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

Gamma-glutamyltransferase-associated glycoprotein patterns in human seminal plasma of normozoospermic men: a new aspect of biomarker heterogeneity

BACKGROUND

Gamma-glutamyltransferase (GGT) is a well-known laboratory biomarker. In spite of high concentration and the possible biomedical importance of estimating GGT in human seminal plasma (hSP), it has not been widely explored in reproductive physiology. This study aimed to complement existing data on its diversity, previously obtained on seminal extracellular vesicles, by analyzing matched soluble fraction of hSP. The GGT-associated patterns of selected glycoproteins were analyzed in order to establish an adjunct referent parameter for differentiation between known high molecular mass forms of GGT. Getting insight into distinct GGT-associated glycoprotein patterns should contribute to define them together as possible multimarkers.

METHODS

GGT forms in soluble, membrane-free-fraction isolated form hSP of normozoospermic men were analyzed using gel filtration and lectin blotting using WGA (wheat germ agglutinin) and Con A (concanavalin A).

RESULTS

Widely distributed GGT (with two to three partially resolved peaks), which may correspond to high molecular mass aggregates, were detected. GGT-associated patterns of selected glycoproteins (at position of big, medium, and small-GGT) all comprised high molecular mass WGA-reactive smears, but differed in the presence of Con A-reactive glycans, as well as mucin-associated antigens CA19-9 and CA125.

CONCLUSIONS

GGT contributes to several molecular patterns that differ between the soluble and extracellular vesicle fractions of hSP. Their glycobiochemical heterogeneity is due to difference in the presence of distinct sialylated and mannosylated glycans. Moreover, GGT-associated glycoprotein patterns differentiate between high molecular mass forms of GGT in the soluble fraction of hSP. They hold promise as possible targets for increasing biomarker potential of GGT.

Intracellular Trafficking of Thyroid Peroxidase to the Cell Surface

For thyroid hormone synthesis, thyroid peroxidase (TPO) molecules must be transported from the endoplasmic reticulum via the Golgi complex to be delivered at the cell surface to catalyze iodination of secreted thyroglobulin. Like other glycoproteins, TPO molecules in transit to the cell surface have the potential to acquire endoglycosidase H resistance as a consequence of Golgi-based modification of their N-linked carbohydrates, and measurement of the intracellular distribution of TPO has often relied on this assumption. To examine TPO surface distribution in thyrocyte cell lines, we prepared new antibodies against rat TPO. Antibody reactivity was first established upon expression of recombinant rat (r) TPO in 293 cells, which were heterogeneous for surface expression as determined by flow cytometry. By cell fractionation, surface rTPO fractionated distinctly from internal pools of TPO (that co-fractionate with calnexin), yet surface TPO molecules remained endoglycosidase H (endo H)-sensitive. Although the FRTL5 (and PC Cl3) rat thyrocyte cell line also exhibits almost no endo H-resistant TPO, much of the endogenous rTPO is localized to the cell surface by immunofluorescence. Similar results were obtained by fractionation of FRTL5 cell membranes on sucrose gradients. We conclude that in FRTL5 cells, a large fraction of rTPO is delivered to the plasma membrane yet does not acquire Golgi-type processing of its N-glycans. Rat and mouse thyroid tissue TPO also shows little or no endo H resistance, although cell fractionation still needs to be optimized for these tissues.

Immunochemical characterization of Na+/H+ exchanger isoform NHE4

Mammalian Na+/H+ exchangers (NHEs) are a family of transport proteins (NHE1-NHE5). To date, the cellular and subcellular localization of NHE4 has not been characterized using immunochemical techniques. We purified a fusion protein containing a portion of rat NHE4 (amino acids 565-675) to use as immunogen. A monoclonal antibody (11H11) was selected by ELISA. It reacted specifically with both the fusion protein and to a 60- to 65-kDa polypeptide expressed in NHE4-transfected LAP1 cells. By Western blot analysis, NHE4 was identified as a 65- to 70-kDa protein that was expressed most abundantly in stomach and in multiple additional epithelial and nonepithelial rat tissues including skeletal muscle, heart, kidney, uterus, and liver. Subcellular localization of NHE4 in the kidney was evaluated by Western blot analysis of membrane fractions isolated by Percoll gradient centrifugation. NHE4 was found to cofractionate with the basolateral markers NHE1 and Na+-K+-ATPase rather than the luminal marker gamma-glutamyl transferase. In stomach, NHE4 was detected by immunoperoxidase labeling on the basolateral membrane of cells at the base of the gastric gland. We conclude that NHE4 is a 65- to 70-kDa protein with a broad tissue distribution. In two types of epithelial cells, kidney and stomach, NHE4 is localized to the basolateral membrane.

