A hidden gene in astroviruses encodes a cell-permeabilizing protein involved in virus release

Valeria Lulla; Andrew E. Firth

The data presented here demonstrate the existence of an additional protein, XP, encoded within the human astrovirus genome. XP is important for virus growth, localizes to the plasma membrane, and plays a role in virus release. Viroporins have been reported for many enveloped and non-enveloped viruses and -although they can play roles in virus entry and modulation of cellular pathways -most often they facilitate virus assembly or release 26 . However, no viroporin candidate had been previously predicted for astroviruses. Identifying a viroporin is challenging due to the lack of homology among viroporins from different viruses. Viroporins are typically small hydrophobic integral membrane proteins of around 100 aa in size, with two motifs -an amphipathic α-helix and an adjacent cluster of positively charged residues; mutation of these residues generally abolishes viroporin activity 27 . Using two different assays and . CC-BY 4.0 International license author/funder. It is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the . https://doi.org/10.1101/661579 doi: bioRxiv preprint computational approaches, we show that XP fulfils these criteria. XP is capable of permeabilizing cellular membranes and has a distinct N-terminal extracellular topology with one TM domain, thus making XP a candidate class IA viroporin, a class which also includes the influenza A virus M2, coronavirus E and HIV-1 Vpu proteins 26, 28 . Future work will be needed to confirm potential ion channel activity, and characterize ion specificity, structural organisation, and processes affected by XP expression in the context of viral infection. The localization of XP not only at the plasma membrane but also in the perinuclear region raises the possibility that XP may also have additional functions. Whereas the C-terminal α-helix appears to be associated with the cell-permeabilizing activity, additional functions (if any) of the extended N-terminal domain of XP remain to be studied.