Evolution of the hepatitis E virus polyproline region: order from disorder.

The hepatitis E virus (HEV) polyproline region (PPR) is an intrinsically unstructured region (IDR). This relaxed structure allows IDRs, which are implicated in the regulation of transcription and translation, to bind multiple ligands. Originally the nucleotide variability seen in the HEV PPR was assumed to be due to high rates of insertion and deletion. This study shows that the mutation rate is about the same in the PPR as in the rest of the nonstructural polyprotein. The difference between the PPR and the rest of the polyprotein is due to the higher tolerance of the PPR for substitutions at the first and second codon positions. With this higher promiscuity there is a shift in nucleotide occupation of these codons leading to translation of more cytosine residues: a shift that leads to more proline, alanine, serine, and threonine being encoded rather than histidine, phenylalanine, tryptophan, and tyrosine. This pattern of amino acid usage is typical of proline-rich IDRs. Increased usage of cytosine also leads to >22% of all amino acids in the PPR being prolines. Alignments of PPR sequences from HEV strains representing all genotypes indicate that all zoonotic isolates share an ancestor, and the carboxyl half of the PPR is more tolerant of mutations than the amino half. The evolution of HEV PPR, in contrast with that of the rest of the nonstructural polyprotein, is molded by pressures that lead toward increased proline usage with a corresponding decrease in the usage of aromatic amino acids, favoring formation of IDR structures.