The integral and peripheral proteins of the zymogen granule membrane

Glucocorticoids regulate L-fucose glycoconjugates in rat pancreatic zymogen granules

Lectin-binding studies were performed on rat pancreatic zymogen granules to investigate the influence of glucocorticoid levels on saccharide membrane composition. The following animal groups were used: (1) control rats; (2) rats treated with hydrocortisone (1, 10 and 25 mg/kg/day) for 1, 3 and 8 days; (3) postadrenalectomized rats at days +1, +3 and +8; and (4) adrenalectomized rats receiving hydrocortisone therapy (10 mg/kg/day) for 8 days. By flow cytometry, fluoresceinated (FITC) lectins were used to measure the amount of Concanavalin A (Con A) (specific for D-mannose), wheat germ agglutinin (WGA) (specific for N-acetyl-D-glucosamine) and sialic acids and Tetragonolobus purpureus (TP) (specific for L-fucose) bound to individual zymogen granules from two subpopulations, Z1 and Z2, identified on the basis of their forward and side scatter properties. The molar ratio of the different FITC-lectins revealed significant differences in the glycoconjugate composition of Z1 and Z2 granules, the Z1 granules showing higher ratios of N-acetyl-D-glucosamine:L-fucose and N-acetyl-D-glucosamine:D-mannose, both in control, adrenalectomized and hydrocortisone-treated rats. It was also observed that N-acetyl-D-glucosamine and/or sialic acids were more abundant than L-fucose and D-mannose in the zymogen granule membrane. Z1 and Z2 granules had different glycosylation patterns. Neither adrenalectomy nor hydrocortisone treatments varied the Con A binding to zymogen granules. An increase in WGA binding was only induced by administration of very high doses of hydrocortisone (25 mg/kg/day) for 8 days, an effect not directly related to glucocorticoids. In contrast, a correlation between the FITC-TP labelling and glucocorticoid levels can be established, so that, in a time-dose dependent way, an increase was observed in zymogen granules of rats treated with hydrocortisone while a decreased TP binding was found in adrenalectomized rats-an effect which was reversed with hydrocortisone therapy. Therefore, glucocorticoids exert a direct influence on the saccharide composition of rat pancreatic zymogen granules, regulating the amount of L-fucose glycoconjugates, with Z2 granules more sensitive than Z1 ones.

Cholecystokinin regulates glycoprotein membrane composition of rat pancreatic zymogen granules

Lectin-binding studies were performed on rat pancreatic zymogen granules to investigate the alterations in the carbohydrate membrane composition under both chronic CCK stimulation and long-term CCK blockade for 3, 7 and 15 days. By flow cytometry using FITC-WGA--which specifically binds to N-acetylglucosamine and sialic acid--we measured the amount of WGA molecules bound to each individual granule. Parallel studies on pancreatic secretion were also carried out. CCK treatment displayed a differential effect on two zymogen granule subpopulations (Z1 and Z2) identified by flow cytometry on the basis of their light scatter properties: no effects on Z2 zymogen granules were observed in CCK-treated rats, while Z1 granules showed a significant increase in WGA binding at day + 7 which coincides with an increase in protein secretion in response to the hormone. On the contrary, a significant decrease in the amount of WGA receptors was observed in zymogen granule membrane of both the Z1 and Z2 subsets of rats subjected to a long-term CCK blockade. Again, these changes parallel to the reduction observed in protein secretion. Our results suggest that glycoconjugates of zymogen granule membrane involved in CCK-regulated exocytosis contain N-acetylglucosamine and sialic acid residues whose quantities are regulated by CCK.

Identification of membrane dipeptidase as a major glycosyl-phosphatidylinositol-anchored protein of the pancreatic zymogen granule membrane, and evidence for its release by phospholipase A

