Enzymatic methylation of carboxyl groups of chromaffin granule membrane proteins.

Carboxyl groups of membrane and soluble proteins from bovine adrenal medulla chromaffin granules were enzymatically methylated. The methylated peptides were resolved using gel electrophoresis under acidic conditions in the presence of N-cetylpyridinium chloride. There was a selective methylation of two groups of membrane peptides which did not correspond to any of the chromaffin granule soluble proteins. Dopamine beta-hydroxylase, an acidic protein accounting for up to 25% of the membrane proteins, was a poor substrate for protein carboxylmethylase. The methyl esters of membrane proteins were more labile than those of the chromaffin granule soluble proteins. At all pH values tested, membrane protein-methyl esters were hydrolyzed three times more rapidly than the soluble protein-methyl esters.     

Calmodulin-binding proteins in granule and plasma membranes from bovine chromaffin cells

Calmodulin-binding proteins in chromaffin granule membrane and chromaffin cell plasma membranes have been investigated and compared. Chromaffin granules were purified by centrifugation over a 1.7 M sucrose layer. Plasma membranes were obtained in a highly purified form by differential and isopycnic centrifugation. Enzymatic determinations of 5'-nucleotidase, a generally accepted plasma membrane marker, showed a 40-50-fold enrichment as compared to the cell homogenate. Marker enzyme studies demonstrated only minimal contamination by other subcellular organelles. After solubilization with Triton X-100, calmodulin-binding proteins were isolated from chromaffin granule membranes and plasma membranes by affinity chromatography on a calmodulin/Sepharose 4B column. On two-dimensional polyacrylamide gelelectrophoresis a prominent protein (Mr = 65,000, pI ranging from 5.1 to 6) consisting of multiple spots, was present in the calmodulin-binding fraction from chromaffin granule membranes as well as from plasma membranes. Besides this 65 kDa protein both fractions had at least four groups of proteins in common. Also, proteins typical for either preparation were observed. In the calmodulin-binding protein preparations from chromaffin granule membranes a prominent spot with Mr = 80,000 and a pH ranging from 5.0 to 5.7 was present. This protein was enzymatically and immunologically identified as dopamine-beta-monooxygenase.     

Conversion of soluble dopamine beta-hydroxylase to a membrane binding form

We have shown that purified bovine soluble dopamine beta-hydroxylase can reconstitute onto preformed phosphatidylserine containing vesicles. The binding is dependent on pH and vesicle phosphatidylserine composition but does not require calcium. Reconstitution appears to be irreversible, with the lipid-bound enzyme possessing hydroxylase activity. Additionally, [14C] phosphatidylserine binds to soluble dopamine beta-hydroxylase and remains bound after several detergent washes. Thus the reconstituted soluble form of the enzyme appears to be functionally analogous to the membranous form. Both the reconstitution data and the lipid binding data suggest that multiple phosphatidylserine molecules bind to the soluble hydroxylase. We propose that noncovalently bound phosphatidylserine moieties, which copurify with the membrane bound form of the enzyme, alone are responsible for anchoring membranous dopamine beta-hydroxylase to chromaffin granule and model membranes.     

Subcellular Distribution of 65,000 Calmodulin-Binding Protein (p65) and Synaptophysin (p38) in Adrenal Medulla

Both neuronal and endocrine cells contain secretory vesicles that store and release neurotransmitters and peptides. Neuronal cells release their secretory material from both small synaptic vesicles and large dense-core vesicles (LDCVs), whereas endocrine cells release secretory products from LDCVs. Neuronal small synaptic vesicles are known to express three integral membrane proteins: 65,000 calmodulin-binding protein (65-CMBP) (p65), synaptophysin (p38), and SV2. A controversial question surrounding these three proteins is whether they are present in LDCV membranes of endocrine and neuronal cells. Sucrose density centrifugation of adrenal medulla was performed to study and compare the subcellular distribution of two of these small synaptic vesicle proteins (65-CMBP and synaptophysin). Subsequent immunoblotting and 125I-Protein A binding experiments performed on the fractions obtained from sucrose gradients showed that 65-CMBP was present in fractions corresponding to granule membranes and intact chromaffin granules. Similar immunoblotting and 125I-Protein A binding experiments with synaptophysin antibodies showed that this protein was also present in intact granules and granule membrane fractions. However, an additional membrane component, equilibrating near the upper portion of the sucrose gradient, also showed strong immunoreactivity with anti-synaptophysin and high 125I-Protein A binding activity. In addition, immunoblotting experiments on purified plasma and granule membranes demonstrated that 65-CMBP was a component of both membranes, whereas synaptophysin was only present in granule membranes. Thus, there appears to be a different subcellular localization between 65-CMBP and synaptophysin in the chromaffin cell.     

