Biosynthesis of salivary proteins in the parotid gland of the subhuman primate, Macaca fascicularis. Cell-free translation of the mRNA for a proline-rich glycoprotein and partial amino acid sequence and processing of its signal peptide

The major anionic proline-rich proteins in the parotid and submandibular secretions of subhuman primates and man perform the important biological function of inhibiting crystal growth of calcium phosphate salts from saliva, which is supersaturated with calcium phosphate salts, thereby preventing excess deposition of hydroxylapatite on tooth surfaces. The present work was initiated as a first step towards investigating proline-rich protein biosynthesis in parotid glands using the subhuman primate, Macaca fascicularis, as a model system. RNA was isolated from macaque parotid glands and separated into poly(A)-enriched and poly(A)-deficient fractions by chromatography on oligo(dT)-cellulose. The mRNAs in both fractions promoted incorporation of radiolabeled amino acids into polypeptides in an mRNA-dependent reticulocyte lysate translation system. Five major proline-rich polypeptides were detected and one of these was shown to be the in vitro precursor of the major anionic macaque proline-rich protein (MPRP), which is the structural and functional counterpart of the major anionic proline-rich proteins in the parotid and submandibular secretions of man (Oppenheim, F.G., Offner, G.D., and Troxler, R.F. (1982) J. Biol. Chem. 257, 9271-9282). Radiosequencing of the material in anti-MPRP immune precipitates showed that the in vitro precursor of MPRP contained an 18-residue signal peptide. The in vitro precursor of MPRP was processed in dog pancreas vesicles to a form with a lower apparent Mr and with an NH2-terminal amino acid sequence identical to that of native MPRP. The phenylthiohydantoin derivatives of Ala and Ile were detected at residue 9 and those of Val and Met were detected at residue 16 of the signal peptide. This indicated that the in vitro precursor of MPRP, which migrated electrophoretically as a single band in anti-MPRP immune precipitates, contained two different in vitro polypeptides derived from two different mRNAs. These results are discussed in the context of the genetic polymorphism among the major anionic proline-rich proteins in the parotid and submandibular secretions of man.     

Immunological comparison of proline-rich proteins from human and primate parotid secretion.

Antisera raised in response to proline-rich proteins purified from parotid secretions of man and the primate Macaca fascicularis were employed to investigate the interrelationships of these proteins by immunodiffusion, immunoelectrophoresis and the combined use of disc gel acrylamide electrophoresis with radial immunodiffusion. The major human proline-rich proteins, PRP I, PRP II, PRP III and PRP IV as well as several minor proline-rich proteins cross-react with antiserum to PRP I or PRP III. Similarly primate parotid saliva contains several components cross-reacting with antiserum directed against a purified primate proline-rich protein, MPRP. Antiserum to PRP I or PRP III cross-reacted with MPRP and primate parotid saliva protein, whereas antiserum to MPRP cross-reacted only with human parotid saliva protein and not with the isolated human proline-rich proteins. The immunological relationships of these salivary proline-rich proteins within and between species suggest their origin from a common precursor molecule.     

Proteome analysis of glandular parotid and submandibular-sublingual saliva in comparison to whole human saliva by two-dimensional gel electrophoresis

The secretions of the salivary parotid and submandibular-sublingual (SMSL) glands constitute the main part of whole human saliva (WS) in which proline-rich proteins (PRPs) and mucins represent dominant groups. Although proteome analysis had been performed on WS, no identification of PRPs or mucins by 2-DE and MS was achieved in WS and no comprehensive analysis of both glandular secretions is available so far. The aim of this study was to compare the protein map of WS to parotid and SMSL secretions for the display of PRPs and mucins. WS and glandular secretions were subjected to 2-DE and spots were analyzed by MALDI-MS. New components identified in WS were cyclophilin-B and prolyl-4-hydroxylase. Also acidic and basic PRPs as well as the proline-rich glycoprotein (PRG) could now be mapped in WS. Acidic PRPs were found equally in parotid and SMSL secretions, whereas basic PRPs and PRG were found primarily in parotid secretion. Salivary mucin MUC7 was identified in SMSL secretion. Thus, the more abundant proteins of WS can be explained mainly by mixed contributions of parotid and SMSL secretions with only few components remaining that may be derived from local sources in the oral cavity.     

The presence and origin of phosphopeptides in human saliva.

The presence of phosphopeptides in whole saliva (saliva expectorated from the mouth) was demonstrated and their origin was evaluated. Whole saliva contained much larger numbers of small phosphopeptides than are found in the glandular secretions. Most of these originated from the acidic proline-rich proteins (PRPs) in the major salivary glands and were formed, after secretion into the oral cavity, as a result of rapid degradation by proteolytic enzymes from extraglandular sources contained in sediment from whole saliva. Some peptides may have been formed by cleavage of basic PRPs, but other phosphoproteins apparently contributed little to the observed phosphopeptides. Most of the enzymes that produced phosphopeptides are serine proteinases. The gel-electrophoretic band patterns of the phosphopeptides obtained from 26 individuals of various acidic-PRP phenotypes were remarkably similar, demonstrating that the enzymes responsible were generally present in the population surveyed and that similar cleavages occur regardless of the nature of the acidic PRPs. Many of these peptides were N-terminal proteolytic cleavage products of acidic PRPs. The N-terminal phosphorylated region of acidic PRPs contains various biological activities, such as inhibition of hydroxyapatite formation, calcium binding and binding to hydroxyapatite, the major mineral of teeth. The demonstration of these phosphopeptides in the saliva that is in contact with the oral surface may therefore be of biological importance.     

PRGL: A cell wall proline-rich protein containning GASA domain in Gerbera hybrida

PRPs (proline-rich proteins) are a group of cell wall proteins characterized by their proline and hy- droproline-rich repetitive peptides. The expression of PRPs in plants is stimulated by wounding and environmental stress. GASA (gibberellic acid stimulated in Arabidopsis) proteins are small peptides sharing a 60 amino acid conserved C-terminal domain containing twelve invariant cysteine residues. Most of GASAs reported are localized to apoplasm or cell wall and their expression was regulated by gibberellins (GAs). It has been reported that, in French bean, these two proteins encoding by two distinct genes formed a two-component chitin-receptor involved in plant-pathogen interactions when plant was infected. We cloned a full-length cDNA of PRGL (proline-rich GASA-like) gene which encodes a protein containing both PRP and GASA-like domains. It is demonstrated that PRGL is a new protein with characteristics of PRP and GASA by analyzing its protein structure and gene expression.     

Isolation and nucleotide sequence of cDNA clone for bovine pancreatic anionic trypsinogen. Structural identity within the trypsin family.

A cDNA clone encoding an anionic form of bovine trypsinogen was isolated from a pancreatic cDNA library. The corresponding 855-nucleotide mRNA contains a short 5' noncoding region of 8 nucleotides and a long 3' noncoding region of 56 nucleotides in addition to a poly(A) tail of at least 50 nucleotides. The deduced amino acid sequence for the anionic pretrypsinogen (247 residues) includes the N-terminal 15-amino-acid signal peptide followed by an 8-amino-acid activation peptide. The zymogen (232 residues) contains an additional C-terminal serine, compared with the amino acid sequence of bovine cationic trypsinogen. The identity between the anionic and cationic forms of bovine trypsinogen (65%) is lower than that existing between the anionic protein and other mammalian anionic trypsinogens (73-85%), suggesting that trypsin gene duplication in mammals occurred prior to the evolutionary events responsible for the species divergence. Bovine pancreatic anionic trypsin possesses all the key amino acids characteristic of the serine protease family.     

Proline-rich proteins and glycoproteins: expression of salivary gland multigene families

Our recent research interests have focused on a group of unusual proteins and glycoproteins high in proline content, or the so-called proline-rich proteins (PRPs). The PRPs are tissue-specific expressions of salivary gland multigene families. Normally PRPs are not detected or are present in very low amounts in rat, mouse and hamster salivary glands, but these unusual proteins are dramatically induced by treatment with the catecholamine isoproterenol. The structures and organizations of several PRP mRNAs and PRP genes have been determined. The amino acid sequences of all PRPs show 4 distinct regions, namely, a signal peptide, a transition region, a repeat region and a carboxyl-terminal region. Glycoproteins induced by isoproterenol treatment may be N-glycosylated or O-glycosylated. The N-glycosylated glycoprotein GP-158 from rat submandibular glands has a 12 amino acid glycopeptide which repeats possibly 49 times. Proline-rich proteins of the parotid glands of rats and mice are also greatly induced by dietary tannins. The apparent unique occurrence of PRPs in saliva suggests that one biological role is to neutralize the detrimental effects of dietary tannins and other polyphenols. The upstream regions of the mouse and hamster PRP genes contain cyclic AMP-regulated sequences as demonstrated by deletions and transient transfections. The PRP multigene family members of mouse are all located on chromosome 8.     

Finding new posttranslational modifications in salivary proline-rich proteins

Proline-rich proteins (PRPs) are the most complex family of salivary peptides with distinct isoforms and PTMs. Up to date, only the serine phosphorylation at positions 8, 17, and 22 have been experimentally observed on acidic PRP (aPRPs), and at position 8 on basic PRP1 and 2. The presence of a glucoronyl group at Ser17 was also noticed on aPRP. The main goal of this study was to identify new PTMs and distinct isoforms of salivary PRPs using LC-MALDI-TOF/TOF. Through the salivary peptidome characterization of 20 different subjects from Control, Diabetic, and Head and Neck Cancer groups, it was possible to identify the following species: (i) N-glycosylation sites: two in basic proline-rich protein 2 (bPRP2), one in bPRP3 and one in bPRP4; (ii) O-glycosylation sites: two in bPRP2 and one in aPRP; (iii) other terminal monosaccharide sites: six in bPRP1, two in bPRP2 and two in bPRP3; (iv) other modifications such as N-terminal pyro-Glu (two in bPRP1, six in bPRP2, eight in bPRP3 and nine in bPRP4); (v) phosphorylation in serine, three in bPRP1, one in bPRP2, one in bPRP3 and one in aPRP1; (vi) bPRP1 (allele S, allele M and variant CP5) and bPRP4 (allele M). In summary, salivary peptidome data analysis allowed the identification of 45 new PRP-modified residues, mainly due to glycosylation, phosphorylation and conversion of Gln to pyro-Glu. Moreover, comparing all subject groups, it was noticed a predominance of N-acetyl hexosamine modification on bPRPs in the Head and Neck Cancer patients.     

Localization of repetitive proline-rich proteins in the extracellular matrix of pea root nodules

Summary Early responses of legume roots toRhizobium inoculation include new cell wall synthesis and induction of some putative wall protein genes. Although the predicted amino acid sequences of several early nodulins indicate that they encode proline-rich proteins (PRPs), the proteins have been neither isolated nor has their presence been demonstrated in cell walls. We have used polyclonal antibodies against PRP2 from soybean to identify and localize proline-rich proteins in pea nodules. On immunoblots, several PRPs were detected, ranging from less than 20 kDa to 110 kDa. Immunocytochemistry revealed that tissues of the vascular cylinder contained abundant PRPs, particularly in the secondary cell walls of xylem elements and phloem fibers. PRPs were also found within the primary wall of the nodule endodermis and within Casparian strips of the vascular endodermis. Of symbiotic importance, PRPs were a prominent component of the infection thread matrix in newly infected root cells and in nodules. PRPs were also secreted by cells in the uninfected nodule parenchyma, where they were found occluding intercellular spaces outside the middle lamella. Despite structural conservation among members of this class of cell wall proteins, PRPs were targeted to distinct layers of the extracellular matrix dependent upon cell type, and may thus play separate roles in the biology of plant cells. The putative functions and the potential for interactions between PRPs and other wall polymers are discussed.Abbreviations DTT dithiothreitol - EDTA ethylenediamine tetraacetate - GRP glycine-rich protein - PCR polymerase chain reaction - PGA polygalacturonic acid - PMSF phenylmethylsulfonyl fluoride - PRP proline-rich protein - SDS-PAGE sodium dodecylsulfate-polyacrylamide gel electrophoresis - Tris tris(hydroxylmethyl) aminomethane - Tween 20 polyoxyethylene sorbitan monolaurate Dedicated to the memory of Professor John G. Torrey     

Changes in mouse whole saliva soluble proteome induced by tannin-enriched diet

Background  

Previous studies suggested that dietary tannin ingestion may induce changes in mouse salivary proteins in addition to the primarily studied proline-rich proteins (PRPs). The aim of the present study was to determine the protein expression changes induced by condensed tannin intake on the fraction of mouse whole salivary proteins that are unable to form insoluble tannin-protein complexes. Two-dimensional polyacrylamide gel electrophoresis protein separation was used, followed by protein identification by mass spectrometry.     

Molecular model for astringency produced by polyphenol/protein interactions

Polyphenols are responsible for the astringency of many beverages and foods. This is thought to be caused by the interaction of polyphenols with basic salivary proline-rich proteins (PRPs). It is widely assumed that the molecular origin of astringency is the precipitation of PRPs following polyphenol binding and the consequent change to the mucous layer in the mouth. Here, we use a variety of biophysical techniques on a simple model system, the binding of beta-casein to epigallocatechin gallate (EGCG). We show that at low EGCG ratios, small soluble polydisperse particles are formed, which aggregate to form larger particles as EGCG is added. There is an initial compaction of the protein as it binds to the polyphenol, but the particle subsequently increases in size as EGCG is added because of the incorporation of EGCG and then to aggregation and precipitation. These results are shown to be compatible with what is known of astringency in foodstuffs.     

The primary structures of six human salivary acidic proline-rich proteins (PRP-1, PRP-2, PRP-3, PRP-4, PIF-s and PIF-f).

Human glandular salivary secretions contain several acidic proline-rich phosphoproteins (PRPs). These proteins have important biological functions related to providing a protective environment for the teeth, and appear to possess other activities associated with modulation of adhesion of bacteria to oral surfaces. These functions and activities depend on the primary structures of the PRPs. Previously determined amino acid sequences of two 150-residue molecules, PRP-1 and PRP-2, and two related 106-residue proteins, PRP-3 and PRP-4, indicated that residue 4 was Asn in PRP-1 and PRP-3, and Asp in PRP-2 and PRP-4, and position 50 was Asn in all four proteins. Recent data from cDNA sequence studies and further structural studies, however, showed that the previously proposed sequences cannot be completely correct. The present work has shown that the protein previously designated as PRP-1 actually consisted of two positional isomers, PIF-s, which has Asn and Asp at positions 4 and 50 respectively, and authentic PRP-1, which has the reverse arrangement. The same isomerism is present in the smaller proteins, PIF-f and PRP-3. Since the isomeric pairs have identical compositions and charges, their presence was not previously detected. Also, by using a more highly purified preparation, it has been found that position 50 in PRP-2 and PRP-4 is Asp, rather than Asn previously reported. These new findings for the six PRPs define their complete primary structures, which are now consistent with those proposed for PRP-1 and PIF-s from cDNA data, and are also consistent with the chromatographic and electrophoretic behaviours of the six PRPs and their derived peptides. These corrected structures are important for understanding the biological functions and activities of these unusual proteins.     

Isoproterenol increases sorting of parotid gland cargo proteins to the basolateral pathway

Exocrine cells have an essential function of sorting secreted proteins into the correct secretory pathway. A clear understanding of sorting in salivary glands would contribute to the correct targeting of therapeutic transgenes. The present work investigated whether there is a change in the relative proportions of basic proline-rich protein (PRP) and acidic PRPs in secretory granules in response to chronic isoproterenol treatment, and whether this alters the sorting of endogenous cargo proteins. Immunoblot analysis of secretory granules from rat parotids found a large increase of basic PRP over acidic PRPs in response to chronic isoproterenol treatment. Pulse chase experiments demonstrated that isoproterenol also decreased regulated secretion of newly synthesized secretory proteins, including PRPs, amylase and parotid secretory protein. This decreased efficiency of the apical regulated pathway may be mediated by alkalization of the secretory granules since it was reversed by treatment with mild acid. We also investigated changes in secretion through the basolateral (endocrine) pathways. A significant increase in parotid secretory protein and salivary amylase was detected in sera of isoproterenol-treated animals, suggesting increased routing of the regulated secretory proteins to the basolateral pathway. These studies demonstrate that shifts of endogenous proteins can modulate regulated secretion and sorting of cargo proteins. amylase; parotid secretory protein; polarized secretion     

