Nbseptin2 Expression Pattern and Its Interaction with Nb PTP 1 during Microsporidia Nosema bombycis Polar Tube Extrusion

Maturation of subtilisin-like protease NbSLP1 from microsporidia Nosema bombycis

Microsporidia are obligate intracellular parasites and possess a unique way of invading hosts, namely germination. Microsporidia are able to infect almost all animal cells by germination. During the process, the polar tube extrudes from the spores within, thus injecting infectious sporoplasm into the host cells. Previous studies indicated that subtilisin-like protease 1 (NbSLP1) of microsporidia Nosema bombycis were located at the polar cap of germinated spores where the polar tube extrusion. We hypothesized that NbSLP1 is an essential player in the germination process. Normally, SLP need to be activated by autoproteolysis under conditions. In this study, we found that the signal peptide of NbSLP1 affected the activation of protease, two self-cleavage sites were involved in NbSLP1 maturation between Ala¹⁰⁴Asp¹⁰⁵ and Ala¹²⁴Asp¹²⁵ respectively. Mutants at catalytic triad of NbSLP1 confirmed the decreasing of autoproteolysis. This study demonstrates that intramolecular proteolysis is required for NbSLP1 maturation. The protease undergoes a series of sequential N-terminal cleavage events to generate the mature enzyme. Like other subtilisin-like enzymes, catalytic triad of NbSLP1 are significant for the self-activation of NbSLP1. In conclusion, clarifying the maturation of NbSLP1 will be valuable for understanding the polar tube ejection mechanism of germination.

Septin homologs cooperating in the Proliferative Stage of Microsporidia Nosema bombycis

The single-celled pathogen Nosema bombycis, that can infect silkworm Bombyx mori and other lepidoptera including Spodoptera, is the first identified Microsporidia which has diplokaryotic nuclei throughout the life cycle. Septin proteins can form highly ordered filaments, bundles or ring structures related to the cytokinesis in fungi. Here, three septin proteins (NbSeptin1, NbSeptin2 and NbSeptin3) from Nosema bombycis CQ I are described. These proteins, appear to be conserved within the phylum Microsporidia. NbSeptins transcripts were detected throughout the pathogen developmental cycle and were significantly enhanced from second days of infection, which lead to our hypothesis that NbSeptins play a role in merogony. Immunofluorescence assay (IFA) revealed a broad distribution of NbSeptins in meronts and partly co-localization of NbSeptins. Interestingly, in some of meronts, NbSeptin2 and NbSeptin3 showed localization between the nuclei of the diplokaryon. Yeast two-hybrid and co-immunoprecipitation analysis verified that NbSeptins can interact with each other. Our findings suggest that NbSeptins can cooperate in the proliferation stage of Nosema bombycis and contribute towards the understanding of the rols of septins in microsporidia development.

Expression of Nosema bombycis polar tube protein 1 in lepidopteran Sf9 cells and its effect on microsporidian proliferation

Nosema bombycis, the first identified microsporidium, causes heavy losses to the sericulture industry in China. During infection, microsporidia discharge a long and hollow polar tube, which delivers the sporoplasm into host cells. Polar tube protein 1 was the major component on the polar tube. Previously, we expressed the polar tube protein 1 from Nosema bombycis (NbPTP1) intercellularly in Drosophila S2 cells. Here, the microsporidian protein was expressed in Lepidopteran Sf9 cells. During heterologous expression, NbPTP1 protein was secreted and glycosylated. Microsporidian proliferation decreased in NbPTP1-expressing Sf9 cells. This confirms that NbPTP1 protein can interact with the host cell membrane receptor protein to facilitate microsporidian invasion.