Validation of the detergent micelle classification for membrane protein crystals and explanation of the Matthews Graph for soluble proteins

Protein crystals are of wide-spread interest because many of them allow structure analyses at atomic resolution. For soluble proteins, the packing density of such crystals is distributed according to the Matthews Graph. For integral membrane proteins, the respective graph is similar but at lower density and much broader. By visualizing the relative positions and orientations of membrane proteins in crystals, it has been suggested that the detergent micelles surrounding these proteins form sheets, filaments, or remain isolated in the crystal giving rise to three distinct packing density distributions that superimpose to form the observed broad distribution. This classification was indirect because detergent is not visible in X-ray crystallography. Given the extensive work involved in analyzing detergent structure directly by neutron diffraction, it seems unlikely that a statistically relevant number of them will be established in the near future. Therefore, the proposed classification is here scrutinized by a simulation in which an average detergent-carrying membrane protein was randomly packed to form crystals. The analysis reproduced the three types of detergent structures together with their packing density distributions and relative frequencies, which validates the previous classification. The simulation program was also run for crystals from soluble proteins using ellipsoids as reference shapes and defining a shape factor that quantifies the deviation from the nearest ellipsoid. This series reproduced and thus explained the Matthews Graph.     

Membranes of chromaffin granules. Isolation and partial characterization of two proteins

Membranes of chromaffin granules were isolated from the adrenal glands of four different species. The solubilized membrane proteins could be resolved into several bands by polyacrylamide-gel electrophoresis (alkaline and acid gel systems). Two major protein components appeared to be common to the chromaffin granule membranes of ox, horse, pig and man. The various membrane proteins of bovine chromaffin granules were separated by filtration on Sephadex G-200 in the presence of sodium dodecyl sulphate. Two major membrane proteins (A and B) were obtained in purified form. Treatment of protein A with 2-mercaptoethanol before electrophoresis resulted in two more rapidly migrating subunits, whereas protein B was unaffected by mercaptoethanol treatment. The amino acid compositions of the two purified proteins were determined. They are very similar to that of the total membrane proteins but significantly different from that of the chromogranins, the soluble proteins of chromaffin granules.     

A monoclonal anticarbohydrate antibody detecting superfast myosin in the masseter muscle

Adrenal medullary chromaffin cells secrete catecholamines through exocytosis of their intracellular chromaffin granules. Osmotic granule swelling has been implicated to play a role in the generation of membrane stress associated with the fusion of the granule membrane. However, controversy exists as to whether swelling occurs before or after the actual fusion event. Using morphometric methods we have determined the granule diameter distributions in rapidly frozen, freeze-substituted chromaffin cells. Our measurements show that intracellular chromaffin granules increase in size from an average of 234 nm to 274 nm or 277 nm in cells stimulated to secrete with nicotine or high external K⁺, respectively. Granule swelling occurs before the formation of membrane contact. Ammonium chloride, an agent which inhibits stimulated catecholamine secretion by approximately 50% by altering the intragranular pH, also inhibits granule swelling. In addition, ammonium chloridetreated secreting cells show more granule-plasma membrane contacts than untreated secreting cells. Sodium propionate induces granule swelling in the absence of secretagogue and has been shown to enhance nicotine- and high K⁺- induced catecholamine release. These results indicate that in adrenal chromaffin cells granule swelling is an essential step in exocytosis before fusion pore formation, and is related to the pH of the granule environment.     

Chromaffin Granule Membrane-F-Actin Interactions and Spectrin-Like Protein of Subcellular Organelles: A Possible Relationship

