Expression of individual forms of peptidylglycine alpha-amidating monooxygenase in AtT-20 cells: endoproteolytic processing and routing to secretory granules

Peptidylglycine alpha-amidating monooxygenase (PAM: EC 1.14.17.3) is a bifunctional protein which catalyzes the COOH-terminal amidation of bioactive peptides; the NH2-terminal monooxygenase and mid-region lyase act in sequence to perform the peptide alpha-amidation reaction. Alternative splicing of the single PAM gene gives rise to mRNAs generating PAM proteins with and without a putative transmembrane domain, with and without a linker region between the two enzymes, and forms containing only the monooxygenase domain. The expression, endoproteolytic processing, storage, and secretion of this secretory granule-associated protein were examined after stable transfection of AtT-20 mouse pituitary cells with naturally occurring and truncated PAM proteins. The transfected proteins were examined using enzyme assays, subcellular fractionation, Western blotting, and immunocytochemistry. Western blots of crude membrane and soluble fractions of transfected cells demonstrated that all PAM proteins were endoproteolytically processed. When the linker region was present between the monooxygenase and lyase domains, monofunctional soluble enzymes were generated from bifunctional PAM proteins; without the linker region, bifunctional enzymes were generated. Soluble forms of PAM expressed in AtT-20 cells and soluble proteins generated through selective endoproteolysis of membrane-associated PAM were secreted in an active form into the medium; secretion of the transfected proteins and endogenous hormone were stimulated in parallel by secretagogues. PAM proteins were localized by immunocytochemistry in the perinuclear region near the Golgi apparatus and in secretory granules, with the greatest intensity of staining in the perinuclear region in cell lines expressing integral membrane forms of PAM. Monofunctional and bifunctional PAM proteins that were soluble or membrane-associated were all packaged into regulated secretory granules in AtT-20 cells.     

Evidence that the ability to respond to a calcium stimulus in exocytosis is determined by the secretory granule membrane: Comparison of exocytosis of injected bovine chromaffin granule membranes and endogenous cortical granules inXenopus laevis oocytes

Summary 1. To understand better the mechanisms which govern the sensitivity of secretory vesicles to a calcium stimulus, we compared the abilities of injected chromaffin granule membranes and of endogenous cortical granules to undergo exocytosis inXenopus laevis oocytes and eggs in response to cytosolic Ca²⁺. Exocytosis of chromaffin granule membranes was detected by the appearance of dopamine--hydroxylase of the chromaffin granule membrane in the oocyte or egg plasma membrane. Cortical granule exocytosis was detected by release of cortical granule lectin, a soluble constituent of cortical granules, from individual cells.2. Injected chromaffin granule membranes undergo exocytosis equally well in frog oocytes and eggs in response to a rise in cytosolic Ca²⁺ induced by incubation with ionomycin.3. Elevated Ca²⁺ triggered cortical granule exocytosis in eggs but not in oocytes.4. Injected chromaffin granule membranes do not contribute factors to the oocyte that allow calcium-dependent exocytosis of the endogenous cortical granules.5. Protein kinase C activation by phorbol esters stimulates cortical granule exocytosis in bothXenopus laevis oocytes andX. laevis eggs (Bement, W. M., and Capco, D. G.,J. Cell Biol. 108, 885–892, 1989). Activation of protein kinase C by phorbol ester also stimulated chromaffin granule membrane exocytosis in oocytes, indicating that although cortical granules and chromaffin granule membranes differ in calcium responsiveness, PKC activation is an effective secretory stimulus for both.6. These results suggest that structural or biochemical characteristics of the chromaffin granule membrane result in its ability to respond to a Ca²⁺ stimulus. In the oocytes, cortical granule components necessary for Ca²⁺-dependent exocytosis may be missing, nonfunctional, or unable to couple to the Ca²⁺ stimulus and downstream events.     

