Evidence of a cellulosic layer in Pandoravirus tegument and the mystery of the genetic support of its biosynthesis

Abstract

11Pandoraviruses are giant viruses of amoebae with 1 µm-long virions. They have an ovoid 12 morphology and are surrounded by a tegument-like structure lacking any capsid protein nor any gene 13 encoding a capsid protein. In this work, we studied the ultrastructure of the tegument surrounding 14Pandoravirus massiliensis virions and noticed that this tegument is composed of a peripheral sugar 15 layer, an electron-dense membrane, and a thick electron-dense layer consisting in several tubules 16 arranged in a helicoidal structure resembling that of cellulose. Pandoravirus massiliensis particles 17 were stained by Calcofluor white, a fluorescent dye of cellulose, and the enzymatic treatment of 18 particles by cellulase showed the degradation of the viral tegument. We first hypothesized that the 19 cellulose tegument could be synthesized by enzymes encoded by Pandoravirus. Bioinformatic 20 analyses revealed in Pandoravirus massiliensis, a candidate gene encoding a putative cellulose 21 synthase, with a homology with the BcsA domain, one of the catalytic subunits of the bacterial 22 cellulose synthase, but with a low level of homology. This gene was transcribed during the 23 replicative cycle of Pandoravirus massiliensis, but several arguments run counter to this hypothesis. 24Indeed, even if this gene is present in other Pandoraviruses, the one of the strain studied is the only 25 one to have this BcsA domain and no other enzymes involved in the synthesis of cellulose could be 26 detected, although we cannot rule out that such genes could have been undetected among the large 27 proportion of Orfans of Pandoraviruses. As an alternative, we investigated whether Pandoravirus 28 could divert the cellulose synthesis machinery of the amoeba to its own account. Indeed, contrary to 29 what is observed in the case of infections with other giant viruses such as mimivirus, it appears that 30 the transcription of the amoeba, at least for the cellulose synthase gene, continues throughout the 31 growth phase of envelopes of Pandoravirus. Finally, we believe that this scenario is more plausible. If 32 confirmed, it could be a unique mechanism in the virosphere. 33

Deep Learning-based structural and functional annotation of Pandoravirus hypothetical proteins

“…For comparison, the genome of Staphylococcus aureus is ~2.9Mb in size, illustrating that Pandoravirus genomes have entered the size realm of prokaryotes (Baba et al, 2008). Pandoravirus morphology is unique amongst giant viruses, being ovoid in shape and possessing an apical pore which facilitates the passing of particle contents into the Acanthamoeba host cytoplasm (Philippe et al, 2013;Legendre et al, 2018;Brahim Belhaouari et al, 2019;Rolland et al, 2019). As of June 2022 the Pandoraviridae family consists of 17 members: P. pampulha, P. quercus, P. celtis, P. inopinatum, P. salinus, P. dulcis, P. massiliensis, P. neocaledonia, P. braziliensis, P. macleodensis, P. hades, P. persephone, P. japonicus, P. tropicalis, P. kadiweu, P. aubagnensis, P. belohorizontensis (Brahim Belhaouari et al, 2022).…”