The membrane of chromaffin granule, the secretory vesicle of adrenal medullary cells storing catecholamines, enkephalins, and many other components, interacts with F-actin. Using low shear falling ball viscometry to estimate actin binding to membranes, we demonstrated that mitochondrial and plasma membranes from chromaffin cells also provoked large increases in viscosity of F-actin solutions. Mitochondrial membranes also had the capacity to cause complete gelation of F-actin. In addition, vasopressin-containing granules from neurohypophysial tissue were shown to bind F-actin and to increase the viscosity of F-actin solutions. Using an antibody directed against human erythrocyte spectrin, it was found that a spectrin-like protein was associated with secretory granule membrane, mitochondrial membrane, and plasma membrane. The chromaffin granule membrane-associated spectrin-like protein faces the cytoplasmic side, is composed of two subunits (240 kD and 235kD ), the alpha-subunit (240 kD, pHi5 .5) being recognized by the antibody. Nonionic detergents such as Triton X-100 or Nonidet P40 failed to release fully active spectrin-like protein. In contrast, Kyro EOB , a different nonionic detergent, was found to release spectrin-like protein while keeping intact F-actin binding capacity, at least below 0.5% Kyro EOB concentration. Chromaffin cells in culture were stained with antispectrin antibody, showing the presence of spectrin-like protein in the cell periphery close to the cell membrane but also in the cytoplasm. We conclude that in living cells the interaction of F-actin with chromaffin granule membrane spectrin observed in vitro is important in controlling the potential function of secretory vesicles.     

Isolation, folding and structural investigations of the amino acid transporter OEP16

Membrane proteins compose more than 30% of all proteins in the living cell. However, many membrane proteins have low abundance in the cell and cannot be isolated from natural sources in concentrations suitable for structure analysis. The overexpression, reconstitution, and stabilization of membrane proteins are complex and remain a formidable challenge in membrane protein characterization. Here we describe a novel, in vitro folding procedure for a cation-selective channel protein, the outer envelope membrane protein 16 (OEP16) of pea chloroplast, overexpressed in Escherichia coli in the form of inclusion bodies. The protein is purified and then folded with detergent on a Ni–NTA affinity column. Final concentrations of reconstituted OEP16 of up to 24 mg/ml have been achieved, which provides samples that are sufficient for structural studies by NMR and crystallography. Reconstitution of OEP16 in detergent micelles was monitored by circular dichroism, fluorescence, and NMR spectroscopy. Tryptophan fluorescence spectra of heterologous expressed OEP16 in micelles are similar to spectra of functionally active OEP16 in liposomes, which indicates folding of the membrane protein in detergent micelles. CD spectroscopy studies demonstrate a folded protein consisting primarily of α-helices. ¹⁵N-HSQC NMR spectra also provide evidence for a folded protein. We present here a convenient, effective and quantitative method to screen large numbers of conditions for optimal protein stability by using microdialysis chambers in combination with fluorescence spectroscopy. Recent collection of multidimensional NMR data at 500, 600 and 800 MHz demonstrated that the protein is suitable for structure determination by NMR and stable for weeks during data collection.     

Calcium-Dependent Calmodulin Binding to Chromaffin Granule Membranes: Presence of a 65-Kilodalton Calmodulin-Binding Protein

The presence of calmodulin-binding sites on chromaffin granule membranes has been investigated. Saturable, high-affinity 125I-calmodulin-binding sites (KD = 9.8 nM; Bmax = 25 pmol/mg protein) were observed in the presence of 10(-4) M free calcium. A second, nonsaturable, calmodulin-binding activity could also be detected at 10(-7) M free calcium. No binding occurred at lower calcium levels. When chromaffin granule membranes were delipidated by solvent extraction, calmodulin binding was observed at 10(-4) M free calcium. However no binding was detected at lower calcium concentrations. Thus it appears that a calcium concentration of 10(-7) M promotes the binding of calmodulin to some solvent-soluble components of the chromaffin granule membrane. Calmodulin-binding proteins associated with the granule membrane identified by photoaffinity cross-linking. A calmodulin-binding protein complex, of molecular weight 82K, was formed in the presence of 10(-4) M free calcium. This cross-linked product was specific because it was not detected either in the absence of calcium, in the presence of nonlabeled calmodulin, or in the absence of cross-linker activation. When solvent-treated membranes were used, a second, specific, calmodulin-binding protein complex (70K) was formed. Since the apparent molecular weight of calmodulin in our electrophoresis system was 17K, these experiments suggested the presence of two calmodulin-binding proteins, of molecular weights 65K and 53K, in the chromaffin granule membrane. This result was confirmed by the use of calmodulin-affinity chromatography. When detergent-solubilized membranes were applied on the column in the presence of calcium, two polypeptides of apparent molecular weights of 65K and 53K were specifically eluted by EGTA buffers. Since detergent treatments or solvent extractions are necessary to detect the 53K calmodulin-binding protein, it is concluded that only the 65K calmodulin-binding polypeptide may play a role in the interaction between calmodulin and secretory granules in chromaffin cells.     

