Pro-inflammatory Cytokines Up-regulate MUC1 Gene Expression in Oral Epithelial Cells

The membrane-bound mucin MUC1 is expressed ubiquitously on epithelial surfaces and is thought to provide protection from bacterial and chemical injury. The present study was undertaken to determine whether MUC1 was expressed in cultured oral epithelial cells and whether expression is modulated by pro-inflammatory mediators released as part of the host response to infection by oral pathogens. Northern and Western blotting experiments showed that KB cells express MUC1 mRNA and protein. When cells were treated with interleukins (IL-1β, IL-6), tumor necrosis factor-alpha (TNF-α), or interferon-gamma (IFN-γ), or combinations of these, real-time PCR demonstrated that MUC1 mRNA increased 1.4- to 3.2-fold. Interestingly, a significant increase in levels of MUC1 protein was also observed. While no effect was observed when KB cells were incubated with LPS from Porphyromonas gingivalis, infection of KB monolayers with this oral pathogen caused a 2.85-fold increase in MUC1 transcript levels. These results suggest that increased MUC1 synthesis may be a key element in the host response to infection with oral pathogens.

Vertical integration of biochemistry and clinical medicine using a near-peer learning model

Vertical integration has been extensively implemented across medical school curricula but has not been widely attempted in the field of biochemistry. We describe a novel curricular innovation in which a near-peer learning model was used to implement vertical integration in our medical school biochemistry course. Senior medical students developed and facilitated a case-based small group session for first year biochemistry students. Students were surveyed before and after the session on their attitudes about biochemistry, as well as the effectiveness of the session. Prior to the session, the students believed biochemistry was more important to understanding the basic science of medicine than it was to understanding clinical medicine or becoming a good physician. The session improved students' attitudes about the importance of biochemistry in clinical medicine, and after the session they now believe that understanding biochemistry is equally important to the basic sciences as clinical medicine. Students would like more sessions and believe the senior student facilitators were knowledgeable and effective teachers. The facilitators believe they improved their teaching skills. This novel combination of near-peer learning and vertical integration in biochemistry provided great benefit to both first year and senior medical students, and can serve as a model for other institutions. © 2016 by The International Union of Biochemistry and Molecular Biology, 44(6):507-516, 2016.

Anti-candidal activity of genetically engineered histatin variants with multiple functional domains

The human bodily defense system includes a wide variety of innate antimicrobial proteins. Histatins are small molecular weight proteins produced by the human salivary glands that exhibit antifungal and antibacterial activities. While evolutionarily old salivary proteins such as mucins and proline-rich proteins contain large regions of tandem repeats, relatively young proteins like histatins do not contain such repeated domains. Anticipating that domain duplications have a functional advantage, we genetically engineered variants of histatin 3 with one, two, three, or four copies of the functional domain by PCR and splice overlap. The resulting proteins, designated reHst3 1-mer, reHist3 2-mer, reHis3 3-mer and reHist3 4-mer, exhibited molecular weights of 4,062, 5,919, 7,777, and 9,634 Da, respectively. The biological activities of these constructs were evaluated in fungicidal assays toward Candida albicans blastoconidia and germinated cells. The antifungal activities per mole of protein increased concomitantly with the number of functional domains present. This increase, however, was higher than could be anticipated from the molar concentration of functional domains present in the constructs. The demonstrated increase in antifungal activity may provide an evolutionary explanation why such domain multiplication is a frequent event in human salivary proteins.

Influence of histatin 5 on Candida albicans mitochondrial protein expression assessed by quantitative mass spectrometry

Individual aspects of the mode of action of histatin 5, a human salivary antifungal protein, have been partially elucidated, but the mechanism likely involves a complex set of events that have not been characterized. Previous evidence points toward histatin-induced alterations in mitochondrial function. The purpose of the present study was to verify and quantify changes in the mitochondrial proteome of Candida albicans treated with histatin 5. Cell killing was determined by plating and differential protein expression levels in the mitochondrial samples were determined by quantitative proteomics approaches employing mTRAQ and ICAT labeling and Western blotting. Relative quantitation ratios were established for 144 different proteins. Up-regulated mitochondrial proteins were predominantly involved in genome maintenance and gene expression, whereas proteins that constitute the respiratory enzyme complexes were mostly down-regulated. The differential expression of ATP synthase gamma chain and elongation factor 1-alpha were confirmed by Western blotting by comparison to levels of cytochrome c which were unchanged upon histatin treatment. The mTRAQ and ICAT proteomics results suggest that key steps in the histatin 5 antifungal mechanism involve a bioenergetic collapse of C. albicans, caused essentially by a decrease in mitochondrial ATP synthesis.

Two-hybrid analysis of human salivary mucin MUC7 interactions

MUC7 is a low molecular weight monomeric mucin secreted by submandibular, sublingual and minor salivary glands. This mucin has been implicated in the non-immune host defense system in the oral cavity since it binds and agglutinates a variety of oral microbes. To investigate interactions between this mucin and other secretory salivary proteins, a submandibular gland prey library was screened with baits encoding the N- and C-terminal regions of MUC7 in the yeast two-hybrid system. The N-terminal region interacted with several secretory salivary proteins, whereas the C-terminal region did not. Interacting proteins included amylase, acidic proline-rich protein 2, basic proline-rich protein 3, lacrimal proline-rich protein 4, statherin and histatin 1. Formation of complexes between these proteins and the N-terminal region of MUC7 was confirmed in Far Western blotting experiments. Interactions between mucin and non-mucin proteins in saliva could protect complex partners from proteolysis, modulate the biological activity of complexed proteins or serve as a delivery system for distribution of secretory salivary proteins throughout the oral cavity.

Suppression of MUC1 synthesis downregulates expression of the epidermal growth factor receptor

The transmembrane mucin, MUC1, is overexpressed on many human carcinoma cells, increasing their metastatic potential through decreased cell-cell and cell-matrix adhesion. These cellular changes are mediated both through the altered physical properties of the mucin itself and through the role of the MUC1 cytoplasmic domain as a signaling molecule. The epidermal growth factor receptor (EGFR) is also overexpressed in many cancers and both it and MUC1 constitute important therapeutic targets. In the present study, expression of MUC1 was downregulated by treatment of KB carcinoma cells with a MUC1 small interfering RNA resulting in an inhibition of cell proliferation and colony formation and an increase in cell-cell aggregation. Surprisingly, suppression of MUC1 also inhibited expression of EGFR at both the mRNA and protein levels whereas the reciprocal effect was not observed. These results demonstrate a role for MUC1 in the regulation of EGFR expression and suggest that MUC1 gene silencing may represent a novel therapeutic approach in the treatment of a variety of human cancers.

MG2 and lactoferrin form a heterotypic complex in salivary secretions

Protein-protein interactions are necessary for homeostasis to be maintained and for biological systems to be integrated. Heterotypic complexes occur in saliva, and a complex between MG2 and SIgA has been suggested to promote microbial clearance from the oral cavity. In this study, we used a peptide display library to investigate previously unrecognized heterotypic complexes involving MG2 and other proteins. The library was panned with MG2 12 times, and analyses of clones identified the sequence Ala-Leu-Leu-Cys-, which occurs in salivary lactoferrin. Blotting experiments confirmed that MG2 and lactoferrin form a heterotypic complex in vitro and in vivo. Periodate treatment of MG2 did not affect the interaction. A synthetic lactoferrin peptide containing the motif Ala-Leu-Leu-Cys-blocked the interaction between MG2 and lactoferrin, confirming the specificity of the interaction identified by panning. This complex may enhance the properties of these salivary components in the oral environment.

