Tricarboxylic acid cycle and proton gradient inPandoravirus massiliensis: Is it still a virus?

Abstract

Since the discovery of Acanthamoeba polyphaga Mimivirus, the first giant virus of amoeba, the historical hallmarks defining a virus have been challenged. Giant virion sizes can reach up to 2.3 µm, making them visible by optical microscopy. They have large genomes of up to 2.5 Mb that encode proteins involved in the translation apparatus. Herein, we investigated possible energy production in Pandoravirus massiliensis, the largest of our giant virus collection. MitoTracker and TMRM mitochondrial membrane markers allowed for the detection of a membrane potential in virions that could be abolished by the use of the depolarizing agent CCCP. An attempt to identify enzymes involved in energy metabolism revealed that 8 predicted proteins of P. massiliensis exhibited low sequence identities with defined proteins involved in the universal tricarboxylic acid cycle (acetyl Co-A synthase; citrate synthase; aconitase; isocitrate dehydrogenase; α-ketoglutarate decarboxylase; succinate dehydrogenase; fumarase). All 8 viral predicted ORFs were transcribed together during viral replication, mainly at the end of the replication cycle. Two of these proteins were detected in mature viral particles by proteomics. The product of the ORF132, a predicted protein of P. massiliensis, cloned and expressed in Escherichia coli, provided a functional isocitrate dehydrogenase, a key enzyme of the tricarboxylic acid cycle, which converts isocitrate to α-ketoglutarate. We observed that membrane potential was enhanced by low concentrations of Acetyl-CoA, a regulator of the tricarboxylic acid cycle. Our findings show for the first time that energy production can occur in viruses, namely, pandoraviruses, and the involved enzymes are related to tricarboxylic acid cycle enzymes. The presence of a proton gradient in P. massiliensis coupled with the observation of genes of the tricarboxylic acid cycle make this virus a form a life for which it is legitimate to question ‘what is a virus?’.

High Transcriptional Activity and Diverse Functional Repertoires of Hundreds of Giant Viruses in a Coastal Marine System

“…isolate has suggested that these enzymes have activity consistent with bioinformatic predictions, and that some are packaged into virions and would therefore influence host physiology soon after initial infection (57). Together with our results, this suggests that viral manipulation of central metabolic processes is a widespread mechanism employed by many Nucleocytoviricota, and that this is a common occurrence in viral infections in the ocean.…”

“…We detected expression of many of these central metabolic enzymes throughout the sampling period; genes involved in glycolysis, the TCA cycle, the pentose phosphate pathway, and beta oxidation were all present in the metatranscriptomes, and most were consistently expressed across timepoints, especially in Mimiviridae clades (Fig 4, Fig S2). A recent study of TCA cycle enzymes encoded by a Pandoravirus isolate has suggested that these enzymes have activity consistent with bioinformatic predictions, and that some are packaged into virions and would therefore influence host physiology soon after infection (54). Together with our results, this suggests that viral manipulation of central metabolic processes is a widespread mechanism employed by many Nucleocytoviricota , and that this is a common occurrence in viral infections in the ocean.…”

Rhizomal Reclassification of Living Organisms

“…Giant viruses are viruses with a very large physical size (larger than some bacteria); they have a large genome compared to classical viruses (up to 1 to 2 megabases), as well as both types of nucleic acids. In addition, a recent study has shown that some giant viruses also have enzymes capable of producing energy (with the detection of the tricarboxylic acid cycle and proton gradient in Pandoravirus massiliensis) [ 75 ]. For this reason, only giant viruses were analyzed in this work.…”

Host Range and Coding Potential of Eukaryotic Giant Viruses

“…Such control can be transcriptional regulation of nucleus- and mitochondrion-encoded genes related to energy metabolism, as in coccolithovirus [ 129 ]. Recently reported genome sequences of environmental MAGs [ 17 ] or isolated viruses even encode their own genes related to glycolysis, tricarboxylic acid cycle, succinate dehydrogenase, β-oxidation, and photosynthesis [ 17 , 77 , 81 , 101 ]. Genes encoding enzymes in cellular fermentation, such as pyruvate formate-lyase, have been found in Tetraselmis virus infecting green algae [ 73 ], which have anaerobic energy metabolism in low-oxygen condition [ 130 ].…”