Appressorial proteomics analysis of the wild-type and GAG biosynthetic mutant of Metarhizium robertsii

Exopolysaccharide galactosaminogalactan (GAG) is a fungal cell wall component composed of α-1,4 linked galactose, N-acetyl galactosamine and galactosamine, which has been demonstrated in Aspergillus fumigatus in association with fungal adhesion, biofilm formation and virulence. The gene cluster responsible for GAG biosynthesis has only been characterized in Aspergillus fungi. We found that the highly conserved gene cluster for GAG biosynthesis is also present in the insect pathogenic fungi Metarhizium species. Functional investigations in M. robertsii revealed that GAG is only produced on fungal cell wall during fungal germination, filamentation and the formation of the infection structure appressoria. Gene deletions revealed that, relative to the wild-type (WT), the appressorial mucilage production was abolished in the null mutant of M. robertsii. Since multiple enzymes are produced in appressorial mucilages, appressorial samples of the WT and mutant formed on cicada wings were collected and subjected to iTRAQ comparative proteomic analysis. We found that different protein families were up- or down-regulated in the null mutant when compared with the WT.