Patterns of secretion of mucins and non-mucin glycoproteins in human submandibular/sublingual secretion

The present investigation has characterised the influence of gustatory stimulation and duration of stimulation on the secretion pattern of salivary mucins MG1 and MG2 and non-mucin glycoproteins in submandibular/sublingual secretion (SMSL). Resting SMSL was collected for three 2 min intervals and stimulated SMSL was collected for ten 1 min intervals from six healthy subjects. Flow rates and total protein were significantly different under the two conditions. The secretion patterns of these proteins under resting and stimulated conditions was examined on periodic acid-Schiff reagent (PAS)-stained polyacrylamide gels using a Kodak Digital-Science Image Station. Image analyses revealed that the level of MG1 increased and the level of MG2 remained nearly the same after stimulation. Six other major glycoproteins (designated Band 1-6) were identified on the basis of their electrophoretic mobilities and immuno-reactivity on Western blots. After stimulation the intensity of Band 1 (lactoferrin and peroxidase) and Band 2 (amylase) decreased whereas the intensity of Band 3 (carbonic anhydrase), Band 4 (proline-rich glycoprotein) and Bands 5 and 6 (basic glycosylated proline-rich proteins) increased. These patterns probably reflect secretion from preformed vesicles since de novo synthesis would be unexpected within the time frame of these experiments. The variable patterns observed suggest that mucins and non-mucin glycoproteins in SMSL derive from different subsets of secretory vesicles, some of which may originate in mucous and others in serous acini, as well as in ductal cells. Quantification of mucins was performed by image analysis technology using purified MG1 and MG2 standards. Finally, the present investigation has shown that the secretory patterns of mucins and non-mucin glycoproteins from submandibular/sublingual glands are complex and represent an important aspect of salivary gland physiology.

Electron microscopic immunogold localization of salivary mucins MG1 and MG2 in human submandibular and sublingual glands

The human salivary mucins MG1 and MG2 are well characterized biochemically and functionally. However, there is disagreement regarding their cellular and glandular sources. The aim of this study was to define the localization and distribution of these two mucins in human salivary glands using a postembedding immunogold labeling method. Normal salivary glands obtained at surgery were fixed in 3% paraformaldehyde-0.1% glutaraldehyde and embedded in Lowicryl K4M or LR Gold resin. Thin sections were labeled with rabbit antibodies to MG1 or to an N-terminal synthetic peptide of MG2, followed by gold-labeled goat anti-rabbit IgG. The granules of all mucous cells of the submandibular and sublingual glands were intensely reactive with anti-MG1. No reaction was detected in serous cells. With anti-MG2, the granules of both mucous and serous cells showed reactivity. The labeling was variable in both cell types, with mucous cells exhibiting a stronger reaction in some glands and serous cells in others. In serous granules, the electron-lucent regions were more reactive than the dense cores. Intercalated duct cells near the acini displayed both MG1 and MG2 reactivity in their apical granules. In addition, the basal and lateral membranes of intercalated duct cells were labeled with anti-MG2. These results confirm those of earlier studies on MG1 localization in mucous cells and suggest that MG2 is produced by both mucous and serous cells. They also indicate differences in protein expression patterns among salivary serous cells.

Interaction of human salivary mucin MG2, its recombinant N-terminal region and a synthetic peptide with Actinobacillus actinomycetemcomitans

The antimicrobial properties of human salivary mucin MG2 against the periodontal pathogen, Actinobacillus actinomycetemcomitans (A. actinomycetemcomitans), were investigated using purified MG2, rNMUC7 (a recombinant polypeptide containing residue 1-144 of MG2) and synthetic peptides PEP1 (residue 1-17) and PEP2 (residue 47-63). MG2 and rNMUC7 bound to A. actinomycetemcomitans strains SUNY75, SUNY465, SUNY523, 652 and JP2 in a liquid phase binding assay. The bactericidal activities of rNMUC7, PEP1 and PEP2 against A. actinomycetemcomitans SUNY523 were examined in a colony forming unit killing assay. The LD50 for rNMUC7 was 9 microM, for PEP2 was 20 microM and PEP1 did not exhibit bactericidal activity. The primary structure of these polypeptides was analyzed and a direct relationship between net positive charge and bactericidal activity was found. Screening of saliva samples from 60 individuals on Western blots probed with an anti-MG2 antibody against PEP2 revealed that a 20 kDa MG2 fragment was present in 66% of subjects and that this fragment was not present in glandular secretions. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of tryptic peptides derived from the 20 kDa fragment confirmed that this fragment contained a portion of the amino terminal region of MG2. The present study showed that the N-terminal region of MG2 and a subdomain within this region are microbicidal against A. actinomycetemcomitans and that a 20 kDa fragment of MG2 occurs in whole saliva. This suggests that cleavage of MG2 in vivo may produce fragments with microbicidal properties and that this may represent a novel mechanism of host defense.

Structural characterisation of cysteines in a bacterial-binding motif of human salivary mucin MG2

Human salivary mucin MG2 is a 180 kDa glycoprotein secreted by submandibular/sublingual and minor salivary glands. Secreted MG2 contains a domain with the only two cysteines (Cys(45) and Cys(50)) present in the polypeptide backbone; in native and recombinant MG2 this domain is involved in mucin binding to oral microbes. As the reduction and alkylation of MG2 has been shown to abolish binding, the present study was undertaken to determine whether the cysteine residues exist in the dithiol or disulphide form. Electrophoretic analysis under reducing and non-reducing conditions showed that intermolecular disulphide bonds do not occur between MG2 molecules. The same incorporation of radiolabelled iodoacetamide into MG2 was obtained with or without prior reduction. When radiolabelled alkylated MG2 was digested with Endoproteinase Lys-C and the derived peptides were separated by reversed-phase high-performance liquid chromatography (RP-HPLC), radioactivity was found in two fractions. Mass spectral analyses of these fractions showed the presence of peptides Cys-Leu-His-Lys and Arg-Cys-Arg-Pro-Lys, both containing carboxymethylated cysteines. These results show that the cysteines in the structural motif associated with bacterial binding exist in the dithiol form, and suggest the potential use of cysteine-containing peptides as agents to modify interactions of MG2 with microbes and oral surfaces.

Expression of membrane-associated mucins MUC1 and MUC4 in major human salivary glands

Mucins are high molecular weight glycoproteins secreted by salivary glands and epithelial cells lining the digestive, respiratory, and reproductive tracts. These glycoproteins, encoded in at least 13 distinct human genes, can be subdivided into gel-forming and membrane-associated forms. The gel-forming mucin MUC5B is secreted by mucous acinar cells in major and minor salivary glands, but little is known about the expression pattern of membrane-associated mucins. In this study, RT-PCR and Northern blotting demonstrated the presence of transcripts for MUC1 and MUC4 in both parotid and submandibular glands, and in situ hybridization localized these transcripts to epithelial cells lining striated and excretory ducts and in some serous acinar cells. The same cellular distribution was observed by immunohistochemistry. Soluble forms of both mucins were detected in parotid secretion after immunoprecipitation with mucin-specific antibodies. These studies have shown that membrane-associated mucins are produced in both parotid and submandibular glands and that they are expressed in different cell types than gel-forming mucins. Although the function of these mucins in the oral cavity remains to be elucidated, it is possible that they both contribute to the epithelial protective mucin layer and act as receptors initiating one or more intracellular signal transduction pathways.

The effects of duration and intensity of stimulation on total protein and mucin concentrations in resting and stimulated whole saliva

The present investigation has characterized the influence of the duration and intensity of stimulation on the secretion pattern of total protein and salivary mucins MG1 and MG2 in whole saliva. Resting and stimulated whole saliva was collected from six healthy subjects on 2 consecutive days. Whole saliva was collected for 2 five-minute intervals under resting conditions followed by collection under masticatory stimulation induced by the chewing of parafilm (1 g) at 10 or 60 strokes/min for 15 min. Flow rates were different under the 2 levels of stimulation. The concentration of total protein was different in resting and stimulated whole saliva but was not affected by the duration or intensity of stimulation. Analysis of mucin concentrations determined by capture ELISAs revealed that the pattern of MG1 secretion was similar to that of total protein. The pattern of MG2 secretion was unique in that no differences were observed in the concentration of this mucin under resting and stimulated conditions. This study shows that the pattern of protein secretion in whole saliva does not reflect the combined pattern observed for protein secretion in parotid and submandibular/sublingual glands, and that the secretion patterns of MG1 and MG2 in whole saliva are quite different from one another.

