Abstract
BackgroundMicrosporidians are opportunistic pathogens with a wide range of hosts, including invertebrates, vertebrates and even humans. Microsporidians possess a highly specialized invasion structure, the polar tube. When spores encounter an appropriate environmental stimulation, the polar tube rapidly everts out of the spore, forming a 50–500 µm hollow tube that serves as a conduit for sporoplasm passage into host cells. The polar tube is mainly composed of polar tube proteins (PTPs). So far, five major polar tube proteins have been isolated from microsporidians. Nosema bombycis, the first identified microsporidian, infects the economically important insect silkworm and causes heavy financial loss to the sericulture industry annually.ResultsA novel polar tube protein of N. bombycis (NbPTP6) was identified. NbPTP6 was rich in histidine (H) and serine (S), which contained a signal peptide of 16 amino acids at the N-terminus. NbPTP6 also had 6 potential O-glycosylation sites and 1 potential N-glycosylation site. The sequence alignment analysis revealed that NbPTP6 was homologous with uncharacterized proteins from other microsporidians (Encephalitozoon cuniculi, E. hellem and N. ceranae). Additionally, the NbPTP6 gene was expressed in mature N. bombycis spores. Indirect immunofluorescence analysis (IFA) result showed that NbPTP6 is localized on the whole polar tube of the germinated spores. Moreover, IFA, enzyme-linked immunosorbent (ELISA) and fluorescence-activated cell sorting (FACS) assays results revealed that NbPTP6 had cell-binding ability.ConclusionsBased on our results, we have confirmed that NbPTP6 is a novel microsporidian polar tube protein. This protein could adhere with the host cell surface, so we speculated it might play an important role in the process of microsporidian infection.
Highlights
Microsporidians are opportunistic pathogens with a wide range of hosts, including invertebrates, vertebrates and even humans
We identified a novel polar tube protein from N. bombycis named as NbPTP6
The sequence is rich in histamine and has a signal peptide consisting of 16 amino acids at the N-terminal
Summary
Microsporidians are opportunistic pathogens with a wide range of hosts, including invertebrates, vertebrates and even humans. When spores encounter an appropriate environmental stimulation, the polar tube rapidly everts out of the spore, forming a 50–500 μm hollow tube that serves as a conduit for sporoplasm passage into host cells. The polar tube is mainly composed of polar tube proteins (PTPs). Since the discovery of the first microsporidian Nosema bombycis from silkworms in the 19th century, Lv et al Parasites Vectors (2020) 13:475 forming a hollow tube that delivers the infectious sporoplasm into its host cell’s cytoplasm [1, 9,10,11]. The polar tube is composed of proteins, and presently, five kinds of polar tube proteins (PTP1-PTP5) have been identified from different microsporidia. The third polar tube protein, PTP3, is identified from an E. cuniculi cDNA library. Transferrin receptor 1 (TfR1) was identified as a potential host cell receptor of EhPTP4, and the microsporidian infection rate in TfR1-knockout mutant host cells was markedly reduced [23]
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