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Microsporidia, amitochondrial protists, possess a 70-kDa heat shock protein gene of mitochondrial evolutionary origin
E Peyretaillade1, V Broussolle, P Peyret
1Laboratoire de Protistologie Moléculaire et Cellulaire des Parasites Opportunistes, Université Blaise Pascal, Aubière, France.
Abstract:
An intronless gene encoding a protein of 592 amino acid residues with similarity to 70-kDa heat shock proteins (HSP70s) has been cloned and sequenced from the amitochondrial protist Encephalitozoon cuniculi (phylum Microsporidia). Southern blot analyses show the presence of a single gene copy located on chromosome XI. The encoded protein exhibits an N-terminal hydrophobic leader sequence and two motifs shared by proteobacterial and mitochondrially expressed HSP70 homologs. Phylogenetic analysis using maximum likelihood and evolutionary distances place the E. cuniculi sequence in the cluster of mitochondrially expressed HSP70s, with a higher evolutionary rate than those of homologous sequences. Similar results were obtained after cloning a fragment of the homologous gene in the closely related species E. hellem. The presence of a nuclear targeting signal-like sequence supports a role of the Encephalitozoon HSP70 as a molecular chaperone of nuclear proteins. No evidence for cytosolic or endoplasmic reticulum forms of HSP70 was obtained through PCR amplification. These data suggest that Encephalitozoon species have evolved from an ancestor bearing mitochondria, which is in disagreement with the postulated presymbiotic origin of Microsporidia. The specific role and intracellular localization of the mitochondrial HSP70-like protein remain to be elucidated.
Insights
This study identified a heat shock protein 70 (HSP70) gene in Encephalitozoon cuniculi, suggesting Microsporidia evolved from a mitochondriate ancestor. The findings challenge previous theories on Microsporidia
Area of Science:
- Molecular Biology
- Evolutionary Biology
- Protistology
Background:
- Microsporidia, including Encephalitozoon cuniculi, are obligate intracellular parasites.
- These organisms are characterized by their reduced genomes and lack of mitochondria.
- Heat shock proteins (HSP70s) are crucial molecular chaperones involved in protein homeostasis.
Purpose of the Study:
- To clone and characterize a heat shock protein 70 (HSP70) gene from Encephalitozoon cuniculi.
- To investigate the evolutionary origins of Microsporidia based on HSP70 gene analysis.
- To determine the potential function and localization of the identified HSP70.
Main Methods:
- Gene cloning and sequencing of the HSP70 homolog from E. cuniculi and E. hellem.
- Southern blot analysis to determine gene copy number and location.
- Phylogenetic analysis using maximum likelihood and evolutionary distances.
- PCR amplification to screen for other HSP70 forms.
Main Results:
- An intronless HSP70 gene was identified in E. cuniculi, encoding a protein with similarity to HSP70s.
- Phylogenetic analysis placed the E. cuniculi HSP70 within the cluster of mitochondrially expressed HSP70s, exhibiting a high evolutionary rate.
- A nuclear targeting signal-like sequence was found, suggesting a role in nuclear protein chaperoning. No evidence for cytosolic or ER forms was found.
Conclusions:
- The presence of a mitochondrially-related HSP70 suggests that Encephalitozoon species evolved from an ancestor that possessed mitochondria.
- This finding contradicts the hypothesis of a presymbiotic origin for Microsporidia.
- Further research is needed to elucidate the precise function and intracellular localization of this unique HSP70-like protein.