus,has been purified to homogeneity and characterized. The inhibitory effect of this protein on the growth of different microorganisms has been studied. Whereas the growth of many of the filamentous fungi assayed is inhibited, no effect has been observed against yeasts or bacteria. The minimal concentration for total inhibition of the growth is in the range 6 to 25 μM. The antifungal polypeptide does not produce any effect on the growth of the producing mould. The polypeptide promotes aggregation of acidic phospholipid vesicles. A remarkable resistance to proteolysis and a low hydrogen × deuterium exchange have been observed for this protein. The protein does not show any thermal transition up to 80°C when studied by differential scanning calorimetry and infrared spectroscopy. The uv absorbance, fluorescence emission, and circular dichroism (CD) characteristics of this protein have been studied. The protein exhibits a strong positive band at 230 nm as a prominent feature of the CD spectrum in the far uv region. All the spectroscopical properties of the antifungal protein are highly influenced by the abundance of tyrosine residues. These can be grouped in two different populations, buried and exposed, based on the results of pH-titration experiments. Fourier-transform infrared spectroscopy reveals a high content of β-structure in AFP. Reduction and carboxyamidomethylation produces a rather unstructured polypeptide as deduced from its spectroscopical properties.