Fig. 4

Human S27 harbors several methylated cysteines in its Zn-finger domain. a Intact mass spectra of S27 reveals three proteoforms with distinct masses (left). Based on the mass differences from the sequence mass of S27 these are identified as di- and tri-methylated forms of the protein (S27-Me₂ and S27-Me₃) and a third proteoform carrying an unknown modification (S27-UNK). An additional lower abundant disulfide-linked form was detected in fragmentation maps of the trimethylated form (S27-Me₃-SS). b Top-down analysis of the S27-Me₃ fragmentation spectra pinpoints the methylation to the cysteine residues 40, 56, and 59. Fragments are mapped along the protein sequence (vertical bars) and methylated amino acids are identified by a mass shift of 14 Da. c Structure of the human 40S subunit with protein S27 in green and its cysteine containing zinc finger domain in magenta (PDB 5A2Q). The zinc-finger cysteines are shown as sticks and the methylated cysteine residues are labeled