Membrane dipeptidase (EC 3.4.13.19) enzyme activity that is inhibited by cilastatin has been detected in pancreatic zymogen granule membranes of human, porcine and rat origin. Immunoelectrophoretic blot analysis of human and porcine pancreatic zymogen granule membranes with polyclonal antisera raised against the corresponding kidney membrane dipeptidase revealed that the enzyme is a disulphide-linked homodimer of subunit mass 61 kDa in the human and 45 kDa in the pig. Although membrane dipeptidase was, along with glycoprotein-2, one of the only two major components of carbonate high pH-washed membranes, no enzyme activity or immunoreactivity was detected in the zymogen granule contents. Digestion with bacterial phosphatidylinositol-specific phospholipase C (PI-PLC), and subsequent recognition by antibodies specific for the cross-reacting determinant, revealed that membrane dipeptidase in human and porcine pancreatic zymogen granule membranes is glycosyl-phosphatidylinositol-anchored. Membrane dipeptidase was released from the pancreatic zymogen granule membranes by an endogenous hydrolase, and the released form migrated as a disulphide-linked dimer on SDS/PAGE under non-reducing conditions. Under reducing conditions it migrated with the same apparent molecular mass as the membrane-bound form, and was still a substrate for bacterial PI-PLC. Treatment of kidney microvillar membranes with phospholipase A2 resulted in the release of membrane dipeptidase in a form that demonstrated electrophoretic and cilastatin-Sepharose binding properties identical to those of the endogenously released form of the enzyme from zymogen granule membranes. These results indicate that the glycosyl-phosphatidylinositol anchor on the pancreatic membrane dipeptidase is cleaved by an endogenous hydrolase, probably a phospholipase A, and that this cleavage may promote the release of the protein from the membrane.

Identification of a glycosylphosphatidylinositol-anchored glycoprotein with nucleoside phosphatase activity on the membrane of pig pancreatic zymogen granules

The molecular events between the second messenger-mediated triggering of regulated exocytosis and the subsequent fusion of the secretory granules with the apical plasma membrane are unclear. The glycoprotein GP-2, the most abundant of the very few proteins of the pancreatic zymogen granule membrane has been cloned and sequenced in dog and rat, but no (enzymatic) function has so far been ascribed to it. Nucleoside phosphatase activities associated with the pig zymogen granule membrane were recently assumed to be related to GP-2. To identify the protein(s) carrying these activities we have used a novel combination of native and denaturing one- and two-dimensional polyacrylamide gel electrophoresis in the detergents CHAPS, Triton X-100 or SDS. Histochemical examination on the gels and incubation with lectins and phosphatidylinositol phospholipase-C have allowed characterization of the protein with the nucleoside di- and tri-phosphatase activities. SDS-PAGE of the single protein spot with nucleoside phosphatase activity excised from Triton X-100 2-dimensional gels showed the presence of 92 kDa and 67 kDa glycoproteins. The isolated protein had an isoelectric point of 5.2, formed high molecular weight complexes, was shown to be glycosylphosphatidylinositol-anchored and contained complex carbohydrate structures. It hydrolyses di- and tri-phosphate nucleotides in dependence of the glycosylphosphatidylinositol anchor and is sensitive to non-mitochondrial diphosphohydrolase inhibitors. In summary, this paper identifies GP-2 as a nucleoside phosphatase within the zymogen granule membrane, suggesting it may be involved in energy-requiring processes on the cytosolic side of the granules.

Nucleoside phosphatase activities on pig pancreas zymogen granule membranes analyzed by non-denaturing polyacrylamide gel electrophoresis

The membrane of the pancreatic zymogen granule plays an important part in the sequence of storage, transport and exocytosis of digestive enzymes. While much is known on stimulus-secretion coupling, very little is understood about how the storage organelles move in the cytoplasm to the luminal plasma membrane and why and how they fuse with it to release the contents. It is assumed that nucleoside phosphatases are involved in these energy consuming processes. Pancreatic zymogen granule membranes contain one major glycoprotein, GP-2, and a few minor proteins all with unknown functions. In order to identify functions we have purified zymogen granule membranes from pig pancreas, solubilized the proteins under non-denaturing conditions with the detergent CHAPS and characterized the extracted proteins by polyacrylamide gel electrophoresis, histochemistry and lectins. Three major protein bands, often fused in one broad band, revealed enzymatic activity for adenosine-, cytidine-, inositol- and guanidine- di- and triphosphates by the precipitation of liberated phosphate by Pb(NO3)2. This activity was sensitive to known ATP diphosphohydrolase inhibitors. The band with activity arises from a 92 kDa glycoprotein. A different narrow band showed monophosphatase activity for AMP, GMP, IMP and CMP. Some of the activities were inhibited by different lectins, indicating glycosyl groups near the active site. Electron microscopical cytochemistry confirmed a nucleoside phosphatase activity on granule membranes. Our results show for the first time that the nucleoside phosphatase activity of the zymogen granule membranes is carried by a 92 kDa glycoprotein, probably the known self-associating form of GP-2. The hydrolysis of tri- and diphosphate nucleotides could provide the energy required by exocytosis.

