The microsporidian polar tube: A highly specialised invasion organelle

Abstract

All of the members of the Microsporidia possess a unique, highly specialised structure, the polar tube. This article reviews the available data on the organisation, structure and function of this invasion organelle. It was over 100 years ago that Thelohan accurately described the microsporidian polar tube and the triggering of its discharge. In the spore, the polar tube is connected at the anterior end, and then coils around the sporoplasm. Upon appropriate environmental stimulation the polar tube rapidly discharges out of the spore pierces a cell membrane and serves as a conduit for sporoplasm passage into the new host cell. The mechanism of germination of spores, however, remains to be definitively determined. In addition, further studies on the characterisation of the early events in the rupture of the anterior attachment complex, eversion of the polar tube as well as the mechanism of host cell attachment and penetration are needed in order to clarify the function and assembly of this structure. The application of immunological and molecular techniques has resulted in the identification of three polar tube proteins referred to as PTP1, PTP2 and PTP3. The interactions of these identified proteins in the formation and function of the polar tube remain to be determined. Data suggest that PTP1 is an O-mannosylated glycoprotein, a post-translational modification that may be important for its function. With the availability of the Encephalitozoon cuniculi genome it is now possible to apply proteomic techniques to the characterisation of the components of the microsporidian spore and invasion organelle.

Evolution of a morphological novelty occurred before genome compaction in a lineage of extreme parasites

“…MS ion channels have a variety of roles in the physiology of eukaryotic cells, such as osmotic gradients, cell swelling, gravitropism, and control of cellular turgor (34). We speculate that the microsporidian-specific MS channel could be used for spore germination, which involves increased intrasporal osmotic pressure (29). The exocyst complex, which is well known in yeast, functions by the interaction of four subunits-Sec6, Sec8, Sec10, and Exo70-delivering proteins that are essential for cell separation after division (35).…”

“…3 and Table S1). Because microsporidia germinate via a highly specialized polar tube extrusion mechanism (29) and grow in vivo by schizogony, the myosin domain might no longer have been necessary as there is no polarized growth phase. If this hypothesis is correct, the related group aphelids, which like Rozella appear to grow into their hosts through the formation of a germ tube (30) with a cell wall, would be predicted to have a chitin synthase with a myosin domain.…”

The Evolution of Sex: a Perspective from the Fungal Kingdom

“…The infection cycle of these pathogens includes several steps that are distinguished from other infectious agents (320). Microsporidian dormant spores contain a cylinder-like tube, called the polar tube, which originates from an anterior part of the spores and is coiled within the spore to serve as a "tunnel" for moving infectious material into the hosts.…”

Branching Network of Proteinaceous Filaments within the Parasitophorous Vacuole of Encephalitozoon cuniculi and Encephalitozoon hellem

“…Similarly, while previously identified SWPs (SWP1, EnP1, snd SWP3) and PTPs (PTP1, -2, and -3) were detected as expected in the DTT-insoluble or -soluble fraction, respectively, each was also detected in both fractions, probably also due to their high abundance or, in the case of the PTPs, incomplete solubilization. The appearance of SWP3 in the DTT-soluble fraction has previously been noted (46).…”

“…The spore wall, polar tube, and other specialized organelles, such as the polaroplast and posterior vacuole, play critical roles in the infectious process of the microsporidia (42,46). In a resting spore, the polar tube lies coiled within the cell; at this stage, it is solid and is thus referred to as a polar filament (28,37).…”