Identification of the secretory vesicle membrane binding region of chromogranin A

Since the conserved near N-terminal region (residues 18-37) of chromogranin A (CGA) has tentatively been identified as the vesicle membrane interacting region using synthetic peptides, it was necessary to confirm this finding with CGA deletion proteins. In order to address this need and to clarify the discrepancies of the published amino acid sequences of CGA, we cloned a CGA gene and produced CGA deletion proteins of various sizes. The recombinant CGA protein lacking the first 16 amino acid residues bound to the vesicle membrane as well as the full-length CGA at pH 5.5. However, the CGA protein lacking the first 39 amino acid residues, which include the conserved near N-terminal region (residues 17-38), failed to interact with the vesicle membrane at pH 5.5, clearly indicating the essential role of the conserved near N-terminal region in the pH-dependent interaction of CGA with the vesicle membrane.

Effects of pH and Ca2+ on heterodimer and heterotetramer formation by chromogranin A and chromogranin B

The two major proteins of the secretory vesicles of neuroendocrine cells, chromogranin A (CGA) and chromogranin B (CGB), have been shown to undergo pH- and Ca2+-dependent conformational changes and aggregation and have been suggested to play essential roles during secretory vesicle biogenesis in the trans-Golgi network. CGA has been shown to exist primarily in a tetrameric state at pH 5.5 and primarily in a dimeric state at pH 7.5, and CGB has been shown to exist in a monomeric state at both pH 5.5 and pH 7.5. Using purified CGA and CGB, it recently has been shown that CGA interacts with CGB at pH 5.5 (Yoo, S. H.(1996) J. Biol. Chem. 271, 1558-1565). In expanding this investigation, we have studied the temperature dependence of the pH-dependent interaction of CGA and CGB by analytical ultracentrifugation and found that two molecules of CGA bound to two molecules of CGB at pH 5.5 with DeltaG0 values of -43.6 kcal/mol in the absence of Ca2+ at 37 degrees C and -40.3 kcal/mol in the presence of 0.1 mM Ca2+. However, one molecule of CGA bound to one molecule of CGB at pH 7.5 with DeltaG0 values of -13.6 kcal/mol in the absence of Ca2+ at 37 degrees C. The magnitude of DeltaG0 values increased with increasing temperatures at both pH values. However, the values for enthalpy and entropy changes decreased with increasing temperatures in both pH levels, suggesting formation of more ordered structures. In the absence of Ca2+ at pH 5. 5, the heterotetramerization reaction at 37 degrees C was entropically driven, whereas in the presence of Ca2+ (0.1 mM) the heterotetramerization was virtually an enthalpic reaction. On the other hand, the heterodimer formation in the absence of Ca2+ at pH 7. 5 showed large negative enthalpy and entropy changes at 37 degrees C, indicating an enthalpic interaction compensated by entropic changes. In view of the interaction of tetrameric CGA with tetrameric inositol 1,4,5-trisphosphate (IP3) receptor and the existence of heterotetrameric IP3 receptor in the cell, the heterotetramer formation by CGA and CGB not only raises the possibility of interaction between the heterotetrameric chromogranin and heterotetrameric IP3 receptor but also appears to reflect their important roles in the cell.

pH- and Ca(2+)-dependent aggregation property of secretory vesicle matrix proteins and the potential role of chromogranins A and B in secretory vesicle biogenesis

Chromogranins A and B (CGA and CGB), the major proteins of the secretory vesicles of the regulated secretory pathway, have been shown to aggregate in a low pH and high calcium environment, the condition found in the trans-Golgi network where secretory vesicles are formed. Moreover, CGA and CGB, as well as several other secretory vesicle matrix proteins, have recently been shown to bind to the vesicle membrane at the intravesicular pH of 5.5 and to be released from it at a near physiological pH of 7.5. The pH- and Ca(2+)-dependent aggregation and interaction of chromogranins, as well as several other matrix proteins, with the vesicle membrane are considered essential in vesicle biogenesis. Therefore, to gain further insight into how vesicle matrix proteins find their way into the secretory vesicles, the pH- and Ca(2+)-dependent aggregation and vesicle membrane binding properties of the vesicle matrix proteins were studied, and it was found that most of the vesicle matrix proteins aggregated in the presence of Ca2+ at the intravesicular pH of 5.5. Furthermore, most of the vesicle matrix proteins bound not only to the vesicle membrane but also to CGA at pH 5.5, with the exception of a few matrix proteins that appeared to bind only to CGA or to vesicle membrane. Purified CGB was also shown to interact with CGA at pH 5.5. The extent and Ca(2+)-sensitivity of the aggregation of vesicle matrix proteins lay between those of purified CGB and CGA, CGB aggregation showing the highest degree of aggregation and being the most Ca2+ sensitive at a given protein concentration. Hence, in view of the abundance of chromogranins in secretory vesicles and their low pH- and high calcium-dependent aggregation property, combined with their ability to interact with both the vesicle matrix proteins and the vesicle membrane, CGA and CGB are proposed to play essential roles in the selective aggregation and sorting of potential vesicle matrix proteins to the immature secretory vesicles of the regulated secretory pathway.

