Characterization and Function of the Microsporidian Polar Tube: a Review

Microsporidia are eukaryotic, obligate intracellular organisms defined by their small spores containing a single polar tube that coils around the interior of the spore. After appropriate stimuli the germination of spores occurs. Conditions that promote germination vary widely among species, presumably reflecting the organism's adaptation to their host and external environment as well as preventing accidental discharge in the environment. It appears that calcium may be a key ion in this process. Regardless of the stimuli required for activation, all microsporidia exhibit the same response to the stimuli, that is, increasing the intrasporal osmotic pressure. This results in an influx of water into the spore accompanied by swelling of the polaroplasts and posterior vacuole. The polar tube then discharges from the anterior pole of the spore in an explosive reaction and is thought to form a hollow tube by a process of eversion. If the polar tube is discharged next to a cell, it can pierce the cell and transfer the sporoplasm into this cell. Polar tubes resist dissociation in detergents and acids but dissociate in dithiothreitol. We have developed a method for the purification of polar tube proteins (PTPs) using differential extraction followed by reverse phase high performance liquid chromatography (HPLC). This method was used to purify for subsequent characterization PTPs from Glugea americanus, Encephalitozoon cuniculi, E. hellem and E. intestinalis. These proteins appear to be members of a protein family that demonstrate conserved characteristics in solubility, hydrophobicity, mass, proline content and immunologic epitopes. These characteristics are probably important in the function of this protein in its self assembly during the eversion of the polar tube and in providing elasticity and resiliency for sporoplasm passage.