The Amount of MUC5B mucin in cervical mucus peaks at midcycle

The physical character and amount of mucus secreted by the endocervix changes dramatically during the menstrual cycle to facilitate sperm migration at the time of midcycle ovulation. Mucins are highly glycosylated, high-molecular-weight proteins, which are the major structural components of the protective mucus gel covering all wet-surfaced epithelia, including that of the endocervix. We have previously demonstrated that the endocervical epithelium expresses messenger RNA (mRNA) of three of the large gel-forming mucins, designated MUC5AC, MUC5B, and MUC6, with mRNA of MUC5B predominating. Because mucin protein levels may be regulated posttranscriptionally, measurement of MUC5B protein levels with cycle are needed for correlation to mRNA levels. Measurement of specific mucin gene products within mucus secretions has been limited by availability of specific, well-characterized antibodies and by volume requirements of the isolation protocols for mucins, which include CsCl density centrifugation and fraction isolation. To measure MUC5B protein within the cervical mucus through the hormone cycle, we developed a polyclonal antibody specific to the mucin. The antibody, designated no. 799, is to a synthetic peptide mimicking a 19-amino-acid segment of an intercysteine-rich region within the D4 domain in the 3' region of the MUC5B protein. It recognizes native as well as denatured MUC5B on immunoblot, is preadsorbable with its peptide, and binds to apical secretory vesicles of epithelia expressing MUC5B. We used the MUC5B antibody along with a cervical mucin standard cervical mucin isolate in enzyme-linked immunosorbent assay to determine the relative amount of MUC5B mucin in samples of human cervical mucus taken through the menstrual cycle. We demonstrate a peak of MUC5B mucin in human cervical mucus collected at midcycle, compared with mucus from early or late in the cycle. This peak in MUC5B content coincides with the change in mucus character that occurs at midcycle, suggesting that this large mucin species may be important to sperm transit to the uterus.

Heterogeneity of high-molecular-weight human salivary mucins

The existence of high-molecular-weight glycoproteins in saliva and salivary secretions has been recognized for nearly 30 years. These proteins, called mucins, are essential for oral health and perform many diverse functions in the oral cavity. Mucins have been intensively studied, and much has been learned about their biochemical properties and their interactions with oral micro-organisms and other salivary proteins. In the past several years, the major high-molecular-weight mucin in salivary secretions has been identified as MUC5B, one of a family of 11 human mucin gene products expressed in tissue-specific patterns in the gastrointestinal, respiratory, and reproductive tracts. MUC5B is one of four gel-forming mucins which exist as multimeric proteins with molecular weights greater than 20-40 million daltons. The heavily glycosylated mucin multimers form viscous layers which protect underlying epithelial surfaces from microbial, mechanical, and chemical assault. Another class of mucin molecules, the membrane-bound mucins, is structurally and functionally distinct from the gel-forming mucins. These proteins do not form multimers and can exist as both secreted and membrane-bound forms, with the latter anchored to epithelial cell membranes through a short membrane-spanning domain. In the present work, we show that two of the membrane-bound mucins, MUC1 and MUC4, are expressed in all major human salivary glands as well as in buccal epithelial cells. While the functions of these mucins in the oral environment are not understood, it is possible that they form a structural framework on the cell surface which not only is cytoprotective, but also may serve as a scaffold upon which MUC5B, and possibly other salivary proteins, assemble.

Salivary mucin: a factor in the lower prevalence of gastroesophageal reflux disease in African-Americans?

OBJECTIVES

Organic and inorganic constituents of saliva have been implicated as protective components in the esophagus, and deficiencies in one or more of these factors in different races may be an important element in the prevalence of gastroesophageal reflux disease (GERD). To determine whether there are differences in the concentration of salivary mucins between different racial groups, we measured the concentration of mucous glycoprotein MG1 and mucous glycoprotein MG2 in whole saliva of African-Americans and Caucasians.

METHODS

Whole saliva was collected from 19 African-American (four male, 15 female; mean age 34 yr, range 19-53 yr) and 25 Caucasian (11 male, 14 female; mean age 31 yr, range 20-51 yr) volunteers under masticatory stimulation (1 g Parafilm, 60 strokes/min) between 11:00 AM and 12:00 noon. Total salivary carbohydrate was measured with a periodic acid-Schiff assay and total protein by absorbance at 215 nm. Immunological reagents were employed to quantify MG2 in a combined enzyme-linked immunosorbent assay/enzyme linked lectin assay (ELISA/ELLA) and to quantify MG1 in a capture ELISA.

RESULTS

The total carbohydrate, protein, MG1 and MG2 values were 24.4 +/- 11.9, 243.5 +/- 62.7, 21.8 +/- 13.4, and 11.6 +/- 9.5 mg% for African-Americans, and the corresponding values were 23.3 +/- 9.3, 221.7 +/- 39.7, 25.7 +/- 16.2, and 10.9 +/- 8.7 mg% for Caucasians. There was no statistical difference for any of the parameters measured between the two groups. Furthermore, it was shown that no correlation existed between salivary flow rate and the concentration of carbohydrate, protein, or salivary mucins in African-Americans and in Caucasians. These results show that flow rate did not influence the measured values for salivary parameters in the two groups.

CONCLUSIONS

No differences were found in the concentration of salivary mucins MG1 and MG2 in whole saliva of African-Americans and Caucasians, and it seems unlikely that variations in mucin levels influence the prevalence of GERD in these groups.

Immunoquantification of human salivary mucins MG1 and MG2 in stimulated whole saliva: factors influencing mucin levels

While more and more is known about the structure and function of human salivary mucins, there is relatively little information on quantification of these glycoconjugates in whole saliva and on factors influencing their secretion. The goal of the present work was to develop capture ELISAs that would allow for rapid, inexpensive, and reliable measurement of the salivary mucins MG1 and MG2, and to use these immunological procedures to investigate the significance of age, gender, flow rate, and protein concentration on mucin levels in whole saliva. Previously, we described a rabbit polyclonal antibody against MG1 (Troxler et al., 1995) and a rabbit polyclonal peptide antibody against an epitope in the N-terminal region of MG2 (Liu et al., 1999) which were used to develop the capture ELISAs. We verified the accuracy and specificity of these assays by showing correct measurement of known quantities of purified MG1 or MG2 added to whole saliva and lack of cross-reactivity between mucins and heterologous antisera on Western blots or in ELISAs. Whole saliva was collected from 60 subjects under conditions of masticatory stimulation, flow rates were recorded, and mucin concentrations were determined. The results showed that the mean concentration of MG1 and MG2 was 23.3 +/- 14.6 mg% and 13.3 +/- 11.6 mg%, respectively, and that mucins constitute approximately 16% of the total protein in whole saliva. No significant correlations were found between mucin levels and age or flow rate; however, a significant correlation was found between MG2 levels and total protein concentration. Furthermore, there were statistically significant gender differences in flow rate and MG1 levels, but not in MG2 levels. The availability of these immunoassays for quantification of MG1 and MG2 will help to elucidate the role of mucin in oral health and disease.

