Purification of membrane proteins

Peripheral Membrane Proteins: Promising Therapeutic Targets across Domains of Life

Membrane proteins can be classified into two main categories-integral and peripheral membrane proteins-depending on the nature of their membrane interaction. Peripheral membrane proteins are highly unique amphipathic proteins that interact with the membrane indirectly, using electrostatic or hydrophobic interactions, or directly, using hydrophobic tails or GPI-anchors. The nature of this interaction not only influences the location of the protein in the cell, but also the function. In addition to their unique relationship with the cell membrane, peripheral membrane proteins often play a key role in the development of human diseases such as African sleeping sickness, cancer, and atherosclerosis. This review will discuss the membrane interaction and role of periplasmic nitrate reductase, CymA, cytochrome c, alkaline phosphatase, ecto-5'-nucleotidase, acetylcholinesterase, alternative oxidase, type-II NADH dehydrogenase, and dihydroorotate dehydrogenase in certain diseases. The study of these proteins will give new insights into their function and structure, and may ultimately lead to ground-breaking advances in the treatment of severe diseases.

Electron microscopy-based semi-automated characterization of aggregation in monoclonal antibody products

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Size-based quantification of small heterogeneous proteins using electron microscopy.

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Electron microscopy as an orthogonal tool for characterizing protein aggregates.

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Quick assessment of small heterogeneous proteins via softEM, a GUI-based algorithm.

Aggregation is a critical parameter for protein-based therapeutics, due to its impact on the immunogenicity of the product. The traditional approach towards characterization of such products is to use a collection of orthogonal tools. However, the fact that none of these tools is able to completely classify the distribution and physical characteristics of aggregates, implies that there exists a need for additional analytical methods. We report one such method for characterization of heterogeneous population of proteins using transmission electron microscopy. The method involves semi-automated, size-based clustering of different protein species from micrographs. This method can be utilized for quantitative characterization of heterogeneous populations of antibody/protein aggregates from TEM images of proteins, and may also be applicable towards other instances of protein aggregation.

Digging More Missing Proteins Using an Enrichment Approach with ProteoMiner

Human Proteome Project (HPP) aims at mapping entire human proteins with a systematic effort upon all the emerging techniques, which would enhance understanding of human biology and lay a foundation for development of medical applications. Until now, 2563 missing proteins (MPs, PE2-4) are still undetected even using the most sensitive approach of protein detection. Herein, we propose that enrichment of low-abundance proteins benefits MPs finding. ProteoMiner is an equalizing technique by reducing high-abundance proteins and enriching low-abundance proteins in biological liquids. With triton X-100/TBS buffer extraction, ProteoMiner enrichment, and peptide fractionation, 20 MPs (at least two non-nested unique peptides with more than eight a.a. length) with 60 unique peptides were identified from four human tissues including eight membrane/secreted proteins and five nucleus proteins. Then 15 of them were confirmed with two non-nested unique peptides (≥9 a.a.) identified by matching well with their chemically synthetic peptides in PRM assay. Hence, these results demonstrated ProteoMiner as a powerful means in discovery of MPs.

Human macrophage ferroportin biology and the basis for the ferroportin disease

Ferroportin (FPN1) is the sole iron exporter in mammals, but its cell-specific function and regulation are still elusive. This study examined FPN1 expression in human macrophages, the cells that are primarily responsible on a daily basis for plasma iron turnover and are central in the pathogenesis of ferroportin disease (FD), the disease attributed to lack-of-function FPN1 mutations. We characterized FPN1 protein expression and traffic by confocal microscopy, western blotting, gel filtration, and immunoprecipitation studies in macrophages from control blood donors (donor) and patients with either FPN1 p.A77D, p.G80S, and p.Val162del lack-of-function or p.A69T gain-of-function mutations. We found that in normal macrophages, FPN1 cycles in the early endocytic compartment does not multimerize and is promptly degraded by hepcidin (Hepc), its physiological inhibitor, within 3-6 hours. In FD macrophages, endogenous FPN1 showed a similar localization, except for greater accumulation in lysosomes. However, in contrast with previous studies using overexpressed mutant protein in cell lines, FPN1 could still reach the cell surface and be normally internalized and degraded upon exposure to Hepc. However, when FD macrophages were exposed to large amounts of heme iron, in contrast to donor and p.A69T macrophages, FPN1 could no longer reach the cell surface, leading to intracellular iron retention.

CONCLUSION

FPN1 cycles as a monomer within the endocytic/plasma membrane compartment and responds to its physiological inhibitor, Hepc, in both control and FD cells. However, in FD, FPN1 fails to reach the cell surface when cells undergo high iron turnover. Our findings provide a basis for the FD characterized by a preserved iron transfer in the enterocytes (i.e., cells with low iron turnover) and iron retention in cells exposed to high iron flux, such as liver and spleen macrophages. (Hepatology 2017;65:1512-1525).