Identification and characterization of carboxyl ester hydrolase as a phospholipid hydrolyzing enzyme of zymogen granule membranes from rat exocrine pancreas

Salt-washed (0.6 m NaCl) zymogen granule membranes (ZGM) of rat pancreatic acinar cells were utilized to identify and characterize membrane protein(s) responsible for phospholipase and lysophospholipase activities. Five major bands were identified in salt-washed ZGM by Coomassie Brilliant Blue. A 70-kDa protein with enzymatic activity was retained in significant quantities after several washes with 0.6 M NaCl but could be displaced from ZGM by 2 m NaCl or by 100 mg/ml heparin. By contrast, GP2, an integral membrane protein, was not displaced under these conditions. These findings suggest that the enzyme is a peripheral membrane protein of ZGM. Renaturation of ZGM proteins following electrophoresis revealed that the 70-kDa protein possessed phospholipase activity. Identification of the 70-kDa protein as a membrane-associated carboxyl ester hydrolase was based upon: (a) the use of a specific polyclonal antiserum, (b) N-terminal sequence, (c) two-dimensional gel analysis, (d) enzymatic characterization, and (e) co-localization to an area of a non-reducing gel containing significant phospholipase activity. Other ZGM proteins, namely GP2 and GP3, could not be demonstrated to possess phospholipase activity under the experimental conditions employed. Our finding that carboxyl ester hydrolase from ZGM exhibits PLA1 and lysophospholipase activities represents the first identification and characterization of a protein responsible for phospholipase activity in secretory granule membranes.

The effect of phospholipase A2 on chloride transport by pancreatic secretory granules

Secretory granules from the rat pancreas contain electrolyte transport pathways that may contribute to exocrine fluid production. The Cl- selective transport pathway was measured indirectly in isolated granules by ionophore-induced lysis after suspension in isotonic KCl. This Cl- transport was shown to respond to alterations in the granule membrane lipid environment. Exogenously added phospholipase A2 (PLA2) caused an increase of up to 193% in the Cl- specific transport by the isolated granules. In addition, the products of PLA2 hydrolysis, lysophospholipids and unesterified fatty acids, directly increased the rate of Cl- transport when incubated with granules in vitro. Lysophospholipids (2.0 micrograms/ml) increased the Cl- transport between 280-450% (depending on the lysophospholipid species). Similarly, free fatty acids (10 microM) increased the granule Cl- transport from 25% with capric acid (10:0) to 255% with arachidonic acid (20:4). The relative extent of stimulation by fatty acids was dependent on their carbon chain length and to a lesser extent, the degree of unsaturation. The inhibition (68%) of PLA2 promoted granule lysis by 4-acetamido-4'-isothiocyanatostilbene 2,2'-disulfonic acid (0.5 mM) also suggests that the effect is specific for the granule Cl- channel. Thus, the data show that zymogen granule Cl- transport is influenced by membrane lipids and supports a role for PLA2 in controlling electrolyte transport during stimulus-secretion coupling.

Reconstitution in vitro of the pH-dependent aggregation of pancreatic zymogens en route to the secretory granule: implication of GP-2

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. To elucidate the biogenesis of the secretory granule in the exocrine pancreas, we reconstituted in vitro the conditions of pH and ions believed to exist in the TGN using the end product of this sorting process, the zymogen granule contents. Protein aggregation was dependent on pH (acidic) and on the presence of cations (10 mM Ca2+, 150 mM K+) to reproduce the pattern of proteins found in the granule. The constitutive secretory protein IgG was excluded from these aggregates. Zymogen aggregation correlated with the relative proportion of the major granule membrane protein GP-2 in the assay. These results show that the glycosylphosphatidylinositol-anchored protein GP-2 co-aggregates with zymogens in the acidic environment believed to exist in the pancreatic TGN, and thus suggest that GP-2 would function as a membrane anchor for zymogen aggregates, facilitating their entrapment in budding vesicles directed towards the regulated secretory pathway.

