Abstract
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 soil-borne 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.