UniProt release 15.6

Microsporidian polar tube: a molecular syringe in UniProtKB/Swiss-Prot

Microsporidia are ubiquitous, obligate intracellular spore-forming fungal parasites which infect a wide range of invertebrates and vertebrates. They are common pathogens responsible for opportunistic infections in immunodeficient humans, such as HIV-infected patients or patients being treated with immunosuppressive drugs. The most common microsporidian associated with AIDS is Enterocytozoon bieneusi which induces chronic diarrhea in HIV-infected individuals. However, since no animal model for E.bieneusi is available, most of the experimental studies on microsporidia have been carried out on Encephalitozoon cuniculi. This microsporidium, which commonly infects rodents, has also been reported to infect humans. Its complete proteome is available in UniProtKB.

Microsporidia are primitive organisms lacking fundamental organelles found in other eukaryotes, such as stacked Golgi apparatus, peroxisomes or mitochondria. However, they have a mitochondrial relic organelle called the mitosome which does not contain any DNA. As a result, to persist in the environment, they have to parasitize the cells of higher organisms.

How do they achieve their goal? The microsporidian intracellular developmental cycle leads to a terminal sporogenic phase producing small spores which are critical for their host-to-host transmission. The unicellular spores have a resistant wall protecting a mononucleate or binucleate sporoplasm (the infectious apparatus of the spore) and an extrusion apparatus consisting of a single polar tube with an anterior attachment complex. Once the target cell is recognized, the polar tube acts as a syringe: it pierces the host cell membrane and rapidly "injects" the sporoplasm into the host cell.

3 polar tube proteins have been identified: PTP1, PTP2, and PTP3. The major polar tube protein, PTP1, accounts for at least 70% of the mass of the polar tube. Before the polar tube can act, the spore has to recognize the host cell and stick to its surface. This role is played by EnP1, which is involved in the adhesion of spores to host cell surface glycoaminoglycans. Orthologous proteins have been identified: EnP1, PTP1 and PTP2 in Encephalitozoon intestinalis and PTP1 and PTP2 in Encephalitozoon hellem. These 2 microsporidian species infect man and cause intestinal infections keratoconjunctivitis, and respiratory infections.

All these infectious proteins are available in UniProtKB with the following accession numbers:

Encephalitozoon cuniculi: PTP1 O76942, PTP2 Q8SRT0, PTP3 Q8MTP3, and EnP1 Q8SWL3.

Encephalitozoon intestinalis: PTP1 Q5F2J0, PTP2 Q5F2J0, and EnP1 A7TZU4.

Encephalitozoon hellem: PTP1 O76273, and PTP2 P0CAT5.

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