Different isoforms and post‐translational modifications of human salivary acidic proline‐rich proteins

Abstract

The human salivary acidic proline‐rich proteins (aPRPs) complex was investigated by different chromatographic and mass spectrometric approaches and the main aPRPs, namely PRP‐1, PRP‐2 and PIF‐s (15 515 amu), Db‐s (17 632 amu) and Pa (15 462 amu) proteins, were detected. All these isoforms are phosphorylated at Ser‐8 and Ser‐22 and have a pyroglutamic moiety at the N‐terminus. Apart from Pa, all the other aPRPs undergo a proteolytic cleavage at Arg‐106 residue (Arg‐127 in Db‐s protein), that generates the small PC peptide (4371 amu) and PRP‐3, PRP‐4, PIF‐f (11 162 amu) and Db‐f (13 280 amu) proteins, all of which were detected. With regard to the Pa protein, the main form detected was the dimeric derivative (Pa 2‐mer, 30 922 amu) originated by a disulfide bond involving Cys‐103 residue. Besides these known isoforms, several previously undetected aPRP derivatives were found (in minor amounts): (i) the triphosphorylated derivatives of PRP‐1/PRP‐2/PIF‐s and Db‐s, showing the additional phosphate group at Ser‐17; (ii) the mono‐phosphorylated forms at either Ser‐22 or Ser‐8 of PRP‐1/PRP‐2/PIF‐s, PRP‐3/PRP‐4/PIF‐f, Db‐s and Db‐f; (iii) a nonphosphorylated form of PRP‐3/PRP‐4/PIF‐f; (iv) the triphosphorylated and diphosphorylated forms of Pa 2‐mer. Moreover, minor quantities of PRP‐3/PRP‐4/PIF‐f lacking the C‐terminal Arg (11 006 amu), and of Pa 2‐mer lacking the C‐terminal Gln (30 793 amu) were found. By this approach the different phenotypes of PRH1 locus in 59 different subjects were characterized.