No significant difference in antigenicity or tissue transglutaminase substrate specificity of Irish and US wheat gliadins

The prevalence of clinical celiac disease has been shown to vary both across time and between genetically similar populations. Differences in wheat antigenicity and transglutaminase substrate properties are a possible explanation for these differences. This study assessed the antigenicity and transglutaminase substrate specificities of gliadins from regions of high and low celiac disease prevalence. Gliadin was extracted from three commercial US wheat sources and two Irish sources. SDS-PAGE and western blotting revealed minor, but significant variations in the gliadin extracts. However, ELISA showed no difference in the antigenicity of these gliadins. Transglutaminase pretreatment of gliadin resulted in no significant change in gliadin antigenicity and kinetic studies showed that the Kms of the various gliadins were very similar. Purified IgA and IgG had no effect on transglutaminase activity. In summary, minor variations in wheat gliadins are unlikely to explain the observed differences in disease expression across genetically similar populations.

The recombinant N-terminal region of human salivary mucin MG2 (MUC7) contains a binding domain for oral Streptococci and exhibits candidacidal activity

MG2 (the MUC7 gene product) is a low-molecular-mass mucin found in human submandibular/sublingual secretions. This mucin is believed to agglutinate a variety of microbes and thus is considered an important component of the non-immune host defence system in the oral cavity. We have shown that MUC7 can bind to cariogenic strains of Streptococcus mutans and that this binding requires a structural determinant in the N-terminal region. In the present study an expression construct, pNMuc7, encoding the N-terminal 144 amino acids of MUC7 was generated, and the recombinant protein rNMUC7 was expressed in Escherichia coli. Purified rNMUC7 was characterized and the binding of this protein to oral bacteria was investigated in an established assay. The results showed that the recombinant protein bound to S. mutans ATCC 25175 and ATCC 33402, and that alkylation of the two cysteine residues (Cys(45) and Cys(50)) resulted in the complete loss of bacterial binding. This suggests that binding of MUC7 to S. mutans occurs between the N-terminal region of the mucin molecule and the bacterial surface, and that this interaction is dependent on a cysteine-containing domain within this region of MUC7. In addition, the killing activity of rNMUC7 was compared with that of the candidacidal salivary protein histatin 5 in an established Candida albicans (ATCC 44505) blastoconidia killing assay. It was found that the LD(50) values of rNMUC7 and histatin 5 were comparable, and that the recombinant protein displayed significant killing activity at the physiological concentration range of MUC7 in whole saliva. This study is the first to show that the N-terminal region of MUC7 contains a structural determinant for bacterial binding and that this region exhibits candidacidal activity.

Molecular mapping of statherin- and histatin-binding domains in human salivary mucin MG1 (MUC5B) by the yeast two-hybrid system

MGI is a high-molecular-weight mucin secreted by mucous acinar cells in human submandibular and sublingual glands. We have recently shown that the tracheobronchial mucin MUC5B is a major component of MG1. MUC5B is organized into cysteine-rich N- and C-terminal regions that flank a central tandem-repeat region containing cysteine-rich subdomains and imperfect 29-residue tandem repeats. In earlier work, we have shown that this mucin selectively forms heterotypic complexes with amylase, proline-rich proteins, statherin, and histatins in salivary secretions, and the aim of this study was to identify specific binding domains within MUC5B using the yeast two-hybrid system. Interactions of cysteine-rich domains in the tandem-repeat region (Cys1-Cys4) and C-terminal region (Cys8a, Cys8b, Cys8c) of MUC5B with statherin and histatins were investigated. These studies indicated that histatin 1 selectively bound to Cysl and Cys2, whereas statherin and histatin 1, 3, and 5 selectively bound to Cys8a. Analysis of the primary sequences of the identified binding domains suggests that these domains most probably can fold into globular-like structures in the native mucin. A ProDom blast search revealed that sequences in Cys1, Cys2, and Cys8a exhibit similarity to domains in evolutionarily diverse extracellular proteins known to participate in a wide variety of protein-protein interactions.

A recombinant bovine gallbladder mucin polypeptide binds biliary lipids and accelerates cholesterol crystal appearance time

BACKGROUND & AIMS

Mucin has a central role in the pathogenesis of cholesterol gallstones, in part because of its ability to bind biliary lipids and accelerate cholesterol crystal appearance time. Previous studies have localized these properties to nonglycosylated mucin domains, and we have recently shown that these domains contain a series of 127-amino acid, cysteine-rich repeats. The aim of this study was to express a recombinant mucin polypeptide containing these repeats and investigate its lipid-binding and pronucleating properties.

METHODS

A recombinant mucin polypeptide was expressed as a glutathione S-transferase fusion protein in Escherichia coli, purified by affinity chromatography, and compared with native bovine gallbladder mucin in lipid-binding and cholesterol crystal appearance time assays.

RESULTS

The recombinant mucin polypeptide bound a hydrophobic fluorescent probe and cholesterol in a concentration-dependent manner. It accelerated the appearance of cholesterol crystals from lithogenic model bile, an effect that was both time and concentration dependent.

CONCLUSIONS

The cysteine-rich repeats in the recombinant mucin polypeptide correspond to the protease-sensitive hydrophobic domains identified in earlier biochemical studies. Further delineation of the lipid-binding site(s) in these repeats will provide new insights into the mechanism of cholesterol crystal nucleation and stone growth.

The amino-terminal sequence of MUC5B contains conserved multifunctional D domains: implications for tissue-specific mucin functions

The MUC5B mucin gene product is expressed in a wide variety of secretory epithelia including the gallbladder, salivary glands, trachea, and colon. Previous studies by us and others have described the C-terminal region as well as the central tandem repeating domain of this mucin. In an effort to understand the functional role of MUC5B in diverse human tissues, the sequence encoding the N-terminal region of this mucin was determined from the sequences of exons in three overlapping genomic clones. Primer extension mapped the site of transcription initiation 25 bp downstream from a putative TATA box element. The N-terminal region of MUC5B contained 1321 amino acids organized into a signal peptide, a short serine-threonine rich region, and three von Willebrand factor-like D domains. Comparison of this sequence with the N-terminal sequences of MUC2 and MUC5AC revealed a much higher degree of identity (46-59%) than that observed in the C-terminal regions of these mucins (33%). The N-terminal sequence of MUC5B also contains a number of sequence motifs common to several groups of extracellular ligand binding and adhesion proteins not previously recognized in mammalian gel-forming mucins. The N-terminal D domains in MUC5B are likely to have important roles in both mucin assembly and in the protective functions of the secreted mucin.

MUC4 is a major component of salivary mucin MG1 secreted by the human submandibular gland

High molecular weight salivary mucin (MG1) is an important component of saliva, contributing to the lubricative and tissue-protective functions of this biological fluid. We have shown previously that the human mucin gene MUC5B is expressed at high levels in sublingual gland and is a significant constituent of MG1. Since many epithelia express multiple mucin genes, it seemed likely that MG1 in salivary secretions is also a heterogeneous mixture of mucin gene products. The aim of this study was to determine whether MUC4, a mucin shown in Northern blotting experiments to be expressed in salivary glands, was a significant protein component of MG1 in salivary secretions. Two cDNA clones containing MUC4 tandem repeats were isolated from a human submandibular gland cDNA library. In addition, recombinant MUC4 produced in a bacterial expression system cross-reacted with an antibody directed against deglycosylated MG1. This shows conclusively that human salivary mucin MG1 contains both MUC5B and MUC4 gene products suggesting that each mucin may perform distinct functions in the oral cavity.

