Microsporidia are obligate intracellular parasites producing spores surrounded by a thick cell wall and containing the polar tube, a unique invasion apparatus coiled within the cytoplasm. Under stimuli, the polar tube is suddenly extruded allowing the transfer of the infectious content, named the sporoplasm, within the cytoplasm of a new host cell. Two polar tube proteins (PTP1 and PTP2), the genes of which are tandemly arranged, have been previously described in the three Encephalitozoon species. A third protein of higher size, PTP3, was only identified in E. cuniculi. To improve our knowledge of the polar tube composition and architecture, the search for new polar tube components has been undertaken. The comparison of E. cuniculi and Antonospora locustae genomes reveals significant gene order conservation (synteny). This syntenic relationship facilitated the characterisation of PTP1 and PTP2 proteins in A. locustae despite their high sequence divergences with those of E. cuniculi. In addition, the A. locustae genome is characterized by the presence of several ptp2 like genes. The ptp1 and ptp2 gene cluster organization is also conserved in Paranosema grylli. An E. cuniculi proteomic analysis of proteins expressed in late sporogonial stages was helpful in the identification of two new PTPs (PTP4 and PTP5) the genes of which are tandemly arranged. These proteins, presenting no homology with other known PTPs, have been also identified in A. locustae and P. grylli. PTP4 is localized at the end terminal region of extruded polar tubes suggesting a role in host cell interactions. Co-expression experiments in E. coli showed the existence of interactions between PTP1 and PTP2. Moreover, molecular data concerning spore wall components indicate the presence of SWP1 proteins in the exospore of both E. cuniculi and E. intestinalis. In the present study, we identified the whole sequences of two SWP1-like proteins in E. hellem. The high variability of their C-terminal regions is of interest for epidemiological studies. Finally, for functional genomic approaches, the development of a transfection strategy of the microsporidia E. cuniculi was initiated. Only few transient transformants have been obtained.