Isolation of the membranes from secretory organelles (trichocysts) of Paramecium tetraurelia

We present for the first time a method for isolation of the membranes of extrusive organelles (trichocysts) from sterile culture of different strains of Paramecium tetraurelia. First, trichocysts are isolated according to a new method (Glas-Albrecht, R. and Plattner, H. (1990) Eur. J. Cell Biol. 53, 164-172) with high purity and yield. Then the organelles are subjected to osmotic swelling. Since trichocysts then easily 'decondense' and entangle membranes, these cannot be isolated directly by centrifugation, but only by passage through a filter and subsequent centrifugation. Purity of membrane fractions is analysed by electron microscopy and SDS-PAGE, combined with silver staining or, after biotinylation, by avidin-peroxidase labelling. Molecular masses resolved in our gels are in a range from less than or equal to 15 to greater than or equal to 105 kDa. Main bands obtained with nd9-28 degrees C trichocyst membranes (most bands also being common to wild type trichocysts) are of about 16.5, 19-21, 27-29, 33-34, 44-45 (strong), 47-48 (strong), 57, 61, 65 (strong), 68-71, 75, 81, 94-95 (strong), 104 and greater than or equal to 110 kDa, from a total of approx. 23 bands resolved. There is no remarkable occurrence of dominant protein bands from trichocyst contents ('trichynins'), though these might represent up to 10(3)-times more of the total trichocyst proteins. The ratio of phospholipid/protein is approx. 0.2 mg/mg. The methodology developed might also be valuable for the isolation of extrusome membranes from some other protozoan species.

In resting conditions, the pancreatic granule membrane protein GP-2 is secreted by cleavage of its glycosylphosphatidylinositol anchor

GP-2 is the major membrane protein of the exocrine pancreatic secretory granule. It is an integral protein which is anchored by a phosphatidylinositolglycan. In addition to being present in the soluble contents of the granule, GP-2 is also actively secreted by the pancreas. Although 93% of the GP-2 in the resting secretions of anaesthetized rats could be pelleted, Triton X-114 phase extraction showed that 70% of this GP-2 had lost its hydrophobic properties. Proteases have been postulated to release GP-2 from the membrane, but phospholipases also have the capacity to release the protein from the membrane by hydrolysis of its peculiar glycosylphosphatidylinositol membrane anchor. These studies show the presence of inositol 1,2-(cyclic)monophosphate on the secreted hydrophilic GP-2, confirming the involvement of an endogenous phospholipase C in the solubilization of GP-2 by the exocrine pancreas. It is therefore concluded that most of the GP-2 secreted by the pancreas of anaesthetized rats under resting conditions is released from the membrane by a phospholipase C which hydrolyses the phosphodiester bond linking GP-2 to its diradylglycerol anchor.

Quantitation and topography of membrane proteins in highly water-permeable vesicles from ADH-stimulated toad bladder

Antidiuretic hormone (ADH) stimulation of toad bladder granular cells rapidly increases the osmotic water permeability (Pf) of their apical membranes by insertion of highly selective water channels. Before ADH stimulation, these water channels are stored in large cytoplasmic vesicles called aggrephores. ADH causes aggrephores to fuse with the apical membrane. Termination of ADH stimulation results in prompt endocytosis of water channel-containing membranes via retrieval of these specialized regions of apical membrane. Protein components of the ADH water channel contained within these retrieved vesicles would be expected to be integral membrane protein(s) that span the vesicle's lipid bilayer to create narrow aqueous channels. Our previous work has identified proteins of 55 (actually a 55/53-kDa doublet), 17, 15, and 7 kDa as candidate ADH water channel components. We now have investigated these candidate ADH water channel proteins in purified retrieved vesicles. These vesicles do not contain a functional proton pump as assayed by Western blots of purified vesicle protein probed with anti-H(+)-ATPase antisera. Approximately 60% of vesicle protein is accounted for by three protein bands of 55, 53, and 46 kDa. Smaller contributions to vesicle protein are made by the 17- and 15-kDa proteins. Triton X-114-partitioning analysis shows that the 55, 53, 46, and 17 kDa are integral membrane proteins. Vectorial labeling analysis with two membrane-impermeant reagents shows that the 55-, 53-, and 46-kDa protein species span the lipid bilayer of these vesicles. Thus the 55-, 53-, and 46-kDa proteins possess characteristics expected for ADH water channel components. These data show that the 55- and 53- and perhaps the 46-, 17-, and 15-kDa proteins are likely components of aqueous transmembrane pores that constitute ADH water channels contained within these vesicles.