Secretory Granule Membrane Protein Recycles through Multivesicular Bodies

The recycling of secretory granule membrane proteins that reach the plasma membrane following exocytosis is poorly understood. As a model, peptidylglycine α‐amidating monooxygenase (PAM), a granule membrane protein that catalyzes a final step in peptide processing was examined. Ultrastructural analysis of antibody internalized by PAM and surface biotinylation showed efficient return of plasma membrane PAM to secretory granules. Electron microscopy revealed the rapid movement of PAM from early endosomes to the limiting membranes of multivesicular bodies and then into intralumenal vesicles. Wheat germ agglutinin and PAM antibody internalized simultaneously were largely segregated when they reached multivesicular bodies. Mutation of basally phosphorylated residues (Thr⁹⁴⁶, Ser⁹⁴⁹) in the cytoplasmic domain of PAM to Asp (TS/DD) substantially slowed its entry into intralumenal vesicles. Mutation of the same sites to Ala (TS/AA) facilitated the entry of internalized PAM into intralumenal vesicles and its subsequent return to secretory granules. Entry of PAM into intralumenal vesicles is also associated with a juxtamembrane endoproteolytic cleavage that releases a 100‐kDa soluble PAM fragment that can be returned to secretory granules. Controlled entry into the intralumenal vesicles of multivesicular bodies plays a key role in the recycling of secretory granule membrane proteins.     

Secretion Stimulates Intramembrane Proteolysis of a Secretory Granule Membrane Enzyme

Regulated intramembrane proteolysis, a highly conserved process employed by diverse regulatory pathways, can release soluble fragments that directly or indirectly modulate gene expression. In this study we used pharmacological tools to identify peptidylglycine α-amidating monooxygenase (PAM), a type I secretory granule membrane protein, as a γ-secretase substrate. PAM, an essential enzyme, catalyzes the final step in the synthesis of the majority of neuropeptides that control metabolic homeostasis. Mass spectroscopy was most consistent with the presence of multiple closely spaced NH₂ termini, suggesting that cleavage occurred near the middle of the PAM transmembrane domain. The luminal domains of PAM must undergo a series of prohormone convertase or α-secretase-mediated cleavages before the remaining transmembrane domain/cytosolic domain fragment can undergo a γ-secretase-like cleavage. Cleavage by γ-secretase generates a soluble fragment of the cytosolic domain (sf-CD) that is known to localize to the nucleus. Although PAM sf-CD is unstable in AtT-20 corticotroph tumor cells, it is readily detected in primary rat anterior pituitary cells. PAM isoform expression, which is tissue-specific and developmentally regulated, affects the efficiency with which sf-CD is produced. sf-CD levels are also modulated by the phosphorylation status of the cytosolic domain and by the ability of the cytosolic domain to interact with cytosolic proteins. sf-CD is produced by primary rat anterior pituitary cells in response to secretogogue, suggesting that sf-CD acts as a signaling molecule relaying information about secretion from the secretory granule to the nucleus.     

Trafficking of a secretory granule membrane protein is sensitive to copper

We explored the effect of copper availability on the synthesis and trafficking of peptidylglycine alpha-amidating monooxygenase (PAM), an essential cuproenzyme whose catalytic domains function in the lumen of peptide-containing secretory granules. Corticotrope tumor cell lines expressing integral membrane and soluble forms of PAM were depleted of copper using bathocuproinedisulfonic acid or loaded with copper by incubation with CuCl(2). Depleting cellular copper stimulates basal secretion of soluble enzyme produced by endoproteolytic cleavage of PAM in secretory granules and transit of membrane PAM though the endocytic pathway and back into secretory granules. Unlike many cuproenzymes, lack of copper does not lead to instability of PAM. Copper loading decreases cleavage of PAM in secretory granules, secretion of soluble enzyme, and the return of internalized PAM to secretory granules. The trafficking and stability of the soluble, luminal domain of PAM and truncated membrane PAM lacking a cytosolic domain are not affected by copper availability. Taken together, our data demonstrate a role for copper-sensitive cytosolic machinery in directing endocytosed membrane PAM back to secretory granules or to a degradative pathway. The response of PAM to lack of copper suggests that it facilitates copper homeostasis.     

