A family of GHF5 endo-1,4-beta-glucanases in the migratory plant-parasitic nematode Radopholus similis

Endo-1,4-beta-glucanases, which can degrade cellulose, have been identified in a number of plant-parasitic nematodes, mainly sedentary endoparasites. This study reports the discovery of four different endoglucanases of glycosyl hydrolase family 5 (GHF5) in the migratory endoparasitic nematode Radopholus similis. Spatial expression of the corresponding genes was analysed by in situ hybridization, which showed the presence of transcripts in the pharyngeal gland cells. A semi-quantitative RT-PCR on different developmental stages was performed to study the temporal expression pattern. Three of the endoglucanase genes showed reduced expression in adult males compared with that in females. This could be explained by the fact that males do not feed and are considered non-parasitic. Only one of the endoglucanase genes was expressed in juveniles. The four corresponding proteins had a putative signal peptide for secretion and a catalytic domain. Two of the proteins had an additional linker and carbohydrate-binding module (CBM). Modelling of the catalytic domain resulted in the alpha/beta-barrel typical for GHF5 endoglucanases. Mapping the conserved amino acids of the four endoglucanases onto the 3D structure revealed that most were positioned near the catalytic centre of the protein, whereas less conserved amino acids occurred more often in the alpha helices, pointing towards the outside of the protein. Analyses of guanine/cytosine (GC) contents and codon adaptation indices indicated that the endoglucanase genes were well adapted to the codon usage of R. similis. The GC and GC3 (GC content of the third position of the codons) contents of the endoglucanases were significantly higher than the average for all Radopholus ESTs (expressed sequence tags).