Nitrogen dioxide exposure activates gamma-glutamyl transferase gene expression in rat lung

Exposure to nitrogen dioxide (NO2) has been shown to activate glutathione metabolism in lung and lung lavage. Since GGT is a key enzyme in glutathione metabolism and we have previously characterized GGT expression in distal lung epithelium and in lung surfactant, we examined the NO2 exposed lung for induction of gamma-glutamyl transferase (GGT) mRNA, protein, and enzyme activity. We found that the GGT gene product is induced in lung by NO2. The GGT mRNA level in lung increases 2-fold within 6 hr and 3-fold after 24 hr of exposure to this oxidant gas, and this 3-fold elevation persists even after 14 days of exposure. The pattern of GGT mRNA expression switches from the single GGT mRNA III transcript in the normal lung to the dual expression of GGT mRNA I and mRNA III. Enzyme activity in whole lung increases 1.6- to 2.5-fold while extracellular surfactant-associated GGT activity accumulates 5.5-fold and GGT protein accumulates in lung surfactant. Induction of GGT mRNA and protein is evident in cells of the bronchioles by in situ hybridization and immunolocalization, respectively. In contrast, alveolar type 2 cells lack an in situ hybridization signal and exhibit a reduction in the intensity of immunostaining with prolonged exposure. Our studies show that NO2 induces GGT mRNA expression, including GGT mRNA1, in lung and GGT protein and enzyme activity in lung and lung lavage in response to the oxidative stress of NO2 inhalation.

Role of NHE3 in mediating renal brush border Na+-H+ exchange. Adaptation to metabolic acidosis

The aims of the present study were to estimate the fraction of renal brush border membrane Na+-H+ exchange activity mediated by the isoform NHE3 and to evaluate whether the increased brush border Na+-H+ exchange observed in metabolic acidosis is due to increased expression of NHE3 protein. Compared with other isoforms, NHE3 is known to have a unique profile of sensitivity to pharmacologic inhibitors, including relative resistance to amiloride analogs and HOE694. We therefore assessed the inhibitor sensitivity of pH gradient-stimulated 22Na uptake in renal brush border vesicles isolated from normal rats. The I50 values for amiloride (30 microM), dimethylamiloride (10 microM), ethylisopropylamiloride (6 microM), and HOE694 (>100 microM) were markedly dissimilar from those reported for NHE1 and NHE2 but were nearly identical to reported values for NHE3. Na+-H+ exchange activity in renal brush border vesicles isolated from rats with 5 days of NH4Cl-induced metabolic acidosis was increased 1.5-fold compared with control rats, with no change in inhibitor sensitivity. Western blot analysis indicated that NHE3 protein expression was greater in brush border membranes from acidotic compared with control rats. We conclude that virtually all measured Na+-H+ exchange activity in brush border membranes from control and acidotic rats is mediated by NHE3 and that metabolic acidosis causes increased expression of renal brush border NHE3 protein.