Membrane and soluble proteins of adrenal chromaffin granules

Bovine adrenal chromaffin granules are useful 'model' neurosecretory vesicles, particularly for biochemical studies. The granule matrix contains three major secretory proteins (chromogranin A and secretogranins I and II) together with peptides derived from them, and smaller amounts of neuropeptides (enkephalins and neuropeptide Y). Several different endo- and exo-proteinases are also present in both soluble and membrane-bound forms. The major membrane proteins are those involved in catecholamine biosynthesis (dopamine beta-monooxygenase and cytochrome b(561)), active transport of granule components (vacuolar-type proton-translocating ATPase, and carriers for monoamines, nucleotides and small ions) and exocytosis (synaptotagmin, synaptobrevin and other proteins). In addition, the functions of a number of major granule membrane proteins remain unknown.     

Calmodulin-Binding Proteins in Chromaffin Cell Plasma Membranes

Abstract: Calmodulin-binding proteins present in chromaffin cell plasma membranes were isolated and directly compared with calmodulin-binding proteins present in chromaffin granule membranes. Chromaffin cell plasma membranes were prepared using Cytodex 1 microcarriers. Marker enzyme studies on this preparation showed a nine- to 10–fold plasma membrane enrichment over cell homogenates and a low contamination of these plasma membranes by subcellular organelles. Plasma membranes prepared in this manner were solubilized with Triton X-100 and applied to a calmodulin-affinity column in the presence of calcium. Several major calmodulin-binding proteins ( 240, 105 , and 65 kilodaltons) were eluted by an EGTA-containing buffer. ¹²⁵I-Calmodulin overlay experiments on nitrocellulose sheets containing both chromaffin plasma and granule membranes showed that these two membranes have several calmodulin-binding proteins in common ( 65, 60, 53 , and 50 kilodaltons), as well as unique calmodulin-binding proteins (34 kilodaltons in granule membranes and 240 and 160 kilodaltons in plasma membranes). The 65–kilodalton calmodulin-binding protein present in both membrane types was shown to consist of two isoforms (pI 6.0 and 6.2) by two-dimensional gel electrophoresis. Previous experiments from our laboratory, using two monoclonal antibodies (mAb 30 and mAb 48) specific for a rat brain synaptic vesicle membrane protein (p65), showed that the monoclonal antibodies reacted with a 65–kilodalton calmodulin-binding protein present in at least three neurosecretory vesicles (chromaffin granules, neurohypophyseal granules, and rat brain synaptic vesicles). When these monoclonal antibodies were tested on chromaffin cell plasma membranes and calmodulin-binding proteins isolated from these membranes, they recognized a 65–kilodalton protein. These results indicate that an immunologically identical calmodulin-binding protein is expressed in both chromaffin granule membranes (as well as other secretory vesicle membranes) and chromaffin cell plasma membranes, thus suggesting a possible role for this protein in granule/plasma membrane interaction.     

Chromogranin, an integral membrane protein

Chromogranin is the major soluble protein of the adrenal medulla chromaffin granule and is secreted upon nervous stimulation. Using antisera to pure chromogranin in immunoblotting procedures, we show that chromogranin is the major integral membrane protein as well. Extraction of chromaffin granule membranes with low salt, high salt, chelating agents, or calcium-containing solutions does not remove the chromogranin from the membranes. The membrane form of chromogranin can be purified on a C-18 semi-preparative column using high pressure liquid chromatography. Amino-terminal sequence data indicate that the membrane and soluble forms of chromogranin are identical or very similar.     

Adrenal Chromaffin Cell Calmodulin: Its Subcellular Distribution and Binding to Chromaffin Granule Membrane Proteins

Bovine adrenal medullae were homogenized in the presence or in the absence of EGTA and different subcellular fractions were prepared by differential and density gradient centrifugations. In the presence of the chelating agent, 69% of the total calmodulin, measured by radioimmunoassay, was present in the cytosol; the rest was bound to different membrane-containing fractions (nuclei, microsomal, and crude granule fraction). When the chelating agent was omitted, 43% of the calmodulin was present in the cytosol, the remaining calmodulin being membrane-bound. Further resolution of the crude granule fraction by sucrose density centrifugation demonstrated that the distribution of calmodulin in the density gradient was similar to the distribution of chromaffin granules rather than to that of mitochondria, Golgi elements, and lysosomes. In this case, there was also more calmodulin bound to chromaffin granules when EGTA was omitted from the density gradient. Experiments with 125I-calmodulin indicated the presence of high-affinity binding sites (KD = 1.3 X 10(-8) M; Bmax = 30 pmol/mg protein) for calmodulin in chromaffin granule membranes. Further, photoaffinity crosslinking experiments with 125I-calmodulin followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography indicated the presence of three calmodulin-binding polypeptide complexes (84,000; 41,000; and 38,000 daltons) in chromaffin granule membranes. These polypeptides were not labelled when either Ca2+ was omitted or an excess of nonradioactive calmodulin was present in the photolysis buffer, indicating the Ca2+ dependency and the specificity of the interaction. On the basis of the results described, it is suggested that the cellular levels of Ca2+ control the cellular distribution of calmodulin and its binding to specific chromaffin granule membrane proteins. Further, it is also suggested that the interactions between calmodulin and granule proteins might play a role in stimulus-secretion coupling.     