Differential regulation of granule-to-granule and granule-to-plasma membrane fusion during secretion from rat pituitary lactotrophs

We used fluorescence imaging of individual exocytic events together with electron microscopy to study the regulation of dense core granule-to-plasma membrane fusion and granule-to-granule fusion events that occur during secretion from rat pituitary lactotrophs. Stimulating secretion with elevated extracellular potassium, with the calcium ionophore ionomycin, or with thyrotropin releasing hormone or vasoactive intestinal polypeptide resulted in abundant exocytic structures. Approximately 67% of these structures consisted of multiple granules fused together sharing a single exocytic opening with the plasma membrane, i.e., compound exocytosis. For all of these stimulation conditions there appeared to be a finite number of plasma membrane fusion sites, approximately 11 sites around each cellular equator. However, a granule could fuse directly with another granule that had already fused with the plasma membrane even before all plasma membrane sites were occupied. Granule-to-plasma membrane and granule-to-granule fusion events were subject to different regulations. Forskolin, which can elevate cAMP, increased the number of granule-to-granule fusion events without altering the number of granule-to-plasma membrane fusion events. In contrast, the phorbol ester PMA, which activates protein kinase C increased both granule-to-granule and granule-to-plasma membrane fusion events. These results provide a cellular mechanism that can account for the previously demonstrated potentiation of secretion from lactotrophs by cAMP- and PKC-dependent pathways.     

Membrane-associated peptidylglycine alpha-amidating monooxygenase in the heart

Recent investigations have shown that the heart atrium is an endocrine tissue. In the present studies, high levels of peptidylglycine alpha-amidating monooxygenase (PAM), which catalyzes the formation of bioactive alpha-amidated peptides from their glycine-extended precursors, have been found in particulate fractions from bovine and rat heart atrium; only low levels of PAM activity were present in soluble fractions. Corresponding fractions from the ventricles contained 20-fold less activity. Immunocytochemical studies demonstrated that PAM was localized primarily to atrial cardiocytes, with a distribution resembling that of atriopeptin. Following differential centrifugation of rat atrial homogenates, most of the PAM activity was associated with crude granule fractions, with lesser amounts of activity associated with crude microsomal fractions. Upon further subcellular fractionation, PAM activity in the rat atrium was found primarily with immunoactive atriopeptin in fractions enriched in secretory granules. Following sodium dodecyl sulfate-polyacrylamide gel electrophoresis, antisera to purified bovine pituitary PAM identified a 113,000-dalton protein in bovine atrial microsomes and secretory granules; the protein predicted from the sequence of the cDNA encoding bovine pituitary PAM is of similar size (Eipper, B. A., Park, L. P., Dickerson, I. M., Keutmann, H. T., Thiele, E. A., Rodriguez, H., Schofield, P. R., and Mains, R. E. (1987) Mol. Endocrinol. 1, 777-790). Northern blot analysis using cDNA probes encoding bovine pituitary PAM demonstrated higher levels of PAM mRNA in heart atrium than in anterior pituitary. Rat heart contains PAM mRNA species of 3.6 and 3.8 kilobases, the smaller mRNA species corresponding in size to the PAM mRNA expressed in rat anterior pituitary.     

Membrane-associated forms of peptidylglycine alpha-amidating monooxygenase activity in rat pituitary. Tissue specificity

Membrane-associated peptidylglycine alpha-amidating monooxygenase (PAM) activity was investigated in rat anterior and neurointermediate pituitary tissues and in pituitary AtT-20/D-16v and GH3 cell lines. A substantial fraction of total pituitary PAM activity was found to be membrane-associated. Triton X-100, N-octyl-beta-D-glucopyranoside, and Zwittergent were effective in solubilizing PAM activity from crude pituitary membranes. The distribution of enzyme activity between soluble and membrane-associated forms was tissue-specific. In the anterior pituitary lobe and pituitary cell lines, 40-60% of total PAM activity was membrane-associated while only 10% of the alpha-amidating activity in the neurointermediate lobe was membrane-associated. Soluble and membrane-associated forms of PAM shared nearly identical characteristics with respect to copper and ascorbate requirements, pH optima, and Km values. Upon subcellular fractionation of anterior and neurointermediate pituitary lobe homogenates on Percoll gradients, 12-18% of total PAM activity was found in the rough endoplasmic reticulum/Golgi fractions and 42-60% was localized to secretory granule fractions. For both tissues, membrane-associated PAM activity was enriched in the rough endoplasmic reticulum/Golgi pool, whereas most of the secretory granule-associated enzyme activity was soluble.     

