Table 2.

MS assignments of measured peptides containing Thr-446 and Thr-451a

PKR alleleMeasured mass (Da)Calculated mass (Da)Sequence (residues)Comments
Wild typeNot observed1,977.3430–447Tryptic peptide; unphosphorylated peptide of residues 430 to 447 was not detected
Wild type2,057.2b , c , d 2,057.3b 430–447 + PO3 Tryptic peptide; Thr-437, Ser-438, or Thr-446 could be phosphorylated; MS-MS sequencing of this peptide unambiguously identified Thr-446 as the site of phosphorylation
Wild type1,717.9b 1,720.0b 430–445Tryptic peptide; Thr-437 and Ser-438 were not phosphorylated
Wild type2,060.2b , d 2,060.3b 450–467Lys-C peptide; Thr-451 was not phosphorylated; verified by MS-MS sequencing
K296R2,525.9b 2,525.8b 442–463Asp-N peptide; Thr 446 and Thr 451 were not phosphorylated
  • a We were able to identify predicted peptides covering 79% of the entire PKR sequence.

  • b Averaged mass.

  • c This peptide loses 80 Da upon CIP treatment of the total trypsin digest.

  • d Peptides sequenced by MS-MS.