Purification and pH-dependent secretory vesicle membrane binding of chromogranin B

Chromogranins A and B have been suggested to play crucial roles in the sorting of vesicular matrix proteins into secretory vesicles during vesicle biogenesis. Chromogranin A (CGA), a high-capacity, low-affinity Ca(2+)-binding protein, is the major protein in the secretory vesicles, while chromogranin B (CGB) is present in the vesicle at a significantly lower concentration. Chromogranin B has not been purified in its native form so far, thus severely limiting detailed studies of this protein. In the present study, chromogranin B was purified to complete homogeneity in its native state from the secretory vesicle lysates of bovine adrenal chromaffin cells using several chromatographic and electrophoresis steps. Recently, several intravesicular matrix proteins including chromogranins A and B have been shown to interact with the vesicle membrane at the intravesicular pH of 5.5 and to be released at a near-physiological pH of 7.5. However, since the experiment was done with the total vesicle lysate proteins, it was not clear whether CGB bound to the vesicle membrane directly or not. Hence, the pH-dependent binding of CGB to the vesicle membrane was tested using purified CGB, and it was found that pure CGB directly bound to the vesicle membrane at the intravesicular pH of 5.5. However, unlike the vesicle membrane-bound CGA, which can be easily eluted by a change of pH in the elution buffer from 5.5 to 7.5, the change of pH from 5.5 to 7.5 was not enough to elute the vesicle membrane-bound CGB.(ABSTRACT TRUNCATED AT 250 WORDS)

pH- and Ca(2+)-induced conformational change and aggregation of chromogranin B. Comparison with chromogranin A and implication in secretory vesicle biogenesis

Chromogranins A and B have been known to undergo pH- and Ca(2+)-dependent aggregation, and this property is considered essential for the proper sorting of the vesicular matrix proteins. In the present study, purified native chromogranin B (CGB) from bovine adrenal medulla was used to study the pH- and Ca(2+)-dependent conformational changes and aggregation property. Similar to chromogranin A (CGA), which had been shown to undergo pH- and Ca(2+)-dependent conformational changes and to be composed of 60-65% random coil with 25-40% alpha-helicity, chromogranin B was also shown to consist of 65-70% random coil, 15-25% alpha-helix, and 10-15% beta-sheet structures. The high percentage of random coil suggests that CGB behaves hydrodynamically as an asymmetric molecule, thus explaining its anomalous migration on SDS-polyacrylamide gels. Further, CGB eluted from a gel filtration column in the volume indicative of a globular protein with molecular weight of approximately 200,000 at both the intravesicular pH of 5.5 and a near physiological pH of 7.5. Considering that dimeric CGA eluted from a gel filtration column in the position suggestive of a 300-kDa protein, this result indicated that CGB exists in a monomeric state at both pH levels. Like CGA, which exhibited greater aggregation at pH 5.5 than at pH 7.5 upon Ca2+ binding, CGB also aggregated much more readily at pH 5.5 than at pH 7.5. However, there was a marked difference in the aggregation properties of CGA and CGB with regard to their sensitivity to Ca2+: CGB was at least 2 orders of magnitude more sensitive to Ca2+ than CGA. This suggested that, in spite of the low concentration of CGB (approximately one-tenth that of CGA) in bovine adrenal chromaffin cells, CGB would start to aggregate well ahead of CGA in the trans-Golgi network. In view of the proposed importance of the pH- and Ca(2+)-induced chromogranin aggregation in vesicle biogenesis, the extreme sensitivity of CGB aggregation to Ca2+ appears to underline the potential importance of CGB aggregation in the early stages of vesicle biogenesis.