Molecular characterization of a major high molecular weight mucin from human sublingual gland

Human submandibular/sublingual gland secretions contain a multimeric high molecular weight mucin (MG1) and a monomeric low molecular weight mucin (MG2). MG2 is the product of the MUC7 gene, whereas the gene for MG1 has not been identified. Previously, we isolated a clone (pSM2-1) from a human sublingual gland cDNA expression library using an antibody against deglycosylated MG1 (Troxler et al., Biochem. Biophys. Res Commun., 217, 1112-1119, 1995). In order to identify the mucin gene from which pPM2-1 was derived, Northern blots of human submandibular and sublingual gland RNA were hybridized with a series of probes for tandem repeats found in the high molecular weight secreted mucins MUC2, MUC3, MUC4, MUC5AC, MUC5B, and MUC6. The only known mucin expressed at high levels in sublingual gland was MUC5B, and no known mucin was expressed at high levels in submandibular gland. A series of overlapping clones was obtained by rescreening the sublingual gland cDNA library and by reverse transcriptase-polymerase chain reaction. The resulting clones connected pSM2-1 to a series of MUC5B tandem repeats at the 3' end of the repeat domain and provided the complete nucleotide and deduced amino sequence of the carboxyl terminal region of MG1. This region is enriched with respect to cysteine (approximately 10 mol %) and contained a D domain and a carboxyl terminal domain that could be aligned with the corresponding domains in human intestinal MUC2, human tracheobronchial MUC5AC, and human von Willebrand factor. The limited expression of known mucin genes, together with the considerable mucin synthesizing capacity of submandibular gland, suggests that a novel (previously not described) mucin gene is expressed in this gland and constitutes a portion of MG1 in salivary secretions.

Molecular cloning of a major human gall bladder mucin: complete C-terminal sequence and genomic organization of MUC5B

Gall bladder mucin has been shown to play a central role in the pathogenesis of cholesterol gallstone disease. While cloning and sequencing studies have provided a wealth of information on the structure of other gastrointestinal and respiratory mucins, nothing is known about the primary structure of human gall bladder mucin. In this study, we show that the tracheobronchial mucin MUC5B is a major mucin gene product expressed in the gall bladder. Antibodies directed against deglycosylated human gall bladder mucin were used to screen a gall bladder cDNA expression library, and most of the isolated clones contained repetitive sequences nearly identical with those in the tandem repeat region of MUC5B. An additional clone (hGBM2-3) contained an open reading frame coding for a 389 residue cysteine-rich sequence. The arrangement of cysteine residues in this sequence was very similar to that in the C-terminal regions of MUC2, MUC5AC and human von Willebrand factor. This cysteine-rich sequence was connected to a series of degenerate MUC5B tandem repeats in a 7.5 kb HincII genomic DNA fragment. This fragment, with ten exons and nine introns, contained MUC5B repeats in exon 1 and a 469 residue cysteine-rich sequence in exons 2-10 that provided a 152 nucleotide overlap with cDNA clone hGBM2-3. Interestingly, the exon-intron junctions in the MUC5B genomic fragment occurred at positions equivalent to those in the D4 domain of human von Willebrand factor, suggesting that these proteins evolved from a common evolutionary ancestor through addition or deletion of exons encoding functional domains.

The pronase resistance of cholesterol-nucleating glycoproteins in human bile

BACKGROUND & AIMS

Many putative pronucleating proteins have been isolated from the biliary concanavalin A (con A)-binding fraction. The pronase resistance of the overall nucleating-promoting activity was almost never taken into consideration. The aim of this study was to identify the major pronase-resistant con A-binding glycoproteins.

METHODS

Pronase-treated and -untreated con A-binding glycoproteins were separated on a Superose 12 gel permeation column (Pharmacia, Uppsala, Sweden) and tested in a crystal growth assay. Proteins were identified by amino-terminal sequencing.

RESULTS

Con A-binding pronucleating activity eluted in two peaks on the Superose column. This activity was unaltered after pronase treatment. Activity peak I contained too little protein to allow amino-terminal sequencing. In activity peak II, the major pronase-resistant con A-binding glycoproteins were identified as alpha 1-antitrypsin and alpha 1-antichymotrypsin. The 130-kilodalton nucleation promoter was identified as aminopeptidase N, but the full pronase resistance of this protein, reported earlier, was not confirmed. Immunoabsorptive removal of alpha 1-antitrypsin and alpha 1-antichymotrypsin and immunopurification showed that only alpha 1-antichymotrypsin had pronucleating activity.

CONCLUSIONS

The pronase resistance of the nucleating-promoting activity of the con A-binding glycoprotein fraction was confirmed. An important part of this activity could be attributed to alpha 1-antichymotrypsin. It is an acute-phase protein, as are many other pronucleating proteins, which might indicate a general mechanism of action in gallstone formation.

Molecular cloning of a novel high molecular weight mucin (MG1) from human sublingual gland

A human sublingual gland cDNA library was screened with a polyclonal antiserum against deglycosylated MG1 and a positive clone, pSM2-1, was isolated which codes for 196 amino acids in the carboxyl-terminal region of this mucin. This region is cysteine-rich and contains a C2-like domain upstream of the extreme carboxyl-terminal domain in which the arrangement of cysteines is nearly identical to that in human von Willebrand factor, human intestinal mucin MUC2, human tracheobronchial mucin MUC5 and porcine and bovine submaxillary gland mucins. Northern analyses with pSM2-1 showed MG1 transcripts are abundant in sublingual gland and barely detectable in submandibular gland. This study provides the first primary sequence data on human salivary mucin MG1 and the significance of the results is discussed with respect to the biosynthesis and differential expression of MG1 in human salivary glands.

Interaction of bovine gallbladder mucin and calcium-binding protein: effects on calcium phosphate precipitation

BACKGROUND & AIMS

Gallstones consist of calcium salts and cholesterol crystals, arrayed on a matrix of gallbladder mucin (GBM), and regulatory proteins like calcium-binding protein (CBP). To determine if interactions between CBP and GBM follow a biomineralization scheme, their mutual binding and effects on CaHPO4 precipitation were studied.

METHODS

Binding of CBP to GBM was assessed by inhibition of the fluorescence of the complex of GBM with bis-1,8-anilinonaphthalene sulfonic acid (bis-ANS). The effects of the proteins on precipitation of CaHPO4 were assessed by nephelometry and gravimetry. Precipitates were analyzed for calcium, phosphate, and protein.

RESULTS

CBP and bis-ANS competitively displaced each other from 30 binding sites on mucin, with a 1:1 stoichiometry and similar affinity. The rate of precipitation of CaHPO4 was retarded by mucin and CBP. Precipitate mass was unaffected by GBM alone but decreased with the addition of CBP. Complexing CBP with GBM abolished or moderated this latter effect, altered precipitate morphology, and changed the stoichiometric ratios of Ca to PO4 in the precipitates from 1:1 to 3:2. Mucin and CBP were incorporated into the precipitates.

CONCLUSIONS

These studies suggest that the formation of calcium-containing gallstones is a biomineralization process regulated by both GBM and CBP.

Mucin-vesicle interactions in model bile: evidence for vesicle aggregation and fusion before cholesterol crystal formation

Nucleation of cholesterol monohydrate crystals from bile is a critical step in the formation of cholesterol gallstones. Measurement of nucleation in model bile system and the characteristics of the initial nucleus have proven elusive. In this study we have used three separate physical chemical techniques to examine vesicle aggregation and fusion, including dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescent biochemical assays. These assays enabled us to quantify the effect of biliary proteins, such as gallbladder mucin, on vesicle fusion and aggregation. In the absence of mucin, fusion is a relatively slow process occurring over 24 hours, whereas physiological concentrations of mucin are able to accelerate almost complete fusion of vesicles within 6 hours. Vesicle fusion and aggregation as characterized by TEM result in the formation of aggregates of multilamellar vesicles and giant fusion bodies associated with a background of mucin. These mucin-vesicle aggregate bodies may represent true nuclei and precede cholesterol monohydrate crystal nucleation. In future studies, these vesicle fusion assays can be used to quantitatively examine the effect of putative pro- and anti-nucleating proteins on the earliest steps of cholesterol crystal nucleation.

