Sawyeria marylandensis (Heterolobosea) Has a Hydrogenosome with Novel Metabolic Properties

Abstract

Protists that live under low-oxygen conditions often lack conventional mitochondria and instead possess mitochondrion-related organelles (MROs) with distinct biochemical functions. Studies of mostly parasitic organisms have suggested that these organelles could be classified into two general types: hydrogenosomes and mitosomes. Hydrogenosomes, found in parabasalids, anaerobic chytrid fungi, and ciliates, metabolize pyruvate anaerobically to generate ATP, acetate, CO 2 , and hydrogen gas, employing enzymes not typically associated with mitochondria. Mitosomes that have been studied have no apparent role in energy metabolism. Recent investigations of free-living anaerobic protists have revealed a diversity of MROs with a wider array of metabolic properties that defy a simple functional classification. Here we describe an expressed sequence tag (EST) survey and ultrastructural investigation of the anaerobic heteroloboseid amoeba Sawyeria marylandensis aimed at understanding the properties of its MROs. This organism expresses typical anaerobic energy metabolic enzymes, such as pyruvate:ferredoxin oxidoreductase, [FeFe]-hydrogenase, and associated hydrogenase maturases with apparent organelle-targeting peptides, indicating that its MRO likely functions as a hydrogenosome. We also identified 38 genes encoding canonical mitochondrial proteins in S. marylandensis, many of which possess putative targeting peptides and are phylogenetically related to putative mitochondrial proteins of its heteroloboseid relative Naegleria gruberi. Several of these proteins, such as a branched-chain alpha keto acid dehydrogenase, likely function in pathways that have not been previously associated with the well-studied hydrogenosomes of parabasalids. Finally, morphological reconstructions based on transmission electron microscopy indicate that the S. marylandensis MROs form novel cup-like structures within the cells. Overall, these data suggest that Sawyeria marylandensis possesses a hydrogenosome of mitochondrial origin with a novel combination of biochemical and structural properties.It is now widely accepted that the most recent common ancestor of extant eukaryotes possessed an endosymbiont-derived mitochondrial organelle of alphaproteobacterial ancestry (see reference 16 for a recent review). Although most wellknown eukaryotes contain mitochondria that aerobically respire to produce ATP, a vast diversity of anaerobic eukaryotic lineages have been discovered that lack classical mitochondrial structures. Instead of mitochondria, biochemically diverse double-membrane-bounded organelles have been found that function under low-oxygen conditions (for recent reviews, see references 5, 6, 17, 23, 54, and 57). The discovery and recent investigations of the functions of these mitochondrion-related organelles (MROs) has greatly expanded our understanding of both the conservation and diversity of functions that these organelles can perform.A number of protist lineages, including the parabasalids, several anaerobic ciliate groups, and anaerobic chy...

Biochemistry and Evolution of Anaerobic Energy Metabolism in Eukaryotes

“…4. Abbreviations: CI to CIV, respiratory complexes I to IV; UQ, ubiquinone; RQ, rhodoquinone; C, cytochrome c; A, ATPase; FRD, fumarate reductase; [2B], acetate:succinate CoA-transferase (subfamily 1B); [3], succinylCoA thiokinase; [5], malic enzyme; [6], pyruvate dehydrogenase complex; [8], fumarase; [9], pyruvate kinase; [11], malate dehydrogenase; [14], methylmalonylCoA mutase; [15], methylmalonyl-CoA epimerase; [16], propionyl-CoA carboxylase; [17], phosphoenolpyruvate carboxykinase (ITP/GTP dependent); [18], glutamate:pyruvate transaminase; [19], glutamate:oxaloacetate transaminase; [26], strombine dehydrogenase; [27] Fig. 9 is based.…”

“…Our focus here is on organisms whose metabolisms have been investigated. Genome sequencing and expressed sequence tag (EST) data are becoming available for many excavate taxa, which allow inferences about biochemical functions in organelles, for example, the hydrogenosome-like organelles of Sawyeria marylandensis (27), the hydrogenosomes of Psalteriomonas lanterna (102), or the organelle of mitochondrial origin in Naegleria gruberi (149,161,371), but there are insufficient direct experimental data available concerning enzyme activities and excreted end products to generate metabolic maps for these organisms.…”

Diversity and origins of anaerobic metabolism in mitochondria and related organelles

“…R. Soc. B 370: 20140326 multiple times in different eukaryote lineages via LGT [34,41,89,90,99,[118][119][120][121].…”

Diversity of Heterolobosea

“…The first lineage is represented by the extreme halophile Pleurostomum flabellatum, the second one is more diversified and comprises Psalteriomonas lanterna, Sawyeria marylandensis, Monopylocystis visvesvarai, and most probably also Percolomonas descissus, Lyromonas vulgaris and Vahlkampfia anaerobica (Broers et al, 1990(Broers et al, , 1993O´Kelly et al, 2003;Smirnov & Fenchel, 1996). Mitochondrial derivates of Psalteriomonas lanterna and Sawyeria marylandensis were studied in detail and it was shown that they have been transformed to hydrogenosomes (Barberà et al, 2010;de Graaf et al, 2009). Interestingly, presumably aerobic Naegleria gruberi recently appeared to be a facultatively anaerobic protist.…”