Hydrophobins from Aspergillus species cannot be clearly divided into two classes

Abstract

Background: Hydrophobins are a family of small secreted proteins with a characteristic pattern of eight cysteine residues found exclusively in filamentous fungi. They have originally been divided into two classes based on their physical properties and hydropathy patterns, and are involved in the attachment of hyphae to hydrophobic structures, the formation of aerial structures and appear to be involved in pathogenicity.

Findings: Analysis of nine genome sequences from seven Aspergilli revealed fifty hydrophobins, where each species displayed between two to eight hydrophobins. Twenty of the identified hydrophobins have not previously been described from these species. Apart from the cysteines, very little amino acid sequence homology was observed. Twenty-three of the identified hydrophobins could be classified as class I hydrophobins based on their conserved cysteine spacing pattern and hydropathy pattern. However twenty-six of the identified hydrophobins were intermediate forms. Notably, a single hydrophobin, ATEG_04730, from Aspergillus terreus displayed class II cysteine spacing and had a class II hydropathy pattern.

Conclusion: Fifty hydrophobins were identified in Aspergillus, all containing the characteristic eight cysteine pattern. Aspergillus terreus exhibited both class I and class II hydrophobins. This is the first report of an Aspergillus species with the potential to express both class I and class II hydrophobins. Many of the identified hydrophobins could not directly be allocated to either class I or class II.

Figure 1

Hydropathy patterns. Hydropathy patterns of SC3 from S. commune, EAS from N. crassa, RodA from A. fumigatus, HFBI and HFBII from T. reesei and proteins ATEG_06492 and ATEG_04730 from A. terreus. The amino acids of the hydrophobins are shown along the x-axis, where cysteines are indicated by vertical lines. Hydrophobic amino acids are shown above the x-axis, while hydrophilic amino acids are shown below. Only the part of the sequence from the first to the eighth cysteine was used to create the hydropathy pattern.

Figure 2

Phylogenetic tree of class I and class II hydrophobins. Sequences of SC3 (S. commune), EAS (N. crassa), RodA (A. fumigatus), HFBI and HFBII (T. reesei) were obtained from the National Center for Biotechnology Information (NCBI). The phylogenetic tree was constructed based on a multiple alignment of identified hydrophobins using Phylogeny.fr [22]. Branches with support values less than 50% were collapsed.