Glutamate uptake by brain synaptic vesicles. Energy dependence of transport and functional reconstitution in proteoliposomes

The dependence of glutamate uptake on ATP-generated proton electrochemical potential was studied in a highly purified preparation of synaptic vesicles from rat brain. At low chloride concentration (4 mM), the proton pump present in synaptic vesicles generated a large membrane potential (inside-positive), associated with only minor acidification. Under these conditions, the rate of L-[3H]glutamate uptake was maximal. In addition, L-glutamate induced acidification of the vesicle interior. D-Glutamate produced only 40% of the effect, and L-aspartate or gamma-aminobutyric acid produced less than 5%. The initial rate of glutamate-induced acidification increased with increasing glutamate concentration. It was saturable and showed first-order kinetics (KM = 0.32 mM). Correspondingly, L-glutamate induced a small reduction in the membrane potential. The rate of ATP hydrolysis was unaffected. In comparison, glutamate had no effect on acidification or membrane potential in resealed membranes of chromaffin granules. At high chloride concentration (150 mM), the vesicular proton pump generated a large pH difference, associated with a small change in membrane potential. Under these conditions, uptake of L-[3H]glutamate by synaptic vesicles was low. For reconstitution, vesicle proteins were solubilized with the detergent sodium cholate, supplemented with brain phospholipids, and incorporated into liposomes. Proton pump and glutamate uptake activities of the proteoliposomes showed properties similar to those of intact vesicles indicating that the carrier was reconstituted in a functionally active form. It is concluded that glutamate uptake by synaptic vesicles is dependent on the membrane potential and that all components required for uptake are integral parts of the vesicle membrane.     

Influence of Membrane Lipid Composition on a Transmembrane Bacterial Chemoreceptor

Most bacterial chemoreceptors are transmembrane proteins. Although less than 10% of a transmembrane chemoreceptor is embedded in lipid, separation from the natural membrane environment by detergent solubilization eliminates most receptor activities, presumably because receptor structure is perturbed. Reincorporation into a lipid bilayer can restore these activities and thus functionally native structure. However, the extent to which specific lipid features are important for effective restoration is unknown. Thus we investigated effects of membrane lipid composition on chemoreceptor Tar from Escherichia coli using Nanodiscs, small (∼10-nm) plugs of lipid bilayer rendered water-soluble by an annulus of “membrane scaffold protein.” Disc-enclosed bilayers can be made with different lipids or lipid combinations. Nanodiscs carrying an inserted receptor dimer have high protein-to-lipid ratios approximating native membranes and in this way mimic the natural chemoreceptor environment. To identify features important for functionally native receptor structure, we made Nanodiscs using natural and synthetic lipids, assaying extents and rates of adaptational modification. The proportion of functionally native Tar was highest in bilayers closest in composition to E. coli cytoplasmic membrane. Some other lipid compositions resulted in a significant proportion of functionally native receptor, but simply surrounding the chemoreceptor transmembrane segment with a lipid bilayer was not sufficient. Membranes effective in supporting functionally native Tar contained as the majority lipid phosphatidylethanolamine or a related zwitterionic lipid plus a rather specific proportion of anionic lipids, as well as unsaturated fatty acids. Thus the chemoreceptor is strongly influenced by its lipid environment and is tuned to its natural one.     

DNA knotting caused by head-on collision of transcription and replication

Obtaining crystals of membrane proteins that diffract to high resolution remains a major stumbling block in structure determination. Here we present a new method for crystallizing membrane proteins from a bicelle forming lipid/detergent mixture. The method is flexible and simple to use. As a test case, bacteriorhodopsin (bR) from Halobacterium salinarum was crystallized from a bicellar solution, yielding a new bR crystal form. The crystals belong to space group P2(1) with unit cell dimensions of a=45.0 A, b=108.9 A, c=55.9 A, beta=113.58 degrees and a dimeric asymmetric unit. The structure was solved by molecular replacement and refined at 2.0 A resolution. In all previous bR structures the protein is organized as a parallel trimer, but in the crystals grown from bicelles, the individual bR subunits are arranged in an antiparallel fashion.     