A single gene encodes membrane-bound and free forms of GP-2, the major glycoprotein in pancreatic secretory (zymogen) granule membranes

GP-2, a 75-kDa glycoprotein, was isolated from dog pancreatic zymogen granule membranes (ZGMs). In a carbohydrate-shift strategy, N-terminal and internal peptide sequences were obtained on glycosylated and deglycosylated forms of GP-2, respectively, by gas-phase sequencing. Sets of mixed oligonucleotides and the polymerase chain reaction were used to obtain a double-stranded cDNA probe, which was used to isolate overlapping cDNA clones from a dog pancreatic cDNA library. The sequence of these clones revealed an open reading frame that encodes a protein of 509 amino acids, eight N-linked oligosaccharide attachment sites, and an N-terminal signal sequence absent from the mature form of GP-2 associated with ZGMs. The C terminus shows a 20-residue hydrophobic transmembrane domain preceded by a decapeptide containing potential phosphatidylinositol-glycan attachment sites. GP-2 completely released from ZGMs by exogenous phospholipase C showed similar immunochemical properties and electrophoretic mobilities compared to the form associated with ZGMs. A similar form of GP-2 was released from zymogen granules permeabilized with saponin and incubated in the absence of added phospholipase C. Kinetic analysis of GP-2 release at 0 degrees C and 37 degrees C suggested the presence of a granule enzyme responsible for endogenous release of GP-2 to granule contents and into the apical medium. The data indicate that GP-2 is a phosphatidylinositol-glycan-linked membrane protein released from the membrane of mature zymogen granules by an enzymatic mechanism. The cDNA structure presented here thus encodes both membrane-bound and free forms of GP-2.

Characterization of phospholipase A2 and acyltransferase activities in purified zymogen granule membranes

Phospholipase A2 and acyltransferase activities were identified in membranes associated with purified pancreatic zymogen granules. In homogenate and granule membranes, phospholipase activity was linearly related to protein concentration and was Ca2(+)-dependent with an alkaline pH optimum. The Ca2+ sensitivity was observed over the range of concentrations through which intracellular ionic Ca2+ is elevated by physiological stimuli in intact cells. Intact zymogen granules and granule membranes also demonstrated reacylating activity in the presence and absence of an exogenous acceptor. Reacylating activity was related to the concentration of lyosphospholipid added and was optimally activated at alkaline pH. A more rapid rate of reacylation was observed when [14C]arachidonoyl CoA was employed as the donor molecule rather than [3H]arachidonate (plus coenzyme A); this suggests the absence of acyl-CoA synthetase in the purified granule membranes. We conclude that granule membrane phospholipase A2 and acyltransferases may be involved in arachidonic acid turnover in exocrine pancreas and perhaps in membrane fusion events associated with exocytosis.

A competition enzyme-linked immunosorbent assay (ELISA) for the measurement of pancreatic GP-2 glycoprotein

A competition enzyme-linked immunosorbent assay has been developed for the quantitative detection of soluble and membrane-bound GP-2, a glycoprotein which is confined to the exocrine pancreas. Zymogen granule membranes fixed to microtiter plates with poly-L-lysine were used as the source of antigen. Detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS] was added to the assay in order to reveal all the antigens, more particularly in membranous samples. Presence of detergent at concentrations as high as 0.5% did not interfere with any particular steps of the ELISA. This competition ELISA can detect 10 ng of GP-2 and will be useful for measuring soluble as well as membrane GP-2 in order to elucidate its role in the secretory process of the pancreas as well as in certain pathologies such as cystic fibrosis.

Properties of rabbit pepsinogen granules

Pepsinogen granules were isolated from the rabbit stomach using isoosmotic Percoll density gradients, low free calcium (10(-7) M), and conditions that minimize physical damage. These granules were enriched approximately eightfold with respect to pepsinogen and were free from contamination by mitochondria and endoplasmic reticulum. Electrophoretic examination shows pepsinogen to account for approximately 80% of the Coomassie Blue-stainable intragranular protein and the granule membrane to yield a simple spectrum of proteins similar to other granule systems. In addition to purity, the isolated granules displayed a high degree of osmotic stability at physiologic conditions of pH, temperature, and ionic strength. This stability suggests strict regulation of the granule electrolyte transport pathways, which are shown to include a Cl- conductance, Cl-/anion exchange, and a K+ conductance. These transport systems in the granule membrane are consistent with the promotion of primary fluid secretion. Furthermore, granule-mediated ion transport would allow the chief cell to couple fluid secretion directly to exocytotic pepsinogen secretion and flush the enzyme from the base of oxyntic glands.

