Characterization of human sublingual-gland protein kinase by phosphorylation of a peptide related to secreted proteins

Molecular Cloning of Mouse Homologue of Enamel Protein C4orf26 and Its Phosphorylation by FAM20C

It is widely accepted that cellular processes are controlled by protein phosphorylation and has become increasingly clear that protein degradation, localization and conformation as well as protein-protein interaction are the examples of subsequent cellular events modulated by protein phosphorylation. Enamel matrix proteins belong to members of the secretory calcium binding phosphoprotein (SCPP) family clustered on chromosome 4q21, and most of the SCPP phosphoproteins have at least one S-X-E motifs (S; serine, X; any amino acid, E; glutamic acid). It has been reported that mutations in C4orf26 gene, located on chromosome 4q21, are associated with autosomal recessive type of Amelogenesis Imperfecta (AI), a hereditary condition that affects enamel formation/mineralization. The enamel phenotype observed in patients with C4orf26 mutations is hypomineralized and partially hypoplastic, indicating that C4orf26 protein may function at both secretory and maturation stages of amelogenesis. The previous in vitro study showed that the synthetic phosphorylated peptide based on C4orf26 protein sequence accelerates hydroxyapatite nucleation. Here we show the molecular cloning of Gm1045, mouse homologue of C4orf26, which has 2 splicing isoforms. Immunohistochemical analysis demonstrated that the immunolocalization of Gm1045 is mainly observed in enamel matrix in vivo. Our report is the first to show that FAM20C, the Golgi casein kinase, phosphorylates C4orf26 and Gm1045 in cell cultures. The extracellular localization of C4orf26/Gm1045 was regulated by FAM20C kinase activity. Thus, our data point out the biological importance of enamel matrix-kinase control of SCPP phosphoproteins and may have a broad impact on the regulation of amelogenesis and AI.

Processing of acidic proline-rich proprotein by human salivary gland convertase

Previously it was found that proproteins for basic and glycosylated salivary proline-rich proteins (PRP) were cleaved prior to secretion from cells by furin, a well-known convertase. In contrast proproteins for acidic PRPs are not cleaved by furin or other convertases. To investigate the convertase responsible for in vivo processing of acidic PRP proproteins, homogenates of human sublingual glands were fractionated by centrifugation at 10,000 x g and 100,000 x g and activity demonstrated in all fractions. The 100,000 x g pellet was fractionated into Golgi, smooth endoplasmic reticulum and microsomal fractions with the latter containing the enzyme. Subfractionation of the microsomes revealed that the activity was located in the membrane proteins. Since the microsomes contain components of the secretory pathway the enzyme in this fraction may be responsible for intracellular cleavage of the acidic PRP proprotein. The enzyme was active at alkaline pH. It was strongly inhibited by metal chelators indicating that it is a metalloprotease. It was not inhibited by an acid protease inhibitor, but partly inhibited by some serine protease inhibitors indicating that serine proteases may play a role in degradation. Co2+ and to some extent Zn2+ activated the enzyme, but it was strongly inhibited by Hg2+ and Cu2+ as well as the organomercurial p-chloromercuribenzenesulfonic acid. Thus it appears that the enzyme contains an important -SH group. These characteristics indicate that the convertase is related to a group of metal- and thiol-dependent proteases known as thimet oligopeptidases, but in contrast to the latter enzymes the sublingual convertase was not inhibited by angiotensin antagonists.

Purification of kinase activity from primate parotid glands

Salivary secretions contain phosphoproteins that contain phosphorylation sites composed of serine residues in acidic environments. The hypothesis of this study is that a protein kinase responsible for phosphorylating these proteins is similar to kinases that phosphorylate proteins in other glandular secretions. Homogenates and subfractions from macaque parotid glands were able to phosphorylate synthetic peptide substrates containing each of the phosphorylation sites in acidic proline-rich proteins, statherin, and histatin 1. Activity was purified from Golgi membranes to greater than 220-fold by extraction with Triton X-100 and affinity chromatography with the use of immobilized ATP. The enzyme preferred substrates containing serine residues in a specific acidic environment, particularly those containing the Ser-Xaa-acidic sequence, preferred ATP over GTP, and was sensitive to high concentrations of heparin. These characteristics are similar to those reported for Golgi casein kinase, which phosphorylates casein in vivo. Based on these observations, the parotid gland kinase may be related to other Golgi-localized serine kinases.

Enkephalin-degrading enzymes and their inhibitors in human saliva

The possible presence of enzymes able to hydrolyze leucine enkephalin has been investigated in human saliva. The data obtained indicate that, in the presence of saliva, Leu-enkephalin is partially hydrolyzed. The disappearance of the substrate is paired with the formation of hydrolysis byproducts whose composition indicates the presence of all three classes of enzymes known to hydrolyze enkephalins: aminopeptidases, dipeptidylaminopeptidases, and dipeptidylcarboxypeptidases. The presence of low molecular weight substances with inhibitory activity on proteolytic enzymes has also been detected. These substances are active on all three classes of enkephalin-degrading enzymes, although the inhibition is more evident on dipeptidylpeptidases than on aminopeptidases. Substrate degradation was found to be higher in male than in female saliva: this seems to be caused by the activities both of enzymes and low molecular weight inhibitors that are different in the two sexes.