Two proteins associated with secretory granule membranes identified in chicken regulated secretory cells

Lately, we have identified two polypeptides of 92–94 kDa (GRL1) and 45–60 kDa (GRL2), expressed in cytoplasmic granules of chicken granulocytes and thrombocytes. Here, we report that GRL1 and GRL2 are widely distributed in all exocrine and several endocrine cell types, but not in neurons of the central nervous system, during late stages of embryonic development, as well as in newly hatched and two-month-old chickens. Immunogold studies in ultrathin frozen sections of pancreatic acinar cells show that GRL1 and GRL2 are co-localized at the periphery of zymogen granules, in granules fused with apical acinar membranes and on apical membranes of acini, while the pregranular compartments of the secretory pathway are weakly or not labeled. Semiquantitative morphometric studies indicate that GRL1 and GRL2 are equally distributed in secretory granules. A variety of physical and metabolic studies reveal that GRL2, a highly N-glycosylated polypeptide, is an intrinsic membrane protein, while GRL1 is a peripheral membrane polypeptide released by Na2CO3 treatment of granulocyte membranes. In all hematopoietic, exocrine or endocrine cells examinated, GRL1 shows identical electrophoretic patterns, while GRL2 is identified as a diffuse band, at 40–65 kDa, in hematopoietic and pancreatic cells. Taken together, the morphological and biochemical studies indicate that GRL1 and GRL2 are components of the secretory granule membrane in chicken exocrine, endocrine and hemopoietic cell types.

Glycosylation of gamma-glutamyltransferase is modified by ethanol in H5-6 hepatoma cell line

The H5-6 cultured rat hepatoma cell line was used to investigate the post-translational maturation of gamma-glutamyltransferase (GGT) and the effects of acute ethanol administration on the expression and glycosylation of this membrane-bound glycoprotein. We found that the two subunits of H5-6 GGT with molecular masses of 55 and 33 kDa were derived from a single glycosylated precursor of 80 kDa. In addition, signals of high molecular mass (more than 90 kDa) were detected. In vitro deglycosylation experiments indicated that N-linked sugars represented about 25% of the molecular weight of the H5-6 enzyme. By use of serial lectin affinity technique, we showed that N-linked sugar chains were mainly of the biantennary complex and hybrid-type, without fucose linkage to the innermost N-acetyl-glucosamine. Ethanol treatment did not seem to affect the expression of GGT and the sialic acid content of the enzyme, but altered its oligosaccharide chain composition both quantitatively and qualitatively.

Staining of pancreatic centroacinar cells, liver bile canaliculi and testicular Leydig cells with a monoclonal antibody against adrenocortical cells

The immunoreactivity of a monoclonal antibody against cell suspensions from guinea pig adrenal glands was examined at light- and electron-microscopic levels. In addition to the cell surface membrane of adrenocortical cells, the antibody labeled specific sites in the pancreas, liver and testis, but did not label any of the other tissues examined. In the pancreas, microvilli-like processes and the cell surface membrane of centroacinar cells were immunoreactive to the antibody. The microvilli of interlobular duct cells and pancreatic duct cells were also immunoreactive. In the liver, bile canalicular microvilli of hepatocytes were exclusively labeled. Membrane structures of cell organelles, mainly mitochondria, in testicular Leydig cells were also labeled. Immunoblot analysis showed that the monoclonal antibody bound to two common bands at molecular weights of approximately 62 kDa and 110 kDa in the pancreas, liver, testis, and adrenal gland. The two bands reacted with the digoxigenin-conjugated lectin, Sambucus nigra agglutinin (SNA), which recognizes sialic acid linked alpha (2-6) to galactose. Reaction patterns of SNA in the pancreas, liver and testis were similar to those of the monoclonal antibody; pancreatic centroacinar cells and interlobular duct cells, hepatocyte bile canaliculi and testicular Leydig cells were densely stained with SNA. Thus, the monoclonal antibody recognizes two common membrane glycoproteins containing sialic acids in the pancreas, liver, testis and adrenal cortex.

Light and electron microscopic studies of lectin binding to the glycocalyx of rat pancreatic cells. II. Light microscopic changes after induction of an olive-oil pancreatitis

Using a battery of 7 horseradish peroxidase marked lectins (WGA, RCA I, PHA, LCA, PNA, UEA I, LPA) or 2 unmarked lectins (Con A, VAA I) and HRP-marked antibodies, the binding to acinar cells with a postembedding technique on semithin sections of rat pancreatic tissue after olive-oil pancreatitis was studied light microscopically. The lectin binding of the normal healthy rat pancreatic tissue (Jonas et al. 1991) changed remarkably. Whereas the apical glycocalyx of acinar cells with the strong binding of WGA, RCA I, and PHA remained unchanged within the first 10 min of damage, the basolateral cell surface lost the typical specific binding of UEA I within the initial phase of pancreatitis just 2 min after injection of olive-oil. Con A and VAA I were found to be very reactive with the necrotic cells 60 min after administration of oil. The results were discussed in relation to the possible functions of the 2 main domains of the pancreatic acinar cell glycocalyx.