The major protein of pancreatic zymogen granule membranes (GP-2) is anchored via covalent bonds to phosphatidylinositol

GP-2, the major integral protein characteristic of the pancreatic zymogen granule membrane can be released from the membrane by the action of a phosphatidylinositol specific phospholipase C (PI-PLC). In a hydrophobic/hydrophilic phase separation system using the non-ionic detergent Triton X-114, the membrane-bound form of the protein went from the detergent phase into the hydrophilic phase upon action of the phospholipase. PI-PLC solubilization of GP-2 unmasked an antigenic determinant similar to the cross-reacting determinant of the trypanosome variant surface glycoproteins. This determinant being a distinctive feature of the glycan moiety of phosphatidyl-inositol anchored membrane proteins, it established the glycosyl-phosphatidyl-inositol nature of the GP-2 membrane anchor. Since soluble GP-2 is also found in the contents of the granule and is secreted intact into the pancreatic juice, it is likely that one of the mechanisms responsible for its release could be a specific phospholipase. GP-2 is the first glycosyl-phosphatidyl-inositol-anchored protein that is integral to the membrane of an organelle and not located at the surface of the cell.

Immunological characterization of a membrane glycoprotein of chromaffin granules: its presence in endocrine and exocrine tissues

A glycoprotein was isolated from detergent solubilized membranes of bovine chromaffin granules by high-performance liquid chromatography. Specific antisera raised against this glycoprotein reacted in one- and two-dimensional immunoblots with a heterogeneous component with a pI of 4.2-4.7 and Mr 100,000. The antiserum against bovine glycoprotein II cross-reacted with an analogous component in several species. The specific localization of glycoprotein II in chromaffin granules was established by density gradient centrifugation followed by immunoblotting. The antiserum, as shown by one- and two-dimensional immunoblotting, reacted with an analogous antigen in the posterior pituitary, in endocrine (anterior pituitary, parathyroid gland) and exocrine (parotid gland, pancreas) organs. In the pancreas the protein reacting with the antiserum was found in the membranes of zymogen granules. The results demonstrate for the first time that secretory vesicles of endocrine and exocrine tissues have at least one common antigen, i.e. the glycoprotein II. It seems likely that this protein is involved in a basic function common to all secretory vesicles.

Labeling of the proteins at the surface of the pancreatic zymogen granule using diazotized [125I]iodosulfanilic acid

Purified pig and rat pancreatic zymogen granules have been covalently labeled with the membrane impermeant agent diazotized [125I]iodosulfanilic acid. Following alkaline lysis, the radioactivity was almost entirely (92%) recovered in a dense protein pellet designated as the 1 M sucrose pellet. The rest (8%) of the label was recovered in the membrane fraction. The specificity of this procedure in labeling the cytoplasmic aspect of the granule is demonstrated by the absence of label from granule content proteins and by the removal of iodinatable proteins following protease treatment of intact granules. No characteristic integral membrane proteins were labeled. In the pig, four major protein bands were labeled in both subfractions at Mr of 15,000, 33,000, 35,000 and 38,000. In the rat, a similar set of protein bands was labeled except for that of 15,000 Mr which was poorly labeled. Due to their location, it is suggested that these proteins may play an important role in the recognition between the granule membrane and the cell membrane and thereby the control of the exocytosis process.