Thermodynamic study of the pH-dependent interaction of chromogranin A with an intraluminal loop peptide of the inositol 1,4,5-trisphosphate receptor

The secretory vesicles of adrenal chromaffin cells have previously been identified as a major inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store, and their Ca2+ store role has been attributed to the presence of chromogranin A, a high capacity, low affinity Ca2+ binding protein. Chromogranin A has since been shown to exist primarily in a dimeric state at pH 7.5 and primarily in a tetrameric state at the intravesicular pH of 5.5 and has also been shown to interact with the membrane proteins of secretory vesicles at pH 5.5, including a 260-kDa protein reactive to IP3 receptor antibody [Yoo, S. H. (1994) J. Biol. Chem. 269, 12001-12006]. In a recent study, chromogranin A was shown to interact with one of the intraluminal loop regions of the IP3 receptor at pH 5.5 but not at pH 7.5 [Yoo, S. H., & Lewis, M. S. (1994) FEBS Lett. 341, 28-32]. To gain further insight, we have studied the temperature dependence of the pH-dependent interaction of chromogranin A with the intraluminal peptide of the the IP3 receptor by analytical ultracentrifugation, using multiwavelength scan analysis, and found that four molecules of the intraluminal domain peptide of the IP3 receptor bound to each chromogranin A tetramer with delta Go values ranging from -23.6 to -27.6 kcal mol-1 in the absence and presence of 35 mN Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent interaction of chromogranin A with integral membrane proteins of secretory vesicle including 260-kDa protein reactive to inositol 1,4,5-triphosphate receptor antibody

Chromogranin A is a high capacity, low affinity Ca(2+)-binding protein suggested to be responsible for the Ca2+ storage function of the secretory vesicle, which has been identified as a major inositol 1,4,5-trisphosphate (IP3)-sensitive intracellular Ca2+ store of adrenal medullary chromaffin cells. Moreover, chromogranin A has recently been shown to interact with the vesicle membrane at the intravesicular pH of 5.5 and to be released from it at a near physiological pH of 7.5 (Yoo, S. H. (1993) Biochemistry 32, 8213-8219). In the present study, chromogranin A is shown to interact with several integral membrane proteins of secretory vesicles at pH 5.5 but not at pH 7.5. One of the chromogranin A-interacting membrane proteins had a mass of 260 kDa and reacted with the IP3 receptor antibody. This result suggested not only the existence of the IP3 receptor in the vesicle membrane but also the existence of direct communication between chromogranin A and the IP3 receptor. In addition, the pH-dependent interaction of chromogranin A with integral membrane proteins implies an important role for chromogranin A in the sorting process of the vesicle membrane proteins during vesicle biogenesis in the trans-Golgi network.

pH-dependent interaction of an intraluminal loop of inositol 1,4,5-trisphosphate receptor with chromogranin A

The inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store role of the secretory vesicles of adrenal medullary chromaffin cells is attributed to the presence of high capacity, low affinity Ca2+ binding protein chromogranin A. Chromogranin A has recently been shown to interact with the protein component(s) on the intraluminal side of the secretory vesicle membrane at the intravesicular pH of 5.5 but to dissociate from them at the near physiological pH of 7.5. Further, one of the chromogranin A-interacting membrane proteins was tentatively identified as the IP3 receptor. Therefore, the pH-dependent potential interaction of the intraluminal loop domains of the IP3 receptor with chromogranin A was studied by analytical ultracentrifugation utilizing synthetic intraluminal loop peptides of the IP3 receptor labeled with 5-hydroxy-tryptophan at the N-terminus as a chromophore. One of the intraluminal loop domains was found to interact with chromogranin A at pH 5.5 but not at pH 7.5, suggesting the importance of the intraluminal loop domain in transmitting Ca2+ mobilization signals to chromogranin A.

pH-dependent binding of chromogranin B and secretory vesicle matrix proteins to the vesicle membrane