Interaction of the salivary low-molecular-weight mucin (MG2) with Actinobacillus actinomycetemcomitans

Periodontitis is associated with the presence of certain Gram-negative bacteria in the oral cavity, among these Actinobacillus actinomycetemcomitans. In order to determine which types of salivary components interact with A. actinomycetemcomitans two strains (HG 1175 and FDC Y4) were incubated with whole saliva and individual glandular secretions, viz. parotid, submandibular, and sublingual saliva. Immunochemical analysis by immunoblotting of bacteria-bound salivary proteins showed that IgA, the low-molecular mucin MG2, parotid agglutinin, and a 300 kDa sublingual and submandibular glycoprotein, were bound to the bacterial strains tested. In addition, adherence of A. actinomycetemcomitans to salivary proteins in a solid-phase was studied. After electrophoresis and transfer of salivary proteins to nitrocellulose membranes A. actinomycetemcomitans adhered only to MG2. In this assay periodate treatment, mild acid hydrolysis or neuraminidase digestion of the saliva glycoproteins abolished binding of two clinical isolates (HG 1175 and NY 664), suggesting that sialic acid residues on MG2 are involved in the binding. In contrast, adherence of the smooth laboratory strain Y4 was not affected by removal of sialic acid residues or even periodate treatment of MG2.Abbreviations S-IgA Secretory IgA - MG1 high-molecular-weight mucin - MG2 low-molecular-weight mucin - EP-GP extra parotid-glycoprotein - PRPs proline-rich proteins - SNA Sambucus nigra agglutinin - MAA Maackia amurensis agglutinin - PNA peanut agglutinin - UEA Ulex europaeus agglutinin     

Basic proline-rich proteins from human parotid saliva: complete covalent structures of proteins IB-1 and IB-6

The complete amino acid sequences of two basic proline-rich proteins, IB-1 and IB-6, from human parotid saliva have been determined. Fragments for sequence analysis were obtained by enzymatic digestions. The proteins have molecular weights of 9571 (IB-1) and 11,530 (IB-6) and contain 34 and 39 mol % proline, respectively. IB-1 and IB-6 contain an identical sequence of 54 residues except for an alanine in position 52 of IB-6, where IB-1 has proline. An unusually high number of repeated sequences occurs in both molecules. IB-1 has a blocked amino-terminal residue, pyroglutamic acid, and also contains one phosphoserine residue in position 8. The relationship of these proteins to the basic proline-rich protein IB-9 [Kauffman, D., Wong, R., Bennick, A., & Keller, P. (1982) Biochemistry 21, 6558-6562] and to other salivary proline-rich proteins is discussed.     

Differential expression of proline-rich proteins in rabbit salivary glands.

Salivary glands synthesize and secrete an unusual family of proline-rich proteins (PRPs) that can be broadly divided into acidic and basic PRPs. We studied the tissue-specific expression of these proteins in rabbits, using antibodies to rabbit acidic and basic PRPs as well as antibodies and cDNA probes to human PRPs. By immunoblotting, in vitro translation, and Northern blotting, basic PRPs could be readily detected in the parotid gland but were absent in other salivary glands. In contrast, synthesis in vitro of acidic PRPs was detected in parotid, sublingual, and submandibular glands. Ultrastructural localization with immunogold showed heavy labeling with antibodies to acidic PRPs of secretory granules of parotid acinar cells and sublingual serous demilune cells. Less intense labeling occurred in the seromucous acinar cells of the submandibular gland. With antibodies to basic PRPs, the labeling of the parotid gland was similar to that observed with antibodies to acidic PRPs, but there was only weak labeling of granules of a few sublingual demilune cells, and no labeling of the submandibular gland. These results demonstrate a variable pattern of distribution of acidic and basic PRPs in rabbit salivary glands. These animals are therefore well suited for study of differential tissue expression of PRPs.