Fractionation of detergent lysates of cells by ammonium sulphate-induced phase separation

A procedure is described for fractionating detergent lysates of cells based on the ability of (NH4)2SO4 to induce phase separation of detergents such as Triton X-100, sodium deoxycholate, and sodium cholate, into detergent-rich and detergent-depleted phases. An analysis of six murine lymphocyte cell surface molecules revealed that the partitioning in Triton X-100 of each molecule was highly dependent upon the (NH4)2SO4 concentration, each antigen partitioning into the detergent-rich phase at a defined salt concentration. In contrast, none of the six molecules appeared in the detergent-rich phase of a Triton X-114 phase separation, even though two of the molecules, namely Ly-2/3 and L3T4, are well-characterized integral membrane proteins. It was also observed that (NH4)2SO4 resulted in the partitioning of many nonmembrane proteins into the detergent-rich phase, indicating that the procedure can be used to fractionate all cellular proteins. By judicious choice of (NH4)2SO4 concentrations, precipitation of cellular proteins at two different (NH4)2SO4 concentrations, and combining the method with subcellular fractionation prior to detergent solubilization, substantial enrichment and concentration of particular cellular proteins could be achieved.

Ca2+-dependent protein phosphorylation associated with microsomal fraction of rat pancreas

Microsomes isolated from cat pancreas were incubated with [gamma-32P]ATP in the presence or absence of Ca2+. Following fractionation of phosphoproteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis a single microsomal protein with an apparent molecular mass of 77,000 dalton (77K) was found to be phosphorylated in a Ca2+-dependent mechanism. Maximal phosphate incorporation into the 77K protein was observed at 10(-6) mol/l [Ca2+] and was 4-fold higher than in the absence of Ca2+. The 77K phosphoprotein showed characteristic of a stable phosphoester rather than an acyl phosphate. Measurable phosphate incorporation into the 77K protein was noted 5 s following addition of [gamma-32P]ATP and reached maximum at 9-10th min. The lack of effect of exogenous cyclic AMP, cyclic AMP-dependent protein kinase, calmodulin, the calmodulin antagonist trifluoperazine, leupeptin and the suppression of phosphorylation by some phospholipid-interacting drugs suggested that the 77K protein is a substrate for cyclic AMP- and calmodulin-independent, Ca2+-activated phospholipid-sensitive kinase activity. Centrifugation of the pancreatic homogenate in a ficoll-sucrose density gradient indicated that both the 77K protein and enzyme were associated in a fraction enriched in rough endoplasmic reticulum.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase activity associated with pancreatic zymogen granules

Purified zymogen granules were prepared from rat pancreas by using an iso-osmotic Percoll gradient. In the presence of [gamma-32P]ATP, phosphorylation of several granule proteins was induced by Ca2+, most notably a Mr-13 000 protein, whereas addition of cyclic AMP was without effect. When phosphatidylserine was also added, Ca2+ increased the phosphorylation of additional proteins, with the largest effect on a protein of Mr 62 000. Purified granules were also able to phosphorylate exogenous substrates. Ca2+-induced phosphorylation of lysine-rich histone was enhanced over 3-fold in the presence of phosphatidylserine, and cyclic AMP-activated protein kinase activity was revealed with mixed histone as substrate. The concentrations of free Ca2+ and cyclic AMP required for half-maximal phosphorylation of both endogenous and exogenous proteins were 1-3 microM and 57 nM respectively. Treatment of granules with 0.25 M-KCl resulted in the release of phosphatidylserine-dependent kinase activity into a high-speed granule supernatant. In contrast, granule-protein substrates of Ca2+-activated kinase activity were resistant to KCl extraction, and in fact were present in purified granule membranes. Kinase activity activated by cyclic AMP was not extracted by KCl treatment. It is concluded that phosphorylation of integral membrane proteins in the zymogen granule can be induced by one or more Ca2+-activated protein kinases. Such a reaction is a potential mechanism by which exocytosis may be regulated in the exocrine pancreas by Ca2+-mediated secretagogues.

Identification of the proteins exposed on the cytoplasmic surface of the pancreatic zymogen granule

Lactoperoxidase-catalyzed 125I-iodination was used to label pancreatic zymogen granules. Membrane proteins facing the cytoplasmic surface were specifically labeled. Two low molecular weight proteins of 17 000 and 15 000 were intensely labeled at 0 degree C. Another small 13 kDa protein was strongly iodinated at 25 degrees C along with some others, including the 29 kDa subunit of the ATP diphosphohydrolase. The major glycoprotein of the granule membrane was not iodinated but the presence of an iodinated 80 kDa protein suggests that proteolytic fragments of the 92 kDa glycoprotein were accessible to iodination on the intact granule. These proteins localized on the cytoplasmic surface of the granule are believed to play a major role in the exocytotic phenomenon of the exocrine pancreas.