Contrary to the notion that the soluble intravesicular matrix proteins of the secretory vesicles of adrenal medullary chromaffin cells freely float in the vesicle, several vesicle matrix proteins of the secretory vesicles, including chromogranins A and B, bound to the vesicle membrane at intravesicular pH (5.5) and were freed from it when the pH was raised to a near physiological pH (7.5). Estimation of the fraction of vesicle matrix proteins that might remain bound to the vesicle membrane in the vesicle suggested that the majority (> 50-80%) of chromogranins A and B, as well as several other proteins, will stay bound to the membrane in the vesicle. Comparison of the amino-acid sequences of chromogranins A and B revealed two highly conserved regions, i.e., one near the N-terminus and the other being the C-terminal region. Since it has been demonstrated with chromogranin A that the conserved near N-terminal region of chromogranin A exhibited the pH-dependent membrane-binding activity (Yoo, S. H. (1993) Biophys. J., 64, A195), the same region in chromogranin B (residues 17-36) was tested using a synthetic chromogranin B peptide, and found to exhibit the pH-dependent membrane-binding activity. The pH-dependent binding of the matrix proteins at pH 5.5 and the automatic untethering at a physiological pH accord well with the rapid release and circulation of the vesicular contents in the bloodstream.

Nature of the pH-induced conformational changes and exposure of the C-terminal region of chromogranin A

Chromogranin A is known to undergo pH induced conformational changes, and the difference in conformation is supposed to be responsible for the difference in Ca2+ binding property. To gain insight regarding the overall structure and the nature of pH-induced conformational changes of chromogranin A, limited trypsin digestions were carried out at pH 5.5 and pH 7.5. The resulting fragments were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the amino acid sequences of the tryptic fragments were determined. From these analyses it was shown that the chromogranin A structure consists of an N-terminal compact core region and a rather loosely organized C-terminal region and that the change of pH from 7.5 to 5.5 loosened the overall structure of chromogranin A, exposing the C-terminal region. Since the conserved C-terminal region (residues 407-431) was shown to exist in monomer-dimer and monomer-tetramer equilibria at pH 7.5 and 5.5, respectively, the conformational changes of the region at pH 7.5 and 5.5 were studied by circular dichroism spectroscopy using a synthetic peptide representing the conserved C-terminal region. When the pH was changed from 7.5 to 5.5, the coil structure of the C-terminal peptide decreased with an accompanying increase of alpha-helicity.

Dimerization and tetramerization properties of the C-terminal region of chromogranin A: a thermodynamic analysis

Chromogranin A, which is a high-capacity, low-affinity Ca2+ binding protein, has recently been shown to exist in monomer-dimer and in monomer-tetramer equilibria at pH 7.5 and 5.5, respectively [Yoo, S. H., & Lewis, M. S. (1992) J. Biol. Chem. 267, 11236-11241]. The pH appeared to be a necessary and sufficient factor determining the types of oligomer formed. In the present study, using 14 synthetic peptides representing various portions of chromogranin A, we have identified a region in chromogranin A which exhibited dimerization and tetramerization properties at pH 7.5 and 5.5, respectively. Of the 14 peptides, only the conserved C-terminal region (residues 407-431), represented by peptide 14, showed the oligomerization property, existing in a dimeric state at pH 7.5 and in a tetrameric state at pH 5.5. The delta G degrees values of tetramerization were approximately -18.0 kcal/mol, and the delta G degrees value of dimerization was -4.6 kcal/mol. Although peptide 14 represented only 6% of the entire sequence, the delta G degrees value of -18.0 kcal/mol accounted for 80-83% of the delta G degrees values (-21.6 to -22.7 kcal/mol) of tetramerization of intact chromogranin A. Unlike the tetramerization mechanisms of intact chromogranin A where the presence of 35 mM Ca2+ changed the tetramerization mechanism from an enthalpically driven to an entropically driven reaction, the tetramerization mechanism of peptide 14 remained entropically driven regardless of the presence of Ca2+. Likewise, dimerization of the peptide was also entropically driven.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent association of chromogranin A with secretory vesicle membrane and a putative membrane binding region of chromogranin A

Chromogranin A is a low-affinity, high-capacity Ca2+ binding protein, postulated to be responsible for the Ca2+ buffering role of secretory vesicles, and has been found only in the soluble portions of the vesicular proteins. Contrary to the generally accepted notion of chromogranin A existing as a soluble matrix protein, chromogranin A bound to the secretory vesicle membrane at the intravesicular pH of 5.5 and freed from the membrane when the pH was raised to a more physiological pH of 7.5. Trypsin digestion studies of the vesicle membrane suggested that chromogranin A interacts with the protein component(s) on the intravesicular side of the membrane. Furthermore, in a study using 14 synthetic chromogranin A peptides which represent various portions of chromogranin A, a segment in the N-terminal region (residues 18-37) was shown to bind to the vesicle membrane in a pH-dependent manner. The pH-dependent vesicle membrane binding property of chromogranin A appears to be of fundamental physiological importance with regard to the potential roles of chromogranin A in secretory vesicle biogenesis, particularly in segregating secretory vesicle membranes from others in the trans-Golgi network, and also in transmitting extravesicular signals such as inositol 1,4,5-trisphosphate or inositol 1,3,4,5-tetrakisphosphate for Ca2+ release or uptake to the inside of vesicles.