Chapter 17: Blood-brain barrier: a molecular approach to its structural and functional characterization

Our approach to analyze molecular components of the blood-brain barrier led to the identification of additional transcripts which can be regarded as "BBB markers". Other candidates are presently analyzed in order to find hitherto unknown cell type-specific transcripts. We investigated the expression of these marker-genes in cell culture and found all genes still being transcribed after 10 days in primary cultures, although at a lower level. This is surprising, since other authors report the disappearance of BBB characteristics under such conditions. Moreover, the BBB marker gamma-GT is found to be not only expressed in BMEC, but also in the closely associated pericytes. The hitherto unknown physiological function of the enzyme, especially the abundance in pericytes is still under investigation. Since the method of subtractive cloning has been proven as a fruitful approach, we consider to establish further subtractive cDNA libraries, using different subtraction parameters. The PCR method is applicable for amplification of subtracted cDNA (Timblin et al., 1990) and we expect to find additional clones, mainly of lower abundance which are of functional importance for the BBB phenomenon. The described characterization of cultured BMEC now allows to proceed to study BBB-specific gene expression with special regard to regulatory elements. We will perform these experiments by use of enhancer trap vectors transfected into BMEC. The isolation of the corresponding genomic DNA fragments of the BBB markers is in progress.

Pericytes of the brain microvasculature express gamma-glutamyl transpeptidase

The expression of gamma-glutamyl transpeptidase (GGT) is a specific property of the brain capillary endothelium that constitutes the blood-brain barrier. We report here the detection of GGT, not only in endothelial cells, but also in pericytes, demonstrating that a brain capillary-specific pericyte population exists. We raised antibodies to GGT using a porcine brain microvessel GGT-protein-A (staphylococcal protein A) fusion protein as antigen which was expressed in Escherichia coli. The immunohistochemical analysis of the subcapillary distribution of GGT in porcine brain cortex and cerebellum sections by both light and electron microscopy revealed the expression of GGT in the capillary-adjacent pericytes in addition to the GGT-positive endothelial layer. We confirmed these data for cultured porcine brain microvascular endothelial cells and pericytes. GGT immunofluorescence could be detected in both cell types in culture. Endothelial cells exhibited a weak staining, whereas pericytes were strongly positive for GGT. Due to the high phagocytotic activity of pericytes and their location on the abluminal surface of the microvessels, we propose a possible protective or detoxifying function of GGT in cerebrovascular pericytes.

Glutathione metabolism in the pancreas compared with that in the liver, kidney, and small intestine

The pancreas plays a major role, along with the kidney, liver, small intestine, and several other organs, in glutathione (GSH) metabolism, as evidenced by the large concentration of GSH in the pancreas, its rapid turnover rate, and the presence, at significant levels, of various enzymes involved in GSH metabolism. The pancreas appears to obtain much of the cysteine that is required for both GSH and protein synthesis by hydrolyzing plasma GSH to its constituent amino acids and then transporting cysteine into the cells. GSH hydrolysis is accomplished by the ectoenzymes gamma-glutamyl transferase (GGTase) and aminopeptidase N, both of which are present in the pancreas. Only the kidney has a greater GGTase activity. Although pancreatic GSH synthesis has not been directly demonstrated, pancreatic secretory protein synthesis is substantial, and these proteins contain significant amounts of cysteine as disulfides. The pancreas also contains significant levels of protein disulfide isomerase, glutathione peroxidase, and NADPH:GSH oxidoreductase. Protein disulfide isomerase, using oxidized glutathione generated by glutathione peroxidase, is important in the formation of disulfide bonds in secretory proteins in the pancreas. No other organ has a higher specific activity of protein disulfide isomerase. By analogy with kidney and liver, the pancreas presumably exhibits a rapid apical secretion of GSH. The purpose of this apical secretion is unknown in the kidney. In the liver, it is important in bile secretion. The large GGTase activity of apical plasma membranes in the pancreas is likely to be instrumental in the hydrolysis, and subsequent recovery of the constituent amino acids of apically secreted GSH, as occurs in the kidney and liver.

