Article

Giardia lamblia Expresses a Proteobacterial-like DnaK Homolog

The Josephine Bay Paul Center of Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, Massachusetts 02543-1015, USA.
Molecular Biology and Evolution (Impact Factor: 14.31). 05/2001; DOI: 10.1093/oxfordjournals.molbev.a003832
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ABSTRACT We identified a novel gene encoding molecular chaperone HSP70 in the amitochondriate parasite Giardia lamblia. The predicted protein is similar to bacterial DnaK and mitochondrial HSP70s. The gene is transcribed and translated at a constant level during trophozoite growth and encystation. Alignment of the sequence with a data set of cytosolic, endoplasmic reticulum (ER), mitochondrial, and DnaK HSP70 homologs indicated that the sequence was extremely divergent and contained insertions unique to giardial HSP70s. Phylogenetic analyses demonstrated that this sequence was distinct from the cytosolic and ER forms and was most similar to proteobacterial and mitochondrial DnaKs. However, a specific relationship with the alpha proteobacterial and mitochondrial sequences was not strongly supported by phylogenetic analyses of this data set, in contrast to similar analyses of cpn60. These data neither confirm nor reject the possibility that this gene is a relic of secondary mitochondrial loss; they leave open the possibility that it was acquired in a separate endosymbiotic event.

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    • "Secondly, the extant eukaryotic lineages diverged after the emergence of the major bacterial lineages, presumably even after the mitochondrial endosymbiosis (Hedges et al., 2001; Hedges, 2002), whereas the SahH sequences reflect the evolution of Proteobacteria . Moreover, the probable lack of SahH in the amitochondriate G. lamblia (though its primary amitochondriate status has been questioned recently, Morrison et al., 2001) might suggest that early diverging eukaryotes were missing this enzyme, which would rather support their bacterial origin. It is also known that some metabolic genes in eukaryotes group with bacteria and some of them lack phylogenetic affinity to the a-Proteobacteria or cyanobacteria, suggesting that they originate from other bacterial groups. "
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    • "Some, such as those of Entamoeba and Cryptosporidium, have been shown to contain chaperonin Cpn60, a protein known to participate in the refolding of imported proteins (Sigler et al., 1998; Mai et al., 1999; Tovar et al., 1999; Riordan et al., 2003); mitosomes of Trachipleistophora contain a mitochondrialtype Hsp70 (mtHsp70), a molecular motor that helps internalize proteins into the organelle (Matouschek et al., 2000; Williams et al., 2002). Other proteins have also been suggested as putative mitosomal components in various amitochondrial lineages but their cellular localization has not been demonstrated experimentally (Clark & Roger, 1995; Bakatselou et al., 2000, 2003; Katinka et al., 2001; Morrison et al., 2001; Zhu & Keithly, 2002; Arisue et al., 2002). Perhaps the most significant finding in relation to the biology of mitosomes is the direct demonstration that Giardia mitosomes function in the biosynthesis of molecular iron–sulphur (Fe–S) clusters and in their subsequent incorporation into functional Fe–S proteins (Tovar et al., 2003). "
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