Multiplication of an ancestral gene encoding secreted fungalysin preceded species differentiation in the dermatophytes Trichophyton and Microsporum

Abstract

Dermatophytes are human and animal pathogenic fungi which cause cutaneous infections and grow exclusively in the stratum corneum, nails and hair. In a culture medium containing soy proteins as sole nitrogen source a substantial proteolytic activity was secreted byTrichophyton rubrum,Trichophyton mentagrophytesandMicrosporum canis. This proteolytic activity was 55–75 % inhibited byo-phenanthroline, attesting that metalloproteases were secreted by all three species. Using a consensus probe constructed on previously characterized genes encoding metalloproteases (MEP) of the M36 fungalysin family inAspergillus fumigatus,Aspergillus oryzaeandM. canis, a five-memberMEPfamily was isolated from genomic libraries ofT. rubrum,T. mentagrophytesandM. canis. A phylogenetic analysis of genomic and protein sequences revealed a robust tree consisting of five main clades, each of them including aMEPsequence type from each dermatophyte species. EachMEPtype was remarkably conserved across species (72–97 % amino acid sequence identity). The tree topology clearly indicated that the multiplication ofMEPgenes in dermatophytes occurred prior to species divergence. In culture medium containing soy proteins as a sole nitrogen source secreted Meps accounted for 19–36 % of total secreted protein extracts; characterization of protein bands by proteolysis and mass spectrometry revealed that the three dermatophyte species secreted two Meps (Mep3 and Mep4) encoded by orthologous genes.