Microsporidian invasion apparatus: Identification of a novel polar tube protein and evidence for clustering of ptp1 and ptp2 genes in three Encephalitozoon species

Microsporidia are unicellular eukaryotes occuring as obligate intracellular parasites which produce resistant spores. A unique motile process is repre sented by the sudden extrusion of the sporal polar tube for initiating entr y of the parasite into a new host cell. The complete sequence of an acidic proline-rich polar tube protein (renamed PTP1) has been previously reported for Encephalitozoon cuniculi and E. hellem. Our immunological investigatio ns provided evidence for an additional PTP in E. cuniculi, termed PTP2. The corresponding gene was sequenced and then expressed in Escherichia coli. A s expected, mouse antibodies raised against the recombinant protein reacted specifically with the polar tube. The singlecopy ptp1 and ptp2 genes of E, cuniculi were tandemly arranged on chromosome VL. Polyadenylation of the m RNAs was demonstrated. Identification and sequencing of homologous genes in the two other human-infecting Encephalitozoon species (ptp2 in E. hellem a nd ptp1 and ptp2 in E. intestinalis) were facilitated by conserved gene clu stering. PTP2 appears as a novel structural protein (30 kDa) with a basic l ysine-rich core and an acidic tail. Unlike PTP1, this protein is devoid of large tandem repeats. The interspecies conservation of cysteine residues su pports a major role of disulfide bridges in polar tube assembly. The two PT Ps should serve as both molecular markers of spore differentiation and diag nostic tools.