High‐resolution high‐performance liquid chromatography with electrospray ionization mass spectrometry and tandem mass spectrometry characterization of a new isoform of human salivary acidic proline‐rich proteins named Roma‐Boston Ser22(Phos) → Phe variant

During a survey of human saliva by a top‐down reversed‐phase high‐performance liquid chromatography with electrospray ionization mass spectrometry approach, two proteins eluting at 27.4 and 28.4 min, with average masses of 15 494 ± 1 and 11 142 ± 1 Da, were detected in a subject from Boston. The Δmass value (4352 Da) of the two proteins was similar to the difference in mass values between intact (150 amino acids, a.a.]) and truncated acidic proline‐rich proteins (aPRPs; 106 a.a.) suggesting an a.a. substitution in the first 106 residues resulting in a strong reduction in polarity, since under the same experimental conditions aPRPs eluted at ～22.5 min (intact) and 23.5 min (truncated forms). Manual inspection of the high‐resolution high‐performance liquid chromatography with electrospray ionization tandem mass spectra of the truncated isoform showed the replacement of the phosphorylated Ser‐22 in PRP‐3 with a Phe residue. Inspection of the tandem mass spectra of the intact isoform confirmed the substitution, which is allowed by the code transition TCT→TTT and is in agreement with the dramatic increase in elution time. The isoform was also detected in two other subjects, one from Boston (unrelated to the previous) and one from Rome. For this reason we propose to name this variant PRP‐1 (PRP‐3) RB (Roma‐Boston) Ser₂₂(phos)→Phe.