Expression of multiple proteins structurally related to gamma-glutamyl transpeptidase in non-neoplastic adult rat hepatocytes in vivo and in culture

Freshly isolated hepatocytes from normal adult rat liver do not express measurable gamma-glutamyl transpeptidase (GGT) mRNA in contrast to the significant GGT mRNA levels expressed by normal adult rat kidney and hyperplastic bile ductular tissue from bile duct-ligated rats. However, the induction of GGT activity in rat hepatocytes by two-thirds hepatectomy was accompanied by the appearance of a high level of GGT mRNA. We are now able to demonstrate that normal adult rat hepatocytes express 5 protein bands which cross-react with 2 different anti-rat kidney GGT antisera. The apparent molecular weights were 26.9, 58.0, 63.9, 73.5, and 83.4 kDa, respectively. Expression of the 26.9- and 58.0-kDa proteins strikingly parallels the pattern of induction of GGT enzymatic activity. This suggests that these 2 proteins correspond to the active dimeric enzyme previously described in kidney and neoplastic hepatocellular tissue. In normal hepatocytes, the 73.5-kDa protein represents 50% of the total GGT-immunoreactive protein, in contrast to kidney, where this band contains less than 4% of the GGT protein. The kinetics of expression of the 73.5-kDa protein upon induction of GGT activity in hepatocytes, as well as in culture turnover studies, suggests that this protein is a precursor form of the active enzyme, such as the described 78/79-kDa single-chain glycoprotein propeptide of GGT. It appears that in normal hepatocytes, this precursor is not processed to the same extent as in kidney or in hyperplastic bile ductular tissue.

Gamma-glutamyltransferase from human hepatoma cell lines: purification and cell culture of HepG2 on microcarriers

After screening different human hepatoma cell lines, we observed that both HepG2 and PLC/PRF/5 naturally produced large amounts of gamma-glutamyltransferase. We optimized HepG2 cell culture conditions and observed that higher cell densities were obtained when cells were cultured on microcarriers, particularly when Cytodex 3 was used and that cell growth was optimal when DMEM, the basic medium, was supplemented with 5% fetal calf serum and 6 mmol/l glutamine. These culture conditions allowed us to produce the highest amounts of GGT after about 150 h of culture. The GGT obtained from HepG2 cells was partially purified and some of its physico-chemical properties characterized. Successive Con A gel chromatography separated the activity into two peaks, suggesting that GGT from HepG2 is not uniformly glycosylated. Papain-treated HepG2 GGT showed a Mr of about 120 kDa and migrated as a single-chain protein in SDS-PAGE. Immunological and kinetic properties of the GGT were similar to other human GGTs (liver, kidney and serum). It appears that HepG2 GGT could be a source for the preparation of a human enzyme reference material.

Electrophoretic mobility of gamma-glutamyltransferase in rat liver subcellular fractions. Evidence for structure difference from the kidney enzyme

Adult rat liver gamma-glutamyltransferase (GGT) has been poorly characterized because of its very low concentration in the tissue. In contrast with the kidney, the liver enzyme is inducible by some xenobiotics, and its relationship to hepatic ontogeny and carcinogenesis seems to be important. Liver GGT polypeptides were identified by immunoblot analysis in subcellular fractions (rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi membranes and plasma membranes). Rat liver GGT appeared as a series of polypeptides corresponding to different maturation steps. Polypeptides related to the heavy subunit of GGT were detected in rough endoplasmic reticulum at 49, 53 and 55 kDa, and in Golgi membranes at 55, 60 and 66 kDa. Two polypeptides related to the light subunit of GGT were also observed in Golgi membranes. In plasma membranes GGT was composed of 100 kDa, 66 kDa and 31 kDa polypeptides. The 66 kDa component could correspond to the heavy subunit of the rat liver enzyme, and if so has a molecular mass higher than that of the purified rat kidney form of GGT (papain-treated). These data suggest different peptide backbones for the heavy subunits of liver GGT and kidney GGT.

