Proteome of the Nematode-Trapping Cells of the Fungus Monacrosporium haptotylum

Many nematophagous fungi use morphological structures called traps to capture nematodes by adhesion or mechanically. To better understand the cellular functions of adhesive traps, the trap cell proteome of the fungus Monacrosporium haptotylum was characterized. The trap of M. haptotylum consists of a unicellular structure called a knob that develops at the apex of a hypha. Proteins extracted from knobs and mycelia were analyzed using SDS-PAGE and liquid chromatography-tandem mass spectrometry (LC-MS-MS). The peptide sequences were matched against predicted gene models from the recently sequenced M. haptotylum genome. In total, 336 proteins were identified, with 54 expressed at significantly higher levels in the knobs than in the mycelia. The upregulated knob proteins included peptidases, small secreted proteins with unknown functions, and putative cell surface adhesins containing carbohydrate-binding domains, including the WSC domain. Phylogenetic analysis showed that all upregulated WSC domain proteins belonged to a large, expanded cluster of paralogs in M. haptotylum. Several peptidases and homologs of experimentally verified proteins in other pathogenic fungi were also upregulated in the knob proteome. Complementary profiling of gene expression at the transcriptome level showed poor correlation between the upregulation of knob proteins and their corresponding transcripts. We propose that the traps of M. haptotylum contain many of the proteins needed in the early stages of infection and that the trap cells can tightly control the translation and degradation of these proteins to minimize the cost of protein synthesis.