Response of an integral granule membrane protein to changes in pH

A key feature of the regulated secretory pathway in neuroendocrine cells is lumenal pH, which decreases between trans-Golgi network and mature secretory granules. Because peptidylglycine alpha-amidating monooxygenase (PAM) is one of the few membrane-spanning proteins concentrated in secretory granules and is a known effector of regulated secretion, we examined its sensitivity to pH. Based on antibody binding experiments, the noncatalytic linker regions between the two enzymatic domains of PAM show pH-dependent conformational changes; these changes occur in the presence or absence of a transmembrane domain. Integral membrane PAM-1 solubilized from rat anterior pituitary or from transfected AtT-20 cells aggregates reversibly at pH 5.5 while retaining enzyme activity. Over 35% of the PAM-1 in anterior pituitary extracts aggregates at pH 5.5, whereas only about 5% aggregates at pH 7.5. PAM-1 recovered from secretory granules and endosomes is highly responsive to low pH-induced aggregation, whereas PAM-1 recovered from a light, intracellular recycling compartment is not. Mutagenesis studies indicate that a transmembrane domain is necessary but not sufficient for low pH-induced aggregation and reveal a short lumenal, juxtamembrane segment that also contributes to pH-dependent aggregation. Taken together, these results demonstrate that several properties of membrane PAM serve as indicators of granule pH in neuroendocrine cells.     

Differential trafficking of soluble and integral membrane secretory granule-associated proteins

The posttranslational processing enzyme peptidylglycine alpha-amidating monooxygenase (PAM) occurs naturally in integral membrane and soluble forms. With the goal of understanding the targeting of these proteins to secretory granules, we have compared the maturation, processing, secretion, and storage of PAM proteins in stably transfected AtT-20 cells. Integral membrane and soluble PAM proteins exit the ER and reach the Golgi apparatus with similar kinetics. Biosynthetic labeling experiments demonstrated that soluble PAM proteins were endoproteolytically processed to a greater extent than integral membrane PAM; this processing occurred in the regulated secretory pathway and was blocked by incubation of cells at 20 degrees C. 16 h after a biosynthetic pulse, a larger proportion of soluble PAM proteins remained cell-associated compared with integral membrane PAM, suggesting that soluble PAM proteins were more efficiently targeted to storage granules. The nonstimulated secretion of soluble PAM proteins peaked 1-2 h after a biosynthetic pulse, suggesting that release was from vesicles which bud from immature granules during the maturation process. In contrast, soluble PAM proteins derived through endoproteolytic cleavage of integral membrane PAM were secreted in highest amount during later times of chase. Furthermore, immunoprecipitation of cell surface-associated integral membrane PAM demonstrated that very little integral membrane PAM reached the cell surface during early times of chase. However, when a truncated PAM protein lacking the cytoplasmic tail was expressed in AtT-20 cells, > 50% of the truncated PAM-1 protein reached the cell surface within 3 h. We conclude that the trafficking of integral membrane and soluble secretory granule-associated enzymes differs, and that integral membrane PAM proteins are less efficiently retained in maturing secretory granules.     

Targeting of membrane proteins to the regulated secretory pathway in anterior pituitary endocrine cells

