Genome-wide identification of genes encoding putative secreted E3 ubiquitin ligases and functional characterization of PbRING1 in the biotrophic protist Plasmodiophora brassicae

The E3 ubiquitin ligases are key regulators of protein ubiquitination, which have been shown to be involved in a variety of cellular responses to both biotic and abiotic stresses in eukaryotes. However, the E3 ubiquitin ligase homologues in the soilborne plant pathogen Plasmodiophora brassicae, the causal agent of clubroot disease of crucifer crops worldwide, remain largely unknown. In this study, we characterized secreted E3 ubiquitin ligases, a group of proteins known to be involved in virulence in many pathogens, in a plasmodiophorid P. brassicae. Genome-wide search in the P. brassicae genome retrieved 139 putative E3 ubiquitin ligases, comprising of 115 RING, 15 HECT, 1 HECT-like, and 8 U-box E3 ubiquitin ligases. Among these E3 ubiquitin ligases, 11 RING, 1 U-box, and 3 HECT were found to harbor signal peptide. Based on published RNA-seq data (Schwelm et al. in Sci Rep 5:11153, 2015), we found that these genes were differentially expressed in distinct life stages including germinating spores, maturing spores, and plasmodia. We characterized one potential secreted E3 ubiquitin ligase, PbRING1 (PBRA_000499). Yeast invertase assay showed that PbRING1 harbors a functional N-terminal signal peptide. PbRING1 also harbors a really interested new gene (RING) domain at its C terminus, which was found to display the E3 ligase activity in vitro. Collectively, this study provides a comprehensive insight into the reservoir of putative secreted E3 ligases in P. brassicae.