Picobirnaviruses encode a protein with repeats of the ExxRxNxxxE motif

Picobirnaviruses possess a bisegmented double-stranded RNA genome. While the segment 2 encodes the RNA-dependent RNA polymerase, the segment 1 displays two open reading frames (ORFs). ORF2 was recently shown to code the capsid precursor and ORF1 product has not been characterized. In this study, we show that the three ORF1 sequences available in databases and representing three phylogenetically distant picobirnaviruses (two from human and one from rabbit hosts) encode proteins of various sizes (106-224 residues and without proline and cysteine) harbouring a particular sequence motif (ExxRxNxxxE) repeated four to ten times, depending on the virus species. Several algorithms predicted the three proteins to be mainly unfolded in the domains containing the repeats. The glycine-rich 25-40 amino acid long C-terminal domains containing hydrophobic residues with a periodicity of 3-4 residues are predicted structurally different of the upstream domains containing the motif repetitions. The ExxRxNxxxE sequence was not previously identified as a short linear motif in eukaryotic and prokaryotic proteins. Its function remains elusive.