Understanding leaf membrane protein extraction to develop a food-grade process

Leaf membrane proteins are an underutilised protein fraction for food applications. Proteins from leaves can contribute to a more complete use of resources and help to meet the increasing protein demand. Leaf protein extraction and purification is applied by other disciplines, such as proteomics. Therefore, this study analysed proteomic extraction methods for membrane proteins as an inspiration for a food-grade alternative process. Sugar beet leaves were extracted with two proteomic protocols: solvent extraction and Triton X-114 phase partitioning method. Extraction steps contributed to protein purity and/or to selective fractionation, enabling the purification of specific proteins. It was observed that membrane proteins distributed among different solvents, buffers and solutions used due to their physicochemical heterogeneity. This heterogeneity does not allow a total membrane protein extraction by a unique method or even combinations of processing steps, but it enables the creation of different fractions with different physicochemical properties useful for food applications.

Chemical crosslinkers enhance detection of receptor interactomes

Receptor function is dependent on interaction with various intracellular proteins that ensure the localization and signaling of the receptor. While a number of approaches have been optimized for the isolation, purification, and proteomic characterization of receptor-protein interaction networks (interactomes) in cells, the capture of receptor interactomes and their dynamic properties remains a challenge. In particular, the study of interactome components that bind to the receptor with low affinity or can rapidly dissociate from the macromolecular complex is difficult. Here we describe how chemical crosslinking (CC) can aid in the isolation and proteomic analysis of receptor-protein interactions. The addition of CC to standard affinity purification and mass spectrometry protocols boosts the power of protein capture within the proteomic assay and enables the identification of specific binding partners under various cellular and receptor states. The utility of CC in receptor interactome studies is highlighted for the nicotinic acetylcholine receptor as well as several other receptor types. A better understanding of receptors and their interactions with proteins spearheads molecular biology, informs an integral part of bench medicine which helps in drug development, drug action, and understanding the pathophysiology of disease.

Solubilization of native integral membrane proteins in aqueous buffer by noncovalent chelation with monomethoxy poly(ethylene glycol) (mPEG) polymers

Highly hydrophobic integral membrane proteins (IMPs)are typically purified in excess detergent media, often resulting in rapid inactivation and denaturation of the protein. One promising approach to solve this problem is to couple hydrophilic polymers, such as monomethoxypolyethylene glycol (mPEG) to IMPs under mild conditions in place of detergents. However, the broad application of this approach is hampered by poor reaction efficiencies, low tolerance of detergent stabilized membrane proteins to reaction conditions, and a lack of proper site-specific reversible approaches. Here, we have developed a straightforward, efficient, and mild approach to site-specific noncovalent binding of long-chain polymers to recombinant IMPs. This method uses the hexa-histidine tag (His-Tag) often used for purification of recombinant proteins as an attachment site for mPEGs. Solubility studies performed using five different IMPs confirmed that all tested mPEG-bound IMPs were completely soluble and stable in detergent free aqueous buffer compared to their precipitated native proteins under the identical circumstances. Activity assays and circular dichroism (CD) spectroscopy confirmed the structural integrity of modified IMPs.

Dry powder vaccines for mucosal administration: critical factors in manufacture and delivery

Dry powder vaccine formulations have proved effective for induction of systemic and mucosal immune responses. Here we review the use of dry vaccines for immunization in the respiratory tract. We discuss techniques for powder formulation, manufacture, characterization and delivery in addition to methods used for evaluation of stability and safety. We review the immunogenicity and protective efficacy of dry powder vaccines as compared to liquid vaccines delivered by mucosal or parenteral routes. Included is information on mucosal adjuvants and mucoadhesives that can be used to enhance nasal or pulmonary dry vaccines. Mucosal immunization with dry powder vaccines offers the potential to provide a needle-free and cold chain-independent vaccination strategy for the induction of protective immunity against either systemic or mucosal pathogens.

Protein aggregation: pathways, induction factors and analysis

Control and analysis of protein aggregation is an increasing challenge to pharmaceutical research and development. Due to the nature of protein interactions, protein aggregation may occur at various points throughout the lifetime of a protein and may be of different quantity and quality such as size, shape, morphology. It is therefore important to understand the interactions, causes and analyses of such aggregates in order to control protein aggregation to enable successful products. This review gives a short outline of currently discussed pathways and induction methods for protein aggregation and describes currently employed set of analytical techniques and emerging technologies for aggregate detection, characterization and quantification. A major challenge for the analysis of protein aggregates is that no single analytical method exