The Structure, Function, and Composition of the Microsporidian Polar Tube

Abstract

The spores of microsporidia possess a unique, highly specialized structure, the polar tube, which is used to inject the parasite from the spore into a new host cell. Several theories have been proposed regarding the method by which the sporoplasm exits the spore and on the function of the polar filament or tube in this process. Electron-dense, particulate material fills the center of the filament. Weidner proposed that this material was unpolymerized polar tube protein (PTP). On the basis of ultrastructural observations, the eversion of the polar tube has been likened to a tube sliding within a tube. This chapter presents details on spore activation and discharge. When sporoblasts form, each one contains five to six coils of the preformed polar filament with the anchoring disk positioned at the anterior end. The major amino acids coded by the Encephalitozoon hellem and Encephalitozoon cuniculi PTP genes were proline and glycine. Application of the techniques of modern biology has resulted in the identification of several PTPs although the interactions and functional significance of these proteins remains to be determined.