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.

Protein-sequence polymorphisms and post-translational modifications in proteins from human saliva using top-down Fourier-transform ion cyclotron resonance mass spectrometry

“…Intact protein maps have been developed and have lead to the recognition that although there are only a few parent genes there is a tremendous diversity of protein/peptide products depending upon post-translational processing [15]. Comparisons between individuals has further shown a variety of human alleles with both sequence and downstream processing diversity [16]. The net result is that a single liquid chromatography mass spectrometry experiment on a quadrupole instrument (100 ppm) reveals a wide range of proteins each with an apparently unique intact mass tag (IMT).…”

Human Salivary Acidic Proline-Rich Proteins (APRP-1/2) In Adult Patients With Dental Caries

“…Currently, a group of 10 distinct APRPs can be distinguished (APRP-1/2, APRP-3/4, parotid isoelectric-focusing slow/fast variant-PIF-s/f, parotid acidic protein -Pa, the double-band slow/fast isoform-Db-s/f and PC peptide), encoded by two genetic loci, PRH1 and PRH2 on chromosome 12 at p13.2 [4]. The PRH2 locus has two alleles and is responsible for the synthesis of principal isoforms of APRP-1/2 representing precursors of APRP-3/4 [3,5]. The PRH1 locus has three different alleles encoding the PIF-s, Pa and Db-s isoforms [5].…”

“…APRPs belong to the highly unique family of proline-rich proteins -PRP, characterized by high proline content (25 to 42% of total amino acids), which affects protein conformation with a prominent preference of β-sheet structure [2,3]. Currently, a group of 10 distinct APRPs can be distinguished (APRP-1/2, APRP-3/4, parotid isoelectric-focusing slow/fast variant-PIF-s/f, parotid acidic protein -Pa, the double-band slow/fast isoform-Db-s/f and PC peptide), encoded by two genetic loci, PRH1 and PRH2 on chromosome 12 at p13.2 [4].…”

Despite sequence homologies to gluten, salivary proline-rich proteins do not elicit immune responses central to the pathogenesis of celiac disease

“…PRPs are posttranslationally modified within the gland and furthermore undergo excessive degradation after secretion in the oral cavity by microbial proteases (16, 37). As a result, human saliva contains a diverse mixture of acidic and basic PRP-derived proteins and peptides (17,20,21).It has been hypothesized that structural mimicry between gluten and other proteins may be a factor in the etiology of CD. Kagnoff and coworkers (23) noted earlier structural similarities between a 12-amino acid residue peptide from E1b protein from adenovirus and ␣-gliadin.…”

“…PRPs are posttranslationally modified within the gland and furthermore undergo excessive degradation after secretion in the oral cavity by microbial proteases (16, 37). As a result, human saliva contains a diverse mixture of acidic and basic PRP-derived proteins and peptides (17,20,21).…”