Ubiquitous presence of chromogranin A in the inner ear of guinea pig

Chromogranin A (CGA), which is supposed to be responsible for the calcium storage of secretory vesicles and is also considered to be a marker protein of neurons and endocrine cells, has been found in a variety of organs and tissues. In the present study, soluble proteins from the organ of Corti, saccule, crista, utricle, tectorial membrane, stria vascularis, and the spiral ligament from the inner ear of guinea pig were extracted, and probed with both polyclonal and monoclonal CGA antibodies to determine the presence of CGA. A 75 kDa protein reactive to both antibodies was found in the organ of Corti, saccule, crista, utricle, stria vascularis, and the spiral ligament, suggesting the widespread presence of CGA in the inner ear.

Identification of the Ca(2+)-dependent calmodulin-binding region of chromogranin A

Chromogranin A (CGA), the most abundant protein in bovine adrenal chromaffin granules, is a high-capacity, low-affinity Ca(2+)-binding protein found in most neuroendocrine cells, and binds calmodulin (CaM) in a Ca(2+)-dependent manner. The binding of chromogranin A to calmodulin was determined by measuring the intrinsic tryptophan fluorescence of chromogranin A in the presence and absence of Ca2+. Binding was specifically Ca(2+)-dependent; neither Mg2+ nor Mn2+ could substitute for Ca2+. Chelation of Ca2+ by EGTA completely eliminated the chromogranin A-calmodulin interaction. CaM binding was demonstrated by a synthetic CGA peptide representing residues 40-65. When the CGA peptide and CaM were mixed in the presence of 15 mM CaCl2, the intrinsic tryptophan fluorescence emission underwent a substantial blue-shift, shifting from 350 to 330 nm. Like the intact CGA, the peptide-CaM binding was specifically Ca(2+)-dependent, and neither Mg2+ nor Mn2+ could induce the binding. Calmodulin bound both to CGA and to the synthetic CGA peptide with a stoichiometry of one to one. The dissociation constants (Kd) determined by fluorometric titration were 13 nM for the peptide-CaM binding and 17 nM for intact CGA-CaM binding. The Kd values are comparable to those (approximately 10(-9) M) of other CaM-binding proteins and peptides, demonstrating a tight binding of CaM by CGA. The CaM-binding CGA residues 40-65 are 100% conserved among all the sequenced CGAs in contrast to 50-60% conservation found in the entire sequence, implying essential roles of this region.

Effects of pH and Ca2+ on monomer-dimer and monomer-tetramer equilibria of chromogranin A

Chromogranin A is a high capacity, low affinity Ca2+ binding protein which undergoes Ca2+- and pH-dependent conformational changes, and has recently been suggested to play a Ca2+-buffering role in the secretory vesicle of adrenal medullary chromaffin cell, the major inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ store of chromaffin cell (Yoo, S.H., and Albanesi, J.P. (1990) J. Biol. Chem. 265, 13446-13448). In the present study, it is shown that chromogranin A exists in a monomer-dimer equilibrium at pH 7.5 and in a monomer-tetramer equilibrium at pH 5.5. The pH appears monomer-tetramer equilibrium at pH 5.5. The pH appears to be a necessary and sufficient factor determining the types of oligomers formed. Although Ca2+ did not change the type of oligomerization, it had a very significant effect on the values of the thermodynamic parameters characterizing the associations. The delta G0 values for a monomer-dimer equilibrium were -7 to -8 kcal/mol, while those for a monomer-tetramer equilibrium were -20 to -23 kcal/mol. At pH 5.5, the values of delta H0, delta S0, and delta C0p were large and negative in the absence of Ca2+ and large and positive in the presence of 35 mM Ca2+, implying markedly different reaction mechanisms. Extrapolation of the results to 37 degrees C and 1 mM chromogranin A suggests that chromogranin A is virtually 100% tetramer at pH 5.5 in the presence of 35 mM Ca2+ but is 96% dimer at pH 7.5 in the absence of Ca2+, the two conditions resembling those seen in vivo. These results suggest that chromogranin A is mostly dimer in the endoplasmic reticulum and cis-Golgi area and is essentially all tetramer in the vesicle.

Inositol 1,3,4,5-tetrakisphosphate-induced Ca2+ sequestration into bovine adrenal-medullary secretory vesicles

Ins(1,3,4,5)P4 induced a rapid sequestration of Ca2+ into both secretory vesicles and microsomes of bovine adrenal medulla. The Ca(2+)-sequestering role of Ins(1,3,4,5)P4 contrasts with the Ca(2+)-releasing role of Ins(1,4,5)P3 in adrenal-medullary secretory vesicles and microsomes. The Ins(1,3,4,5)P4-induced Ca2+ sequestration into secretory vesicles was not inhibited by heparin (50 micrograms/ml), whereas Ins(1,4,5)P3-induced Ca2+ release was completely inhibited, indicating two different receptors for Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Furthermore, Ins(1,3,4,5)P4 was as effective at 4 degrees C as at 24 degrees C in sequestering Ca2+ into secretory vesicles, implying Ca2+ sequestration through receptor-operated Ca2+ channels or activation of the Ca(2+)-exchange mechanism by Ins(1,3,4,5)P4. The Ca(2+)-sequestering activity of Ins(1,3,4,5)P4 has also been demonstrated with 45Ca2+; 10 microM-Ins(1,3,4,5)P4 induced rapid uptake of 45Ca2+ into secretory vesicles optimized for Ca2+ uptake, whereas 10 microM-Ins(1,4,5)P3 induced 45Ca2+ release from secretory vesicles in similar experiments.

High capacity, low affinity Ca2+ binding of chromogranin A. Relationship between the pH-induced conformational change and Ca2+ binding property

Chromogranin A, the major intravesicular protein of adrenal chromaffin granules, bound Ca2+ in a pH-dependent manner. Both the maximal binding and affinity of chromogranin A for Ca2+ were dependent on pH. Chromogranin A bound 670 nmol of Ca2+/mg (32 mol/mol) and 1150 nmol of Ca2+/mg (55 mol/mol) at pH 7.5 and 5.5, respectively, with dissociation constants (Kd) of 2.7 and 4 mM. This pH dependence probably reflects different conformations of the protein at the two pH values. Conformational differences of chromogranin A at two different pH values were demonstrated by limited tryptic digestion patterns confirming previous results obtained by circular dichroism spectroscopy (Yoo, S. H., and Albanesi, J. P. (1990) J. Biol. Chem. 265, 14414-14421). Sedimentation equilibrium studies revealed the native molecular mass of chromogranin A to be 100 kDa at pH 7.5 and 192 kDa at pH 5.5, indicating dimeric and tetrameric states of the protein at the two pH levels. We postulate that the pH- and Ca2(+)-induced conformational changes of chromogranin A may have a role both in the regulation of Ca2+ release of chromaffin granules and in the early stages of secretory vesicle biogenesis.

Ca2(+)-induced conformational change and aggregation of chromogranin A

Chromogranin A, the most abundant protein in bovine adrenal chromaffin granules, bound calmodulin in a Ca2(+)-dependent manner, and the calmodulin-binding property was utilized to purify chromogranin A. Chromogranin A has been described in the past as a "random-coil polypeptide" with little alpha-helix or beta-sheet conformation. However, circular dichroism measurements with pure, native chromogranin A revealed relatively high alpha-helical contents (40% at the intravesicular pH of 5.5). Fluorescence studies confirmed previous observations that chromogranin A binds Ca2+ with low affinity. Considering the high concentration of Ca2+ in the secretory vesicle, the effect of Ca2+ on the secondary structure and self-association of chromogranin A was examined. Ca2+ induced a decrease of alpha-helicity of chromogranin A from 40 to 30% at pH 5.5. In contrast, at pH 7.5 the same amount of Ca2+ increased alpha-helicity of the protein from 25 to 40%. Boiling of the adrenal extract, a commonly used purification procedure for chromogranin A, resulted in the isolation of conformationally distinct chromogranin A molecule. Unlike secretory protein-I of the parathyroid gland (Gorr, S.-V., Dean, W. L., Radley, T. L., and Cohn, D. V. (1988) Bone Mineral 4, 17-25), chromogranin A aggregated rapidly in the presence of Ca2+. The extent and rate of aggregation were highly dependent on Ca2+ concentration. Although both the rate and extent of aggregation at pH 7.5 were much lower than those at pH 5.5, aggregation of chromogranin A proceeded at both pH's. In this respect, chromogranin A differs from human chromogranin C which was shown by Gerdes et al. (Gerdes, H.-H., Rosa, P., Phillips, E., Baeuerle, P. A., Frank, R., Argos, P., and Huttner, W. B. (1989) J. Biol. Chem. 264, 12009-12015) to aggregate at pH 5.2 but not at pH 7.4.

