Euglena fatty acid synthetase multienzyme complex is a unique structure

Evolution of metabolic capabilities and molecular features of diplonemids, kinetoplastids, and euglenids

Background

The Euglenozoa are a protist group with an especially rich history of evolutionary diversity. They include diplonemids, representing arguably the most species-rich clade of marine planktonic eukaryotes; trypanosomatids, which are notorious parasites of medical and veterinary importance; and free-living euglenids. These different lifestyles, and particularly the transition from free-living to parasitic, likely require different metabolic capabilities. We carried out a comparative genomic analysis across euglenozoan diversity to see how changing repertoires of enzymes and structural features correspond to major changes in lifestyles.

Results

We find a gradual loss of genes encoding enzymes in the evolution of kinetoplastids, rather than a sudden decrease in metabolic capabilities corresponding to the origin of parasitism, while diplonemids and euglenids maintain more metabolic versatility. Distinctive characteristics of molecular machines such as kinetochores and the pre-replication complex that were previously considered specific to parasitic kinetoplastids were also identified in their free-living relatives. Therefore, we argue that they represent an ancestral rather than a derived state, as thought until the present. We also found evidence of ancient redundancy in systems such as NADPH-dependent thiol-redox. Only the genus Euglena possesses the combination of trypanothione-, glutathione-, and thioredoxin-based systems supposedly present in the euglenozoan common ancestor, while other representatives of the phylum have lost one or two of these systems. Lastly, we identified convergent losses of specific metabolic capabilities between free-living kinetoplastids and ciliates. Although this observation requires further examination, it suggests that certain eukaryotic lineages are predisposed to such convergent losses of key enzymes or whole pathways.

Conclusions

The loss of metabolic capabilities might not be associated with the switch to parasitic lifestyle in kinetoplastids, and the presence of a highly divergent (or unconventional) kinetochore machinery might not be restricted to this protist group. The data derived from the transcriptomes of free-living early branching prokinetoplastids suggests that the pre-replication complex of Trypanosomatidae is a highly divergent version of the conventional machinery. Our findings shed light on trends in the evolution of metabolism in protists in general and open multiple avenues for future research.

Early catalytic steps of Euglena gracilis chloroplast type II fatty acid synthase

Euglena gracilis is a very ancient eukaryote whose chloroplast acquisition and evolution has been independent of higher plants. The organism in unique in possessing two de novo fatty acid synthases, a true multienzyme complex of great size in the cytosol and a plastid-localized type II fatty acid synthase composed of discrete enzymes and acyl carrier protein (ACP). The enzymology of the early steps of fatty acid biosynthesis differed in the Euglena type II fatty acid synthase compared to those of Escherichia coli and plants. The enzymes of Euglena participating in both priming and elongation reactions to form a new carbon-carbon bond were acetyl-CoA-ACP transacylase, malonyl-CoA-ACP transacylase, and beta-ketoacyl-ACP synthase I. The effects of inhibitors on the three different enzymes were noted. All carbon-carbon bond formation was inhibited by cerulenin. Although neither fatty acid biosynthesis nor any of the isolated enzymes were sensitive to diisopropylphosphofluoridate, the three Euglena enzymes studied were sensitive to different sulfhydryl-alkylating agents. Acetyl-ACP supported fatty acid biosynthesis as effectively as did comparable amounts of ACPSH and acetyl-CoA. There was no evidence for a beta-ketoacyl-ACP synthase III for priming such as has been reported in type II fatty acid synthase of higher plants and bacteria. The roles of the acetyl-CoA-ACP transacylase and beta-ketoacyl-ACP synthase I appear to be unique in the type II fatty acid synthase of Euglena. Acetyl-CoA-ACP transacylase, malonyl-CoA-ACP transacylase, and beta-ketoacyl-ACP synthase I were separated from one another and shown to have different molecular weights.

