VIRiON

The first "giant" virus, Mimivirus was discovered in 2003. Its capsid, easily visualized by light microscopy was found to contain a 1.2-Mb-genome as complex as that of many bacteria. In the last 5 years, 3 additional families of giant viruses have been discovered. The amphora-shaped Pandoraviridae have genomes in the 1.5-2.8 Mb range possibly encoding 1500 to 2500 proteins most of them unique to this family. The first representative of two other families, Mollivirus and Pithovirus sibericum were both revived from a 30,000-year-old permafrost layer. They have 600 kb DNA genomes predicted to encode about 500 proteins, 2/3rds of which are again unique to each family and absent from the cellular world. This rapid pace of unexpected discoveries strongly suggests that the viral world remains an untapped reservoir of unknown biochemical processes, cellular manipulation tools and enzymes. The discovery of 4 distinct families of giant viruses very different by their genome structures and contents as well as their replication cycles, revived the question on the origin of viruses in general and on the role they might have played in the emergence and evolution of life on earth. The paucity of shared genes between different groups of giant viruses rules out their origin from a common giant ancestor. We thus hypothesized that these four virus lineages might have originated from different ancestral "protocell" types that were once in competition prior to the emergence of the Last Universal Cellular Ancestor (LUCA) that led to the Eubacteria, Archaea and Eukarya. These ancestral lineages would have lost the race, only managing to survive by becoming parasites (or symbionts) of the winning cellular lineage that led to LUCA, finally giving raise to the diversity of today’s DNA viruses. Following a billion years of co-evolution in a variety of extinct organisms, today's DNA viruses could thus have inherited a vast array of different metabolic pathways absent from the modern cellular world. In the context of the "iron-sulfur world" hypothesis for the development of the earliest life forms, the characterization of iron-sulfur proteins found in giant viruses may shed light on fundamental processes at the origin of life. The present project thus proposes to focus on the cysteine-rich proteins which may present original iron-sulfur coordination patterns, as they could be the remnant of ancestral metabolic pathways bequeathed by the ancestors of these viruses during their transition from proto-cell state to obligate parasite of the extant cellular world. We will combine "omics" techniques with interdisciplinary approaches including bioinformatics, cell biology, biochemistry, physical chemistry and biophysics to elucidate the role of these proteins in the physiology of giant viruses. We will first develop the methodologies necessary to study iron-sulfur proteins by characterizing the most expressed protein during the Mimiviridae infectious cycle, the conserved GG-FeS protein. This 59 amino-acids long protein is also present in the virions and thus important for the initiation of the infectious cycle and presents an atypical Fe-S signature. To achieve its functional characterization we will combine in vitro with in vivo studies, physical chemistry analysis, biochemistry (protein expression and interaction studies), biophysics (NMR, EPR, Mössbauer, X-ray, Microscopy). We will use bioinformatics to select a subset of the most intriguing/promising cysteine-rich proteins encoded by the 4 families of giant viruses and apply the knowledge we developed on the GG-FeS family of proteins to characterize them using the same combination of approaches. We expect this project to lead to the discovery of original metabolic pathways that may in turn find important applications in biotechnology and biomedicine.