Bovine gall-bladder mucin contains two distinct tandem repeating sequences: evidence for scavenger receptor cysteine-rich repeats

Gall-bladder mucin is a densely glycosylated macromolecule which is the primary secretory product of the gall-bladder epithelium. It has been shown to bind cholesterol and other biliary lipids and to promote cholesterol crystal nucleation in vitro. In order to understand the molecular basis for mucin-lipid interactions, bovine gall-bladder mucin cDNAs were identified by expression cloning and were isolated and sequenced. The nucleotide sequences of these cDNAs revealed two distinct tandem repeating domains. One of these domains contained a 20-amino acid tandem repeating sequence enriched in threonine, serine and proline. This sequence was similar to, but not identical with, the short tandem repeating sequences identified previously in other mammalian mucins. The other domain contained a 127-amino acid tandem repeating sequence enriched in cysteine and glycine. This repeat displayed considerable sequence similarity to a family of receptor- and ligand-binding proteins containing scavenger receptor cysteine-rich repeats. By analogy with other proteins containing these cysteine-rich repeats, it is possible that, in gall-bladder mucin, this domain serves as a binding site for hydrophobic ligands such as bilirubin, cholesterol and other biliary lipids.

Identification of a 130-kilodalton human biliary concanavalin A binding protein as aminopeptidase N

BACKGROUND/AIMS

Human gallbladder bile contains a group of nonmucin glycoproteins that binds to the lectin concanavalin A (con A) and has been reported to promote cholesterol monohydrate crystal nucleation, an event preceding the formation of gallstones. Several of these proteins, including a 130-kilodalton protein, have been isolated and shown to promote nucleation in vitro. The aim of this study was to identify this and other major biliary con A binding glycoproteins.

METHODS

Gallbladder bile was chromatographed on con A agarose, and the eluted proteins were electrophoresed, blotted, and subjected to amino-terminal sequence analysis.

RESULTS

The major con A binding proteins were identified as aminopeptidase N (a 130-kilodalton protein), alpha 2 macroglobulin, hemopexin, immunoglobulin heavy chains, and the beta chain of haptoglobin. After further purification, aminopeptidase N was found to be enzymatically active and to promote cholesterol crystallization at its approximate physiological concentration in bile.

CONCLUSIONS

It is likely that aminopeptidase N is the previously characterized 130-kilodalton biliary crystallization promoting protein. Aminopeptidase N is probably released from the biliary canalicular membrane by the detergent activity of bile salts and may be one factor that promotes cholesterol crystallization in the gallbladder.

Localization of the gene for human heart fatty acid binding protein to chromosome 1p32-1p33

Heart fatty acid binding protein (hFABP) is an abundant 14-kDa cytosolic protein thought to be involved in trafficking of fatty acids from the plasma membrane to sites of beta-oxidation in mitochondria and peroxisomes and to the endoplasmic reticulum for lipid synthesis. A human hFABP cDNA isolated by polymerase chain reaction was used as a probe for in situ hybridization to metaphase chromosomes. A fragment of the gene for human hFABP was used as a probe for fluorescence in situ hybridization to metaphase chromosomes. The cDNA and genomic probes both localized the gene for human hFABP to chromosome 1p32-1p33.

Cholesterol cholelithiasis in the prairie dog: role of mucin and nonmucin glycoproteins

The aims of this study were to examine the effect of exogenous prostaglandin on mucin secretion and to determine the role of nonmucin glycoproteins on gallstone formation in the prairie dog model of cholesterol cholelithiasis. The concentration of total glycoprotein and nonmucin glycoproteins was measured in gallbladder bile from four groups of prairie dogs fed a control diet or a diet supplemented with 1.2% cholesterol with or without simultaneous subcutaneous administration of prostaglandin E1. Cholesterol feeding resulted in an increased concentration of concanavalin-A binding-proteins in gallbladder bile associated with an increase in pronucleating activity in vitro. Treatment with prostaglandin E1 and cholesterol feeding was associated with a significant increase in the incidence of cholesterol gallstone formation. Prostaglandin E1 treatment in the cholesterol-fed animals increased biliary concentrations of total glycoprotein and concanavalin-A-binding glycoproteins. Therefore the increased biliary glycoprotein level in cholesterol-fed, prostaglandin E1-treated prairie dogs, which reflects higher levels of mucin and nonmucin glycoproteins, appears to be an important factor in gallstone formation.

Amino acid sequence of human plasma galactoglycoprotein: identity with the extracellular region of CD43 (sialophorin)

The amino acid sequence of galactoglycoprotein purified from human plasma was elucidated to 75% completeness by using chemical degradation of peptides and glycopeptides derived from digests of the protein with seven specific proteases. This sequence represents a polypeptide chain of approximately 220 amino acid residues including a high content of serine, threonine, alanine, and proline with one N-linked and multiple O-linked glycans. Comparison of peptide sequences from the native galactoglycoprotein and the deglycosylated derivative demonstrated the locations of 25 sites of O-glycosylation and three serine sites that are not glycosylated. The homogeneous N terminus was established as serine. C-terminal analysis revealed multiple C-terminal residues, suggesting that galactoglycoprotein molecules are of varying lengths. A search of a protein data base revealed that the galactoglycoprotein polypeptide is identical to the N-terminal (extracellular) polypeptide region of the blood-cell surface molecule CD43 (sialophorin, leukosialin). Further support of the relatedness of these molecules was obtained by immunoprecipitation of 125I-labeled galactoglycoprotein by monoclonal anti-CD43 antibodies. The composition and properties of the molecules together with the known structure of the gene encoding CD43 suggest that galactoprotein is derived by proteolytic cleavage from transmembrane "hexasaccharide CD43," known to be expressed on neutrophils, activated T lymphocytes, and platelets.

Characterization of bovine gallbladder mucin. Amino acid sequences of tryptic peptides from the glycosylated domain of the protein core

Gallbladder mucin is a densely glycosylated macro-molecule that promotes cholesterol gallstone formation in experimental animals and in humans. Bovine gallbladder mucin structure was studied after chemical deglycosylation by treatment with anhydrous hydrogen fluoride at 23 degrees C for 3 hours. Deglycosylated mucin contained less than 5% of the amino sugar and neutral hexose content of native mucin. Electrophoretic and molecular sieve chromatographic analyses indicated that significant cleavage of the mucin polypeptide core had occurred during deglycosylation. Deglycosylated mucin was separated into three major fractions by reverse-phase chromatography, one of which was enriched with respect to threonine and proline. Tryptic peptides prepared from this fraction were purified by molecular sieve and reverse-phase chromatography, and the amino acid sequences (8-20 residues) of the four principal tryptic peptides were determined. These peptides contained 65%-75% threonine and proline residues and demonstrated 80%-100% sequence similarity. These data provide the first information on the primary structure of gallbladder mucin and suggest that repeating amino acid sequences occur in this protein. Comparison of gallbladder mucin peptide structure with the consensus repeat sequence of human intestinal mucin showed approximately 60% sequence similarity. It was concluded that mammalian gastrointestinal mucins may be derived from a common ancestral gene.