Unlike the neuroendocrine cell lines widely used to study trafficking of soluble and membrane proteins to secretory granules, the endocrine cells of the anterior pituitary are highly specialized for the production of mature secretory granules. Therefore, we investigated the trafficking of three membrane proteins in primary anterior pituitary endocrine cells. Peptidylglycine alpha-amidating monooxygenase (PAM), an integral membrane protein essential to the production of many bioactive peptides, is cleaved and enters the regulated secretory pathway even when expressed at levels 40-fold higher than endogenous levels. Myc-TMD/CD, a membrane protein lacking the lumenal, catalytic domains of PAM, is still stored in granules. Secretory granules are not the default pathway for all membrane proteins, because Tac accumulates on the surface of pituitary endocrine cells. Overexpression of PAM is accompanied by a diminution in its endoproteolytic cleavage and in its BaCl(2)-stimulated release from mature granules. Because internalized PAM/PAM-antibody complexes are returned to secretory granules, the endocytic machinery of the pituitary endocrine cells is not saturated. As in corticotrope tumor cells, expression of PAM or Myc-TMD/CD alters the organization of the actin cytoskeleton. PAM-mediated alterations in the cytoskeleton may limit maturation of PAM and storage in mature granules.     

Control of exocytosis from adrenal chromaffin cells

1. Calcium-dependent exocytosis of catecholamines from intact and digitonin-permeabilized bovine adrenal chromaffin cells was investigated. 2. 45Ca2+ uptake and secretion induced by nicotinic stimulation or depolarization in intact cells were closely correlated. The results provide strong support for Ca2+ entry being the trigger for exocytosis. 3. Experiments in which the H+ electrochemical gradient across the intracellular secretory granule (chromaffin granule) membrane was altered indicated that the gradient does not play an important role in exocytosis. 4. Ca2+ entry into the cells is associated with activation of phospholiphase C and a rapid translocation of protein kinase C to membranes. 5. The plasma membrane of chromaffin cells was rendered permeable to Ca2+, ATP, and proteins by the detergent digitonin without disruption of the intracellular secretory granules. In this system in which the intracellular milieu can be controlled, micromolar Ca2+ directly stimulated catecholamine secretion. 6. Treatment of the cells with phorbol esters and diglyceride, which activate protein kinase C, enhanced phosphorylation and subsequent Ca2+-dependent secretion in digitonin-treated cells. 7. Phorbol ester-induced secretion could be specifically inhibited by trypsin. The experiments indicate that protein kinase C modulates but is not necessary for Ca2+-dependent secretion.     

Enzymatic activity of soluble and membrane tethered peptide pro-hormone convertase 1

Pro-hormone convertases PC1 and PC2 perform endoproteolytic cleavages of precursors in peptide-containing secretory granules. PC1 and PC2 are soluble, secreted with bioactive peptides. Evolutionarily related PCs have membrane tethers, not secreted. We tethered PC1 to the transmembrane-cytoplasmic domains (CD) of a granule enzyme (peptidylglycine-alpha-amidating monooxygenase; PAM) and Golgi-localized PC8. The tethered PC1 is far more stable to elevated temperature and denaturants than soluble PC1, and more active. Both tethers allow PC1 to visit the cell surface transiently, cleaving soluble molecules outside the cell. Both membrane-bound PC1 chimeras cleave membrane PAM into soluble active fragments when PAM is expressed on adjacent cells.     

Interleukin 1 potentiates agonist-induced secretion of beta-endorphin in anterior pituitary cells

Interleukin 1 (IL-1) has been shown to potentiate the release of beta-endorphin induced by secretagogues, including corticotropin releasing factor (CRF) and phorbol ester (TPA), in the mouse AtT-20 pituitary tumor cell line (Fagarasan et al., PNAS, 1989, 86, 2070-2073). In cultured rat anterior pituitary cells, pretreatment with IL-1 caused only a small increase in beta-endorphin release but significantly potentiated CRF-and vasopressin-stimulated beta-endorphin secretion. Vasopressin stimulates the secretion of beta-endorphin in normal pituitary cells but not in AtT-20 cells. However, treatment of AtT-20 cells with IL-1 induced the expression of vasopressin-mediated beta-endorphin release; this effect of IL-1 was reduced after depletion of protein kinase C by prolonged treatment with TPA. The enhancement of CRF-stimulated beta-endorphin release by IL-1 was also reduced in AtT-20 cells after depletion of protein kinase C, and after treatment with staurosporine. These findings indicate that treatment with IL-1 amplifies receptor-mediated responses to the major physiological secretagogues in normal corticotrophs, and initiates a secretory response to vasopressin in AtT-20 cells.     

