Fig 1.

Identification of serine 222 as a phosphorylation acceptor. (A) A comprehensive list of NS5A peptides identified following LC-MS-MS analysis. Peptide sequences are listed along with their theoretical mass, theoretical and observed m/z values, and associated mass error (ppm). The charge state, start residue of each peptide, and identified modifications are also provided. Modifications observed are indicated, including the phosphorylation of serine 222. (B) MS-MS spectrum of phosphorylated NS5A peptide, residues 221 to 240. The [M + 2H]⁺² ion of the phosphorylated NS5A peptide (m/z 1012.47) was selected for dissociation, and the amino acid sequence of the peptide is shown above the spectrum. The masses above and below the sequence correspond to the theoretical, singly protonated b- and y-type product ions, respectively. The observed singly protonated product ions are underlined, and doubly protonated ions are denoted with filled circles. Asterisks indicate product ions that result from neutral loss of H₃PO₄. As indicated in the sequence, the phosphorylated residue was identified as serine residue 222.