Structural studies on bovine spleen heme oxygenase. Immunological and structural diversity among mammalian heme oxygenase enzymes

Heme oxygenase is an Mr 32,000 microsomal enzyme which catalyzes the rate-limiting step in the oxidative catabolism of heme to yield equimolar quantities of biliverdin IX alpha, carbon monoxide, and iron. In the present investigation, evidence is presented suggesting that immunochemical and structural differences exist between bovine spleen heme oxygenase and heme oxygenase enzymes from other mammalian species. Using an antibody directed against bovine spleen heme oxygenase, enzyme-linked immunosorbent assays, Western blotting experiments, and cell-free translation immunoprecipitation studies showed that bovine spleen heme oxygenase is only weakly immunochemically related to heme oxygenase from rat spleen. This observation was supported by the fact that a rat spleen heme oxygenase cDNA probe did not hybridize significantly to bovine spleen heme oxygenase mRNA in Northern analyses nor to restriction fragments containing the bovine heme oxygenase gene in Southern analyses. Tryptic peptides were prepared from bovine spleen heme oxygenase and the amino acid sequences of nine peptides comprising 94 amino acid residues were determined, providing the first information on the primary structure of bovine spleen heme oxygenase. Comparison of the sequences of these tryptic peptides with regions of the deduced amino acid sequences of rat spleen and human macrophage heme oxygenase revealed sequence similarities ranging from 55 to 100%. Several peptides displaying the highest degree of sequence similarity were found to occur in regions of the heme oxygenase molecule postulated to contain the heme binding site, indicating that despite the immunochemical and apparent structural differences between bovine spleen heme oxygenase and the rat and human enzymes, functionally important amino acid residues have been conserved in the evolution of mammalian heme oxygenase genes.

Isolation and characterization of peptides from the protein core of bovine gallbladder mucin

Gallbladder mucin may promote cholesterol gallstone formation by accelerating cholesterol monohydrate crystal nucleation in supersaturated bile. In this study, peptides were isolated from the mucin protein core by protease digestion and molecular-sieve high-performance liquid chromatography. Tryptic peptides were purified by anion exchange or reverse-phase high-performance liquid chromatography, and amino acid compositions were determined. Tryptic peptides were (a) nonglycosylated, (b) selectively enriched in serine, glutamic acid plus glutamine, and glycine, and (c) depleted in threonine and proline compared with native gallbladder mucin. Bilirubin derivatized with Woodward's reagent K covalently bound to purified mucin. Tryptic digestion of the mucin-bilirubin complex yielded low-molecular-weight nonglycosylated peptides with covalently bound bilirubin. These data indicate that the mucin protein core contains at least two distinct domains. One domain is rich in threonine and proline and contains the majority of covalently bound carbohydrate. A second domain, possibly internally located, is nonglycosylated, enriched in serine, glutamic acid plus glutamine, and glycine, and binds hydrophobic ligands such as bilirubin and 1-anilino-8-naphthalene sulfonate. Hydrophobic domains on the mucin protein core may contribute to the pathogenesis of cholesterol cholelithiasis.

Studies on Cyanidium caldarium Phycobiliprotein Pigment Mutants

Phycobiliprotein biosynthesis was investigated in four strains of the unicellular rhodophyte, Cyandium caldarium, with different pigment phenotypes. All strains were incapable of synthesizing phycobiliproteins when grown in the dark. Western blotting experiments showed that dark-grown cells of the wild-type and mutant GGB synthesized the alpha and beta subunit polypeptides of allophyocyanin and phycocyanin after exposure to light for 24 hours, whereas cells of mutant IIIC and GGBY did not. Similarly, light promoted the appearance of allophycocyanin and phycocyanin mRNAs in the wild-type and GGB but not in IIIC and GGBY. However, Southern blots of restricted genomic DNA from the wild type, IIIC, GGBY, and GGB, all hybridized with heterologous phycobiliprotein gene probes and revealed that all four strains contained identical Pst, EcoRI, and Dral restriction fragments containing allophycocyanin and phycocyanin genes. Cells of the wild type and GGB incubated in the dark with the heme precursor. delta-aminolevulinate, synthesized allophycocyanin and phycocyanin apoproteins providing strong evidence for the role of a tetrapyrrole in regulation of phycobiliprotein gene expression. However, cells of IIIC and GGBY incubated in the dark with delta-aminolevulinate did not contain detectable quantities of allophycocyanin or phycocyanin apoproteins. The possible role of a tetrapyrrole in phycobiliprotein gene expression and basis for the genetic lesion in mutants IIIC and GGBY is discussed.

Cloning of rat aorta lysyl oxidase cDNA: complete codons and predicted amino acid sequence

Lysyl oxidase cDNA clones were identified by their reactivity with anti-bovine lysyl oxidase in a neonatal rat aorta cDNA lambda gt11 expression library. A 500-bp cDNA sequence encoding four of six peptides derived from proteolytic digests of bovine aorta lysyl oxidase was found from the overlapping cDNA sequences of two positive clones. The library was rescreened with a radiolabeled cDNA probe made from one of these clones, thus identifying an additional 13 positive clones. Sequencing of the largest two of these overlapping clones resulted in 2672 bp of cDNA sequence containing partial 5'- and 3'-untranslated sequences of 286 and 1159 nucleotides, respectively, and a complete open reading frame of 1227 bp encoding a polypeptide of 409 amino acids (46 kDa), consistent with the 48 +/- 3 kDa cell-free translation product of rat smooth muscle cell RNA that was immunoprecipitated by anti-bovine lysyl oxidase. The rat aorta cDNA-derived amino acid sequence contains the sequence of each of the six peptides isolated and sequenced from the 32-kDa bovine aorta enzyme, including the C-terminal peptide with sequence identity of 96%. Northern blots screened with lysyl oxidase cDNA probes identified hybridizing species of 5.8 and 4.5 kb in mRNA of rat aorta and lung, while dot blot analyses were negative for lysyl oxidase mRNA in preparations of rat brain, liver, kidney, and heart. A 258-bp segment of the 3'-untranslated region of lysyl oxidase cDNA is 93% identical with a highly conserved region of the 3'-untranslated sequence of rat elastin cDNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of human submandibular histatins

Histatins are a group of histidine-rich polypeptides found in human parotid and submandibular gland secretions. These polypeptides are microbiocidal, possibly involved in maintaining the acquired enamel pellicle, and enhance the glycolytic activity of certain oral micro-organisms. Histatins 1, 3 and 5 are homologous proteins with 38, 32 and 24 amino acid residues, respectively; the cDNAs coding for histatins 1 and 3 have now been isolated and sequenced. The cDNA sequences were highly homologous but contained differences throughout their length, indicating that they arise from different genes that may be derived from a common ancestral gene. Northern blots were hybridized to a series of oligonucleotide probes, designed on the basis of histatin cDNA sequences, and these positively identified mRNAs for histatins 1 and 3. In addition, there was a third mRNA, which hybridized to several histatin oligonucleotide probes, suggesting that histatin 5 might be derived from a distinct mRNA and not by proteolytic processing of histatin 3. A Northern blot of macaque parotid gland total RNA also showed three histatin mRNAs, indicating that similar histatins exist in a non-human primate.

Structural relationship between human salivary histatins

Histatins are a group of electrophoretically distinct histidine-rich polypeptides with microbicidal activity found in human parotid and submandibular gland secretions. Recently, we have shown that histatins 1, 3, and 5 are homologous proteins that consist of 38, 32, and 24 amino acid residues, respectively, and that these polypeptides kill the pathogenic yeast, Candida albicans. We now describe the isolation and structural characterization of histatins 2, 4, 6, and 7-12, the remaining members of this group of polypeptides. Histatin 2 was found to be identical to the carboxyl terminal 26 residues of histatin 1; histatin 4 was found to be identical to the carboxyl terminal 20 residues of histatin 3; and histatin 6 was found to be identical to histatin 5, but contained an additional carboxyl terminal arginine residue. The amino acid sequences of histatins 7-12 formally correspond to residues 12-24, 13-24, 12-25, 13-25, 5-11, and 5-12, respectively, of histatin 3, but could also arise proteolytically from histatin 5 or 6. These results establish, for the first time, the complete structural relationships between all members of this group of microbicidal proteins in human parotid saliva. The relationship of histatins to one another is discussed in the context of their genetic origin, biosynthesis and secretion into the oral cavity, and potential as reagents in anti-candidal studies.