Intramolecular crosslinking of gamma-glutamyl transpeptidase

gamma-Glutamyl transpeptidase (rat kidney) is a heterodimeric glycoprotein (subunit molecular weights 52,000 and 25,000). In addition to its single-chain biosynthetic precursor (Mr 78,000), glycosylated high molecular weight forms (Mr 85,000-95,000) have been reported in various rat tissues as well as during in vitro translation of its mRNA. Studies reported here suggest that these might be attributed to the anomalous behavior of intramolecularly crosslinked species. Thus, chemical crosslinking of the purified enzyme (as well as enzyme on the renal brush border membranes) by bifunctional reagents such as dimethyl suberimidate and by an active site-directed reagent, diazotized p-amino-hippurate, produces stable heterodimers which exhibit molecular weights identical to that of the native enzyme when subjected to gel filtration. However, when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the crosslinked species exhibit apparent Mr values of 85,000 to 110,000, depending upon the crosslinking agent used. Protein glycosylation alone does not account for such anomalous electrophoretic behavior; the extent and the regions of the enzyme involved in formation of crosslinks appear to exert considerable constraints upon their conformation even in denaturing media.

Localization of Na+,K+-ATPase alpha-subunit to the sinusoidal and lateral but not canalicular membranes of rat hepatocytes

Controversy has recently developed over the surface distribution of Na+,K+-ATPase in hepatic parenchymal cells. We have reexamined this issue using several independent techniques. A monoclonal antibody specific for the endodomain of alpha-subunit was used to examine Na+,K+-ATPase distribution at the light and electron microscope levels. When cryostat sections of rat liver were incubated with the monoclonal antibody, followed by either rhodamine or horseradish peroxidase-conjugated goat anti-mouse secondary, fluorescent staining or horseradish peroxidase reaction product was observed at the basolateral surfaces of hepatocytes from the space of Disse to the tight junctions bordering bile canaliculi. No labeling of the canalicular plasma membrane was detected. In contrast, when hepatocytes were dissociated by collagenase digestion, Na+,K+-ATPase alpha-subunit was localized to the entire plasma membrane. Na+,K+-ATPase was quantitated in isolated rat liver plasma membrane fractions by Western blots using a polyclonal antibody against Na+,K+-ATPase alpha-subunit. Plasma membranes from the basolateral domain of hepatocytes possessed essentially all of the cell's estimated Na+,K+-ATPase catalytic activity and contained a 96-kD alpha-subunit band. Canalicular plasma membrane fractions, defined by their enrichment in alkaline phosphatase, 5' nucleotidase, gamma-glutamyl transferase, and leucine aminopeptidase had no detectable Na+,K+-ATPase activity and no alpha-subunit band could be detected in Western blots of these fractions. We conclude that Na+,K+-ATPase is limited to the sinusoidal and lateral domains of hepatocyte plasma membrane in intact liver. This basolateral distribution is consistent with its topology in other ion-transporting epithelia.

Evidence for stable homodimers and heterodimers of gamma-glutamyltranspeptidase subunits under protein-denaturing conditions