OmpA: Gating and dynamics via molecular dynamics simulations

Outer membrane proteins (OMPs) of Gram-negative bacteria have a variety of functions including passive transport, active transport, catalysis, pathogenesis and signal transduction. Whilst the structures of ∼ 25 OMPs are currently known, there is relatively little known about their dynamics in different environments. The outer membrane protein, OmpA from Escherichia coli has been studied extensively in different environments both experimentally and computationally, and thus provides an ideal test case for the study of the dynamics and environmental interactions of outer membrane proteins. We review molecular dynamics simulations of OmpA and its homologues in a variety of different environments and discuss possible mechanisms of pore gating. The transmembrane domain of E. coli OmpA shows subtle differences in dynamics and interactions between a detergent micelle and a lipid bilayer environment. Simulations of the crystallographic unit cell reveal a micelle-like network of detergent molecules interacting with the protein monomers. Simulation and modelling studies emphasise the role of an electrostatic-switch mechanism in the pore-gating mechanism. Simulation studies have been extended to comparative models of OmpA homologues from Pseudomonas aeruginosa (OprF) and Pasteurella multocida (PmOmpA), the latter model including the periplasmic C-terminal domain.     

Characterization of the chromobindins. Soluble proteins that bind to the chromaffin granule membrane in the presence of Ca2+

A group of proteins that bind to the chromaffin granule membrane in the presence of Ca2+ has been isolated by affinity chromatography of bovine adrenal medullary cytosol on granule membranes coupled to Sepharose 4B. Twenty-two of these proteins were resolved into classes depending upon the Ca2+ concentration at which they were eluted from the affinity column (40 or 0.1 microM), upon their affinities for native granule membranes or for liposomes prepared from extracted granule lipids, and upon the requirement of seven of the proteins for ATP in the cytosol fraction and column buffers to promote binding. The molecular weights and isoelectric points of these proteins were determined by two-dimensional electrophoresis. Two of the granule-binding proteins were identified: synexin and calmodulin. Calmodulin was found to bind to seven specific granule membrane proteins after diffusion of 125I-labeled calmodulin into an acrylamide gel of membrane proteins separated by electrophoresis in the presence of sodium dodecyl sulfate. A phospholipid-activated protein kinase activity, possibly due to protein kinase C, was present in the granule-binding fraction. Two major granule-binding proteins were found to present a pattern in two-dimensional electrophoresis that was very similar to but shifted slightly toward the basic end of the gel from the pattern generated by light chains associated with clathrin in adrenal medullary coated vesicles. In the chromaffin cell, these proteins, by associating with the granule membrane in the presence of an increased cytosolic Ca2+ concentration, might play a variety of roles in the process of exocytosis.     

LACTOPEROXIDASE-COUPLED IODINATION OF BOVINE CHROMAFFIN GRANULES

Abstract— Chromaffin granules were iodinated with lactoperoxidase at either their external or internal membrane surfaces. When iodination of internal soluble granule proteins and membrane phospholipids was minimized, the majority of the membrane proteins, including the 83,000 component, were iodinated. Components with molecular weights 63,000, 61,000, 51,000, 44,000, 32,000, 26,000 and 19,000 had a higher ¹²⁵I specific activity than did the other membrane components, suggesting they were more accessible at the outer membrane surface than were the other components. In the presence of detergent, the iodination of all membrane components was increased more than 10-fold; the incorporation of ¹²⁵I was now similar to their Coomassie Blue staining intensity in disc gels, indicating that all components were equally accessible to lactoperoxidase. In the presence of detergent, iodine incorporation into the MW 83,000 and 16,000 components was stimulated approx 100-fold.
The MW 83,000, 63,000, 61,000 and 37,000 components incorporated significant amounts of ¹²⁵I when granule membranes were iodinated from their internal surface, suggesting these components have a portion of their polypeptide chain accessible at the inner membrane surface. Thus the MW 83,000 component, which we identified as dopamine β hydroxylase, and the MW 63,000/61,000 components, which are part of the membrane ATPase, can be iodinated from both membrane surfaces. This would suggest that these are transmembrane proteins. However, the major portion of all the proteins in this membrane were inaccessible to lactoperoxidase at either membrane surface.