Secretory membranes of the rat parotid gland: preparation and comparative characterization

Proteomic analysis of pancreatic zymogen granules: identification of new granule proteins

The composition of zymogen granules from rat pancreas was determined by LC-MS/MS. Enriched intragranular content, peripheral membrane, and integral membrane protein fractions were analyzed after one-dimensional SDS-PAGE and tryptic digestion of gel slices. A total of 371 proteins was identified with high confidence, including 84 previously identified granule proteins. The 287 remaining proteins included 37 GTP-binding proteins and effectors, 8 tetraspan membrane proteins, and 22 channels and transporters. Seven proteins, pantophysin, cyclic nucleotide phosphodiesterase, carboxypeptidase D, ecto-nucleotide phosphodiesterase 3, aminopeptidase N, ral, and the potassium channel TWIK-2, were confirmed by immunofluorescence microscopy or by immunoblotting to be new zymogen granule membrane proteins.

Complementary hydropathy identifies a cellular prion protein receptor

Prions, the etiological agents for infectious degenerative encephalopathies, act by entering the cell and inducing conformational changes in PrPC (a normal cell membrane sialoglycoprotein), which result in cell death. A specific cell-surface receptor to mediate PrPC and prion endocytosis has been predicted. Complementary hydropathy let us generate a hypothetical peptide mimicking the receptor binding site. Antibodies raised against this peptide stain the surface of mouse neurons and recognize a 66-kDa membrane protein that binds PrPC both in vitro and in vivo. Furthermore, both the complementary prion peptide and antiserum against it inhibit the toxicity of a prion-derived peptide toward neuronal cells in culture. Such reagents might therefore have therapeutic applications.

Evidence for G proteins in rat parotid plasma membranes and secretory granule membranes

G proteins were identified in rat parotid plasma membrane-enriched fractions and in two populations of isolated secretory granule membrane fractions. Both [32P]ADP-ribosylation analysis with bacterial toxins and immunoblot analysis with crude and affinity-purified antisera specific for alpha subunits of G proteins were utilized. Pertussis toxin catalysed the ADP-ribosylation of a 41 kDa substrate in the plasma membrane fraction and both secretory granule membrane fractions. Cholera toxin catalysed the ADP-ribosylation of two substrates with molecular masses of 44 kDa and 48 kDa in the plasma membrane fraction but not in the secretory granule fractions. However, these substrates were detected in the secretory granule fractions when recombinant ADP-ribosylating factor was present in the assay medium. Immunoblot analysis of rat parotid membrane fractions using both affinity-purified and crude antisera revealed strong immunoreactivity of these membranes with anti-Gs alpha, -Gi alpha 1/alpha 2 and -Gi alpha 3 sera. In contrast Gs alpha was the major substrate found in both of the secretory granule fractions. Granule membrane fractions also reacted moderately with anti-Gi alpha 3 antiserum, and weakly with anti-Gi alpha 1/alpha 2 and -G(o) alpha sera. The results demonstrate that the parotid gland membranes express a number of G proteins. The presence of G proteins in secretory granule membranes suggests that they may play a direct role in regulating exocytosis in exocrine glands.

Isolated secretion granules from parotid glands of chronically stimulated rats possess an alkaline internal pH and inward-directed H+ pump activity

Secretion granules have been isolated from the parotid glands of rats that have been chronically stimulated with the beta-adrenergic agonist, isoproterenol. These granules are of interest because they package a quantitatively different set of secretory proteins in comparison with granules from the normal gland. Polypeptides enriched in proline, glycine, and glutamine, which are known to have pI's greater than 10, replace alpha-amylase (pI's = 6.8) as the principal content species. The internal pH of granules from the treated rats ranges from 7.8 in a potassium sulfate medium to 6.9 in a choline chloride medium. The increased pH over that of normal parotid granules (approximately 6.8) appears to reflect the change in composition of the secretory content. Whereas normal mature parotid granules have practically negligible levels of H+ pumping ATPase activity (Arvan, P., G. Rudnick, and J. D. Castle, 1985, J. Biol. Chem., 260, 14945-14952) the isolated granules from isoproterenol-treated rats undergo a time-dependent internal acidification (approximately 0.2 pH unit) that requires the presence of ATP and is abolished by an H+ ionophore. Additionally, an inside-positive granule transmembrane potential develops after ATP addition that depends upon ATP hydrolysis. Two independent methods have been used that exclude the possibility that contaminating organelles are the source of the H+-ATPase activity. Together these data provide clear evidence for the presence of an H+ pump in the membranes of parotid granules from chronically stimulated rats. However, despite the presence of H+-pump activity, fluorescence microscopy with the weak base, acridine orange, reveals that the intragranular pH in live cells is greater than that of the cytoplasm.

Identification of high molecular weight antigens structurally related to gamma-glutamyl transferase in epithelial tissues

Heterologous antibodies to gamma-glutamyl transferase (gamma GT), an ectoenzyme associated with the apical surface of many types of epithelial cells involved in secretion and transport, have been used to identify and partially characterize the spectrum of antigens in a series of epithelial tissues that exhibit a range of enzyme activities. In addition to antigens corresponding to the subunits of the active enzyme (mol wt 55K, 30K), antigens of mol wt approximately 85-greater than or equal to 95K have been detected using an antibody raised against the enzyme purified in nonionic detergent. The latter species are shown to share antigenic determinants with and to be structurally related to the enzyme subunits; however, they do not blind significantly to antibodies raised to protease-solubilized gamma GT. Further, they constitute the major antigens in tissues that exhibit relatively low levels of enzyme activity. These polypeptides are apparently larger than a recently characterized biosynthetic precursor of the gamma GT subunits. Although they do not have gamma GT activity themselves and their function is undefined, the possibility that they may represent highly glycosylated polypeptides related either to gamma GT precursors (that persist without processing) or to the large enzyme subunit merits consideration.