Inositol 1,4,5-trisphosphate-triggered Ca2+ release from bovine adrenal medullary secretory vesicles

The effect of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and calcium ionophore A23187 on Ca2+ release from bovine adrenal medullary secretory vesicles and microsomes was examined. Ins(1,4,5)P3 released 3.5 nmol of Ca2+/mg protein from secretory vesicles and 1.5 nmol of Ca2+/mg protein from microsomes as measured by a Ca2(+)-selective electrode. However, A23187 promoted Ca2+ uptake into vesicles while releasing Ca2+ from microsomes. Ins(1,4,5)P3-induced Ca2+ release from secretory vesicles was rapid, but the released Ca2+ was absorbed within 3 min during which the Ins(1,4,5)P3-releasable pools were refilled. The in situ calcium content of secretory vesicle measured by atomic absorption spectrometry was 112 +/- 6.3 nmol/mg protein indicating the potential importance of secretory vesicles as an intracellular Ca2+ store. The high Ca2(+)-buffering capacity of secretory vesicles is presumed to be due to the high Ca2(+)-binding capacity of chromogranin A, the major intravesicular protein, which has calsequestrin-like properties.

Fluorescence studies of nucleotide interactions with bovine adrenal chromogranin A

The binding of the fluorescent probe bis-ANS to chromogranin A, the major protein of adrenal chromaffin vesicles, caused a marked enhancement and blue shift in the fluorescence emission spectrum. The emission maximum shifted from 515 nm to 480 nm and the yield increased approx. 75-fold upon addition of 10 microM chromogranin A to 1 microM bis-ANS. Adenine nucleotides had clear effects on the bis-ANS fluorescence signal, while other nucleotides such as GTP, UTP and CTP had no discernible effect. Specifically, ATP caused a decrease in the fluorescence, whereas ADP and AMP caused a fluorescence increase. These results indicate adenine nucleotide binding to chromogranin A. Substitution of ATP with epsilon-ATP, an ATP derivative with a modification on the six-membered ring of the adenine base, failed to reduce the fluorescence intensity. Therefore, it was concluded that adenine bases play an important role in the chromogranin A-adenine nucleotide interaction.

A case of pulmonary malignant fibrous histiocytoma associated with pulmonary artery obstruction

Malignant fibrous histiocytoma (MFM) is among the most common soft tissue sarcoma of adults, but primary MFH of the lung is very rare. We report a case of primary pulmonary MFH associated with pulmonary artery obstruction.

Evidence for a direct role of tRNA in an amino acid transport system

The transport of phenylalanine by the general aromatic transport system in spheroplasts of Escherichia coli 9723 has been found to be stimulated by exogenous tRNA. Neither periodate-treated tRNA nor phenylalanine-charged tRNA stimulated, and the latter inhibited, phenylalanine uptake. Among preparations of specific tRNAs, tRNAPhe and tRNATyr were effective in stimulating the uptake of phenylalanine and tyrosine, respectively, and tRNAGlu and tRNAVal gave no detectable stimulation of phenylalanine or tyrosine transport. The preparation of tRNATyr was 10 times as active as unfractionated tRNA and gave as much as 167% stimulation of tyrosine transport. Correspondingly, the preparation of tRNAPhe was at least 3.5 times as active as the unfractionated tRNA and 2.5 times as active as the preparation of tRNATyr in stimulation of phenylalanine transport. Preliminary results in fractionation of the active component of tRNA for stimulating phenylalanine uptake show that the major activity resides in minor isoacceptor(s) tRNAPhe rather than the major component tRNAPhe, and the slight activity of preparations of tRNATyr is probably due to a contamination of the active tRNAPhe. Other preliminary results indicate that this type of stimulation occurs with uptake of other amino acids and their tRNA.