Identification of two new polar tube proteins related to polar tube protein 2 in the microsporidian Antonospora locustae.

Microsporidia are obligate intracellular eukaryotic parasites with a broad host spectrum characterized by a unique and highly sophisticated invasion apparatus, the polar tube (PT). In a previous study, two PT proteins, named AlPTP1 (50 kDa) and AlPTP2 (35 kDa), were identified in Antonospora locustae, an orthoptera parasite that is used as a biological control agent against locusts. Antibodies raised against AlPTP2 cross-reacted with a band migrating at ~70 kDa, suggesting that this 70-kDa antigen is closely related to AlPTP2. A blastp search against the A. locustae genome database allowed the identification of two further PTP2-like proteins named AlPTP2b (568 aa) and AlPTP2c (599 aa). Both proteins are characterized by a specific serine- and glycine-rich N-terminal extension with elastomeric structural features and share a common C-terminal end conserved with AlPTP2 (~88% identity for the last 250 aa). MS analysis of the 70-kDa band revealed the presence of AlPTP2b. Specific anti-AlPTP2b antibodies labelled the extruded PTs of the A. locustae spores, confirming that this antigen is a PT component. Finally, we showed that several PTP2-like proteins are also present in other phylogenetically related insect microsporidia, including Anncaliia algerae and Paranosema grylli.