Characterization of a cytosolic aminopeptidase from Encephalitozoon intestinalis

Proteolytic activity in Encephalitozoon cuniculi sporogonial stages: predominance of metallopeptidases including an aminopeptidase-P-like enzyme

A fraction enriched in spore precursor cells (sporoblasts) of the microsporidian Encephalitozoon cuniculi, an intracellular parasite of mammals, was obtained by Percoll gradient centrifugation. Soluble extracts of these cells exhibited proteolytic activity towards azocasein, with an alkaline optimum pH range (9-10). Prevalence of some metallopeptidases was supported by the stimulating effect of Ca2+, Mg2+, Mn2+ and Zn2+ ions, and inhibition by two chelating agents (EDTA and 1,10-phenanthroline), a thiol reductant (dithiothreitol) and two aminopeptidase inhibitors (bestatin and apstatin). Zymographic analysis revealed four caseinolytic bands at about 76, 70, 55 and 50 kDa. Mass spectrometry of tryptic peptides from one-dimensional gel slices identified a cytosol (leucine) aminopeptidase homologue (M17 family) in 50-kDa band and an enzyme similar to aminopeptidase P (AP-P) of cytosolic type (M24B subfamily) in 70-kDa band. Multiple sequence alignments showed conservation of critical residues for catalysis and metal binding. A long insertion in a common position was found in AP-P sequences from E. cuniculi and Nosema locustae, an insect-infecting microsporidian. The expression of cytosolic AP-P in sporogonial stages of microsporidia may suggest a key role in the attack of proline-containing peptides as a prerequisite to long-duration biosynthesis of structural proteins destined to the sporal polar tube.

A comparison of homologous genes encoding aminopeptidases among bird and human Encephalitozoon hellem isolates and a rabbit E. cuniculi isolate

Encephalitozoon cuniculi and E. hellem are often recognized as the agents of human microsporidiosis, but less than optimal therapy is available for treatment. The identification of enzymes critical to the parasitic life cycle is an important step in finding targets for potential drug development. Aminopeptidase gene sequences were obtained from cDNA and gDNA from avian and human E. hellem isolates and from a rabbit E. cuniculi isolate. At the amino acid level, the aminopeptidase sequences from the E. hellem human and bird isolates share >99% identity and are nearly 70% identical with the E. cuniculi sequence. Conserved HEXXH and GAMEN motifs classify the predicted aminopeptidase in the MA clan of the M1 family. The obtained aminopeptidase gene sequences are likely homologous to the previously reported E. cuniculi glutamyl aminopeptidase. The conservation of this aminopeptidase between species and divergence from mammalian aminopeptidases indicate that this enzyme may be a valid target for drug therapy.

Characterization of aminopeptidase activity from three species of microsporidia: Encephalitozoon cuniculi, Encephalitozoon hellem, and Vittaforma corneae

Microsporidia are obligate intracellular parasites of the phylum Microspora. To date, more than 1,200 species within 144 genera have been described, with 14 infecting humans. Currently, no effective treatment exists for human microsporidiosis. In this study, the biochemical properties of the aminopeptidases were investigated within several species of microsporidia. Aminopeptidase activity was detected in 3 species of microsporidia, Encephalitozoon cuniculi, E. hellem, and Vittaforma corneae, using a fluorometric substrate assay. Each species exhibited distinct aminopeptidase properties. The cytosolic neutral aminopeptidase activities of the Encephalitozoon spp. were characterized as preferentially cleaving leucine, whereas those of V. corneae cleaved arginine. Native polyacrylamide gel electrophoresis estimated the molecular mass of E. cuniculi, E. hellem, and V. corneae as 74, 72, and 79 kDa, respectively. Enzymatic activity was inhibited by bestatin and it's analogue, nitrobestatin, indicating that the enzyme was an aminopeptidase for all species. Inhibition with the chelating agents ethylenediaminetetraacetic acid and 1,10phenanthroline characterized the enzymes as metalloaminopeptidases. Subcellular fractionation of the 3 microsporidial species suggested that the enzyme activity was localized in the cytosolic fraction. Optimal enzyme activity was observed at pH 7.2 for all species. This is the first report of enzyme characterization from these 3 species of microsporidia.