Microsporidia are a group of obligate intracellular pathogens infecting nearly all animal phyla. The microsporidian Nosema bombycis has been isolated from several lepidopteran species, including the economy-important silkworms as well as several crop pests.
Proteins secreted by parasites can be important virulent factors in modulating host pathways.
Ricin is a two-chain
lectin best known for its extreme vertebrate toxicity.
Ricin B
lectin-like
proteins are widely distributed in microsporidia, especially in N. bombycis. In this study, we identify 52
Ricin B
lectin-like
proteins (RBLs) in N. bombycis. We show that the N. bombycis RBLs (NbRBLs) are classified into four subfamilies. The subfamily 1 was the most conserved, with all members having a
Ricin B
lectin domain and most members containing a
signal peptide. The other three subfamilies were less conserved, and even lost the
Ricin B
lectin domain, suggesting that NbRBLs might be a multi-functional family. Our study here indicated that the NbRBL family had evolved by producing tandem duplications firstly and then expanded by segmental duplications, resulting in concentrated localizations mainly in three genomic regions. Moreover, based on
RNA-seq data, we found that several Nbrbls were highly expressed during
infection. Further, the results show that the NbRBL28 was secreted into host nucleus, where it promotes the expressions of genes involved in cell cycle progression. In summary, the great copy number, high divergence, and concentrated genome distribution of the NbRBLs demonstrated that these
proteins might be adaptively evolved and played a vital role in the multi-host N. bombycis.