Heme regulates expression of phycobiliprotein photogenes in the unicellular rhodophyte, Cyanidium caldarium

Allophycocyanin and phycocyanin in the red alga (Cyanidium caldarium) are chloroplast-encoded, light-harvesting accessory pigments composed of alpha and beta subunit polypeptides (17-19 kDa) to which 1 or more residues of the heme-derived bile pigment chromophore phycocyanobilin are attached by cysteinyl thioether linkages (Offner, G.D., and Troxler, R.F. (1983) J. Biol. Chem. 258, 9931-9940). Western blot experiments utilizing phycobiliprotein antisera revealed that immunoreactive allophycocyanin and phycocyanin apoproteins were absent in cells grown in the dark and present in cells exposed to light. Northern blot experiments using genomic DNA hybridization probes indicated that phycobiliprotein mRNAs were absent in the dark, whereas cells exposed to light contained two allophycocyanin mRNA transcripts, 1.4 and 1.6 kilobases in length, and one phycocyanin mRNA transcript, 3.0 kilobases in length, providing evidence that phycobiliproteins are encoded in photogenes which are only transcriptionally active in the light. Northern and Western analyses demonstrated that cells incubated in the dark with the heme precursor delta-aminolevulinate contained allophycocyanin and phycocyanin mRNAs and apoproteins, indistinguishable in size, number, and quantity from those made in the light. Cells incubated in the dark with delta-aminolevulinate, protoporphyrin IX, or heme, but not biliverdin or phycocyanobilin, synthesized allophycocyanin and phycocyanin alpha and beta apoproteins, suggesting a role for heme in the control phycobiliprotein gene expression. Cells incubated with heme in the dark produced allophycocyanin and phycocyanin mRNA transcripts, but did not produce mRNAs for four other photogenes coding for a P-700 reaction center protein, a 32-kDa herbicide-binding protein, and the large and small subunits of ribulose-bisphosphate carboxylase. These results show, for the first time, that heme is a regulatory factor specifically involved in transcriptional regulation of chloroplast genes for phycobiliproteins.

Role of tropoelastin fragmentation in elastogenesis in rat smooth muscle cells

Neonatal rat aortic smooth muscle cell cultures produce two major soluble elastin molecules termed protropoelastin (77 kDa) and tropoelastin (71 kDa). Cell layer extracts are protroproelastin-enriched, while protropoelastin, tropoelastin, and significant amounts of discrete elastin fragments (Mr of 66,000, 61,000, 56,000, and 45,000) are present in preparations from the medium of these cultures. To determine the role of the various elastin molecules in the metabolism of elastin in neonatal rat aortic smooth muscle cell cultures, the amino termini of these proteins were sequenced. All soluble elastin components present in the medium were purified as a single peak by high performance liquid chromatography; further separation of the components was achieved by polyacrylamide gel electrophoresis and electroblotting. The bands were excised and sequenced. The amino-terminal sequences of protropoelastin, tropoelastin, and the 66-kDa, 61-kDa, and 56-kDa fragments were identical: Gly-Gly-Val-Pro-Gly-Ala-Val-Pro-Gly-Gly. This sequence is identical with published amino-terminal sequences of tropoelastins from several other species. As expected, when the cell cultures were pulsed with [3H]valine, all the soluble elastin molecules were radioactive, while only protropoelastin appeared radioactive after [35S] cysteine pulsing. Since cysteine is present only in the carboxyl-terminal end of the molecule, all the data indicate that the cleavage of the elastin fragments identified in the culture are occurring at the carboxyl end of protropoelastin. These results are consistent with the original hypothesis that a precursor-product relationship exists between the 77-kDa and 71-kDa soluble elastin molecules. Based on known tropoelastin sequences and the molecular weights of the discrete fragments, additional fragmentation of protropoelastin and/or tropoelastin most likely occurs at the lysine/alanine-enriched domains presumably involved in cross-link formation.

Histatins, a novel family of histidine-rich proteins in human parotid secretion. Isolation, characterization, primary structure, and fungistatic effects on Candida albicans

Histatins 1, 3, and 5 from human parotid secretion were isolated by gel filtration on Bio-Gel P-2 and reverse phase high performance liquid chromatography. The complete amino acid sequences of histatins determined by automated Edman degradation of the proteins, Staphylococcus aureus V8 protease, and tryptic peptides, are as follows: (Sequence: see text). Histatins 1, 3, and 5 contain 38, 32, and 24 amino acid residues, have molecular weights of 4929, 4063, and 3037, respectively, and contain 7 residues of histidine. Histatin 1 contains 1 mol of phosphate/mol of protein; histatins 3 and 5 lack phosphate. With the exception of Glu (residue 4) and Arg (residue 11) in histatin 1, the first 22 amino acid residues of all three histatins are identical, and the carboxyl-terminal 7 residues of histatins 1 and 3 are also identical. The sequence, -Glu-Phe-Pro-Phe-Tyr-Gly-Asp-Tyr-Gly- (residues 23-29), in histatin 1 is absent in histatin 3; and the sequence, -Gly-Tyr-Arg- (residues 23-25), in histatin 3 is absent in histatin 1. The complete sequence of histatin 5 is contained within the amino terminal 24 residues of histatin 3. The structural data suggest that histatins 1 and 3 are derived from different structural genes, whereas histatin 5 is a proteolytic product of histatin 3. All three histatins exhibit the ability to kill the pathogenic yeast, Candida albicans.

Characterization and amino acid sequence of a fatty acid-binding protein from human heart

The complete amino acid sequence of a fatty acid-binding protein from human heart was determined by automated Edman degradation of CNBr, BNPS-skatole [3'-bromo-3-methyl-2-(2-nitrobenzenesulphenyl)indolenine], hydroxylamine, Staphylococcus aureus V8 proteinase, tryptic and chymotryptic peptides, and by digestion of the protein with carboxypeptidase A. The sequence of the blocked N-terminal tryptic peptide from citraconylated protein was determined by collisionally induced decomposition mass spectrometry. The protein contains 132 amino acid residues, is enriched with respect to threonine and lysine, lacks cysteine, has an acetylated valine residue at the N-terminus, and has an Mr of 14768 and an isoelectric point of 5.25. This protein contains two short internal repeated sequences from residues 48-54 and from residues 114-119 located within regions of predicted beta-structure and decreasing hydrophobicity. These short repeats are contained within two longer repeated regions from residues 48-60 and residues 114-125, which display 62% sequence similarity. These regions could accommodate the charged and uncharged moieties of long-chain fatty acids and may represent fatty acid-binding domains consistent with the finding that human heart fatty acid-binding protein binds 2 mol of oleate or palmitate/mol of protein. Detailed evidence for the amino acid sequences of the peptides has been deposited as Supplementary Publication SUP 50143 (23 pages) at the British Library Lending Division, Boston Spa, Yorkshire LS23 7BQ, U.K., from whom copies may be obtained as indicated in Biochem. J. (1988) 249, 5.

Human serum alpha 2HS-glycoprotein modulates in vitro bone resorption

We previously isolated a family of bone-resorbing proteins from human cancer ascites fluid and established that the three purified bone-resorbing proteins were chemically and immunochemically related to each other and to alpha-2HS glycoprotein (alpha 2HS). After this finding we purified the normal human serum counterpart of these ascites proteins and studied its effects on bone resorption. The bone-resorbing properties of normal human serum alpha 2HS were examined in vitro over a wide dose range. This normal human serum glycoprotein had a biphasic effect on 45Ca2+ release from bone. More specifically, this protein stimulated bone resorption at the lower concentrations tested, with a maximum effect [treated over control ratio of 2.5 +/- 0.30 (+/- SE); P less than 0.01] at 40 micrograms/mL. In contrast, at doses above 40 micrograms/mL, a sharp decline in calcium mobilization occurred, with a return to baseline occurring above 80 micrograms/mL. These results suggest that serum alpha 2HS may participate in the regulation of bone metabolism in vivo.