The effects of phorbol esters on fluid transport and blood flow in the small intestine

Studies were designed to examine the effects of phorbol esters on intestinal fluid transport and blood flow in the anesthetized cat and enteropooling in the conscious rat. Intraluminal administration of phorbol ester into a segment of isolated small bowel produced a copious intestinal secretion and a concomitant mesenteric hyperemia in the cat. Net fluid movement in the intestine was converted from absorption in the control state to secretion following phorbol ester administration. Intravenous atropine reduced the phorbol ester-induced secretion by 56%; clonidine abolished the remaining secretory response. In the rat, intragastric administration of phorbol ester produced enteropooling comparable to that of other potent intestinal secretagogues. Since phorbol esters are known to activate protein kinase C, these studies suggest that activation of protein kinase C in the small intestine may lead to a full secretory response. The evidence suggests that this secretion is accompanied by a metabolic hyperemia. These results suggest that protein kinase C plays an important role in the regulation of intestinal fluid transport.     

Involvement of the Rab27 binding protein Slac2c/MyRIP in insulin exocytosis

Rab27a is a GTPase associated with insulin-containing secretory granules of pancreatic beta-cells. Selective reduction of Rab27a expression by RNA interference did not alter granule distribution and basal secretion but impaired exocytosis triggered by insulin secretagogues. Screening for potential effectors of the GTPase revealed that the Rab27a-binding protein Slac2c/MyRIP is associated with secretory granules of beta-cells. Attenuation of Slac2c/MyRIP expression by RNA interference did not modify basal secretion but severely impaired hormone release in response to secretagogues. Although beta-cells express Myosin-Va, a potential partner of Slac2c/MyRIP, no functional link between the two proteins could be demonstrated. In fact, overexpression of the Myosin-Va binding domain of Slac2c/MyRIP did not affect granule localization and hormone exocytosis. In contrast, overexpression of the actin-binding domain of Slac2c/MyRIP led to a potent inhibition of exocytosis without detectable alteration in granule distribution. This effect was prevented by point mutations that abolish actin binding. Taken together our data suggest that Rab27a and Slac2c/MyRIP are part of a complex mediating the interaction of secretory granules with cortical actin cytoskeleton and participate to the regulation of the final steps of insulin exocytosis.     

Phosphorylation of cytosolic domain Ser(937) affects both biosynthetic and endocytic trafficking of peptidylglycine alpha-amidating monooxygenase.

Peptidylglycine alpha-amidating monooxygenase (PAM), a bifunctional enzyme, catalyzes the COOH-terminal amidation of bioactive peptides. In test tube assays, PAM is phosphorylated by protein kinase C at Ser(937). The roles of phosphorylation and dephosphorylation of Ser(937) in the biosynthetic and endocytic trafficking of integral membrane PAM were examined using an antiserum specific for the phosphorylation of Ser(937) and using AtT-20 cells expressing membrane PAM in which Ser(937) was mutated to Ala or Asp. Although phosphorylation at Ser(937) can occur while PAM is in the endoplasmic reticulum, early steps in the biosynthetic trafficking of membrane PAM were not affected by Ser(937) phosphorylation. The inability to phosphorylate PAM/S937A increased its intracellular degradation and decreased secretion of the soluble monooxygenase portion of PAM. In contrast, the biosynthetic trafficking of PAM/S937D was indistinguishable from wild-type PAM. Despite the fact that Ser(937) is adjacent to the only Tyr-based internalization motif in PAM, internalization and trafficking through early endosomes were unaffected by phosphorylation. However, PAM antibody internalized by wild-type PAM acquired a perinuclear localization, while antibody internalized by PAM/S937A was routed to lysosomes, and antibody bound to PAM/S937D maintained a dispersed, punctate pattern. In cells stimulated with phorbol ester, phosphorylation of Ser(937) increased and phosphorylated PAM accumulated in large vesicular structures. Therefore, phosphorylation of PAM-1 at Ser(937) directs newly synthesized and internalized protein away from lysosomes, while dephosphorylation is needed for a different step in the late endocytic pathway.     