Isolation of a novel type-I fatty-acid synthetase from Euglena gracilis. Specific derepression in streptomycin-bleached cells

A novel, high-molecular-mass fatty-acid synthetase (FAS) complex has been isolated from streptomycin-bleached Euglena gracilis cells. The enzyme was purified 250-fold from the crude cell homogenate and subsequently migrated upon SDS/PAGE as a single band of molecular mass 270 kDa. This apparent subunit size of the purified protein contrasted with a smaller size of only 200 kDa which was exhibited by the same protein upon immunoblotting of the crude cell extract. The purified Euglena FAS complex cosediments in a sucrose density gradient with yeast FAS and, from this, both enzymes were concluded to have the same overall molecular mass of 2.3 MDa. The enzyme described in this paper appears to be a typical type-I FAS multienzyme which clearly differs from the E. gracilis FAS so far described. Instead, it appears to be organized structurally similar to the type-I FAS multienzymes of lower fungi. In vitro, the purified Euglena FAS complex synthesizes mainly palmitic acid, or its CoA ester, from acetyl CoA and malonyl CoA as substrates. The Km values for acetyl CoA and malonyl CoA are 20 microM and 31 microM, respectively. Similar to the FAS enzymes of other lower eucaryotes, the Euglena type-I FAS is a flavoprotein. In contrast to yeast FAS, however, the flavin cofactor appears to be covalently attached to the enzyme protein. By immunological techniques, the enzyme was shown to be absent in green as well as in etiolated E. gracilis cells, while being rapidly induced upon streptomycin bleaching of heterotrophically growing green cells. The data suggest an inverse correlation between organellar development and derepression of this FAS complex.

Purification and partial characterization of acyl carrier protein from Euglena gracilis variety bacillaris

Acyl carrier protein (ACP) was purified from Euglena gracilis variety bacillaris in yields of about 1 mg/100 g (wet wt) of cells. Antibodies against the purified protein were raised in hens and isolated from eggs. Antibodies raised against Euglena ACP inhibited the Euglena chloroplast nonaggregated fatty acid synthetase using either Euglena or Escherichia coli ACP as a substrate. Comparisons with other ACPs included the following items: biologic activity, acidic pI, size, behavior in size exclusion media, and amino acid sequence of the N-terminal portion of the molecule.

Acyl carrier protein interacts with melittin

Acyl carrier protein (ACP) from Escherichia coli has been shown to form complexes with melittin, a cationic peptide from bee venom. ACP is a small (Mr 8847), acidic, Ca2(+)-binding protein, which possesses some characteristics resembling those of regulatory Ca2(+)-binding proteins including interaction with melittin. Complexing between melittin and ACP which occurred both in the presence and absence of Ca2+ was evident by chemical cross-linking the two peptides, fluorescence changes (including anisotropy measurements), and inhibition by melittin of the activity of a nonaggregated fatty acid synthetase from Euglena. Also, anti-Apis mellifera antibodies which contained antibodies against melittin specifically inhibited the same enzyme system activity relative to non-immune IgG.

Behavior of acyl carrier proteins on western blots

Acyl carrier proteins (ACPs) from Escherichia coli and Euglena were analyzed on Western blots using rabbit antibodies raised against E. coli ACP. Euglena ACP, unlike that from E. coli, behaves upon electrophoresis under denaturing conditions as its size would predict. Oligomeric forms of both ACPs were evident on Western blots, but the bacterial ACP had more tendency to aggregate. That the oligomeric forms were not due to impurities was shown by their regeneration from low-Mr protein, reaction with antibodies isolated from low-Mr protein, and by molecular weight determination of the ACP by low-angle laser light scattering.

Chemical cross-linking and its effect on fatty acid synthetase activity in intact chloroplasts from Euglena gracilis

Intact chloroplasts were isolated from Euglena gracilis variety bacillaris, aliquots were exposed to several different chemical cross-linking reagents. The reagents penetrated the triple membrane of Euglena chloroplasts. This was shown by gradient acrylamide gel electrophoresis under denaturing conditions. The activity of the nonaggregated fatty acid synthetase of Euglena was located within the chloroplast stroma, and the effects of dimethylsuberimidate cross-linking on the activity of the enzyme system were examined. The acyl-carrier protein concentration in the chloroplast was measured at about 0.24 mM.