gamma-Glutamyltranspeptidase is synthesized as a core glycosylated propeptide (Mr 75,000) which is subsequently cleaved to yield a stable heterodimeric structure (subunit Mr 50,000 and 30,000). The propeptide represents an insignificant mass of the transpeptidase but higher molecular weight bands designated H1 (Mr 85,000) and H2 (Mr 100,000) are readily observed by protein staining or immunoblot analysis of the enzyme or crude membranes after SDS-polyacrylamide gel electrophoresis. Although H1 and H2 represent the predominant antigenic forms of transpeptidase in tissues which exhibit relatively low specific enzyme activity, neither their structure nor their physiological function is known. In order to determine the relationship between H1 and H2, and the large (L) and small (S) subunits of the transpeptidase, individual bands (H1, H2, L and S) of the purified renal enzyme were cut from a Coomassie-stained SDS gel, eluted and re-electrophoresed. Isolated S produced S and dimers of S (Mr 60,000), while isolated L produced L and dimers of L corresponding to H2. Equivalent mixtures of L and S also produced H1. Utilizing IgG affinity-purified against either L or S, immunoblot analysis confirmed that H2 is a dimer of L, and H1 is a heterodimer of L and S. However, monoclonal IgG which recognizes both transpeptidase propeptide and native heterodimer did not react with H1. Thus, it is clear that isolated L and S can form and maintain unique dimeric structures during SDS-polyacrylamide gel electrophoresis. With this information it should now be possible to ascertain the basis for the apparent predominance of H1 and H2 in non-renal tissues.

A common spectrum of polypeptides occurs in secretion granule membranes of different exocrine glands

A highly purified membrane preparation from rat parotid secretion granules has been used as a comparative probe to examine the extent of compositional overlap in granule membranes of three other exocrine secretory tissues--pancreatic, lacrimal, and submandibular--from several standpoints. First, indirect immunofluorescent studies using a polyclonal polyspecific anti-parotid granule membrane antiserum has indicated a selective staining of granule membrane profiles in all acinar cells of all tissues. Second, highly purified granule membrane subfractions have been isolated from each exocrine tissue; comparative two-dimensional (isoelectric focusing; SDS) PAGE of radioiodinated granule membranes has identified 10-15 polypeptides of identical pI and apparent molecular mass. These species are likely to be integral membrane components since they are not extracted by either saponin-sodium sulfate or sodium carbonate (pH 11.5) treatments, and they do not have counterparts in the granule content. Finally, the identity among selected parotid and pancreatic radioiodinated granule membrane polypeptides has been documented using two-dimensional peptide mapping of chymotryptic and tryptic digests. These findings clearly indicate that exocrine secretory granules, irrespective of the nature of stored secretion, comprise a type of vesicular carrier with a common (and probably refined) membrane composition. Conceivably, the polypeptides identified carry out general functions related to exocrine secretion.

Temperature-sensitive steps in the transport of secretory proteins through the Golgi complex in exocrine pancreatic cells

The effect of temperature on secretory protein transport was studied by cell fractionation of rat pancreatic lobules, pulse-labeled in vitro with [35S]methionine and chased for 60 min at 16, 20, or 37 degrees C. Chase at 37 degrees C allowed secretory proteins to reach a zymogen granule fraction, whereas chase at 16 or 20 degrees C led to their extensive retention in a total microsomal fraction. To pinpoint the sites of transport inhibition, total microsomes were subfractionated by flotation in a sucrose density gradient. Five bands were resolved, of which the heaviest or B1 (density = 1.20 g/ml) consisted primarily of rough microsomes. The lighter fractions, B2 (1.17 g/ml), B3 (1.15 g/ml), and B4 (1.14-1.13 g/ml), consisted primarily of smooth vesicles derived from Golgi elements. B4 had the highest specific activity for galactosyltransferase, a trans Golgi cisternal marker; B2, B3, and B4 are assumed to represent cis, middle, and trans Golgi subcompartments, respectively. At the end of a 2-min pulse, a single peak of labeled proteins colocalized with B1. During subsequent 60-min chases, labeled proteins advanced to B2 at 16 degrees C and to B3 at 20 degrees C. At 37 degrees C the radioactivity remaining in the total microsomal fraction was distributed among four peaks (B1-B4). The results indicate that transport from the endoplasmic reticulum to the Golgi complex is strongly inhibited below 20 degrees C. At 16 degrees C, the bulk of the cohort of labeled secretory proteins is still in the rough endoplasmic reticulum, but its advancing front reaches cis Golgi elements. At 20 degrees C, the front advances to a middle Golgi compartment, and at 37 degrees C most of the cohort (approximately 70%) reaches condensing vacuoles and zymogen granules. It is concluded that transport steps along the endoplasmic reticulum-plasmalemma pathway have distinct temperature requirements.