Elastic behavior of zymogen granule membranes in response to changes in pH and pCa.

In the process of secretion, the membrane of secretory granules is expected to change its elastic behavior. Elastic modulus of the membrane of zymogen granules, prepared from the rat pancreas acinar cell, was measured by an osmotic swelling method. The elastic modulus of the granule membrane at pCa 8 reduced from the maximal value of 230 dyn/cm at pH 6.0 to almost zero at pH 7.5. In a cytosol of an acinar cell, calcium ions play an important role as a second messenger in secretion. The elastic modulus of the granule membrane reduced in a sigmoidal fashion at pCa between 7.0 and 6.0. This range of pCa corresponds to a physiological rise of free Ca2+ concentrations in the cell cytosol when stimulated by external secretagogues. Reduction of the elastic modulus indicates that the state of the granule membrane switches to a more flexible one in which the granule is easy to appose to the cell plasma membrane and then swell as a final step of exocytosis.     

Not all secretory granules are created equal: Partitioning of soluble content proteins

Secretory granules carrying fluorescent cargo proteins are widely used to study granule biogenesis, maturation, and regulated exocytosis. We fused the soluble secretory protein peptidylglycine alpha-hydroxylating monooxygenase (PHM) to green fluorescent protein (GFP) to study granule formation. When expressed in AtT-20 or GH3 cells, the PHM-GFP fusion protein partitioned from endogenous hormone (adrenocorticotropic hormone, growth hormone) into separate secretory granule pools. Both exogenous and endogenous granule proteins were stored and released in response to secretagogue. Importantly, we found that segregation of content proteins is not an artifact of overexpression nor peculiar to GFP-tagged proteins. Neither luminal acidification nor cholesterol-rich membrane microdomains play essential roles in soluble content protein segregation. Our data suggest that intrinsic biophysical properties of cargo proteins govern their differential sorting, with segregation occurring during the process of granule maturation. Proteins that can self-aggregate are likely to partition into separate granules, which can accommodate only a few thousand copies of any content protein; proteins that lack tertiary structure are more likely to distribute homogeneously into secretory granules. Therefore, a simple "self-aggregation default" theory may explain the little acknowledged, but commonly observed, tendency for both naturally occurring and exogenous content proteins to segregate from each other into distinct secretory granules.     

A Histidine-rich Linker Region in Peptidylglycine α-Amidating Monooxygenase Has the Properties of a pH Sensor

Decreasing luminal pH is thought to play a role in the entry of newly synthesized and endocytosed membrane proteins into secretory granules. The two catalytic domains of peptidylglycine α-amidating monooxygenase (PAM), a type I integral membrane protein, catalyze the sequential reactions that convert peptidyl-Gly substrates into amidated products. We explored the hypothesis that a conserved His-rich cluster (His-Gly-His-His) in the linker region connecting its two catalytic domains senses pH and affects PAM trafficking by mutating these His residues to Ala (Ala-Gly-Ala-Ala; H3A). Purified recombinant wild-type and H3A linker peptides were examined using circular dichroism and tryptophan fluorescence; mutation of the His cluster largely eliminated its pH sensitivity. An enzymatically active PAM protein with the same mutations (PAM-1/H3A) was expressed in HEK293 cells and AtT-20 corticotrope tumor cells. Metabolic labeling followed by immunoprecipitation revealed more rapid loss of newly synthesized PAM-1/H3A than PAM-1; although release of newly synthesized monofunctional PHM/H3A was increased, release of soluble bifunctional PAM/H3A, a product of the endocytic pathway, was decreased. Surface biotinylation revealed rapid loss of PAM-1/H3A, with no detectable return of the mutant protein to secretory granules. Consistent with its altered endocytic trafficking, little PAM-1/H3A was subjected to regulated intramembrane proteolysis followed by release of a small nuclear-targeted cytosolic fragment. AtT-20 cells expressing PAM-1/H3A adopted the morphology of wild-type AtT-20 cells; secretory products no longer accumulated in the trans-Golgi network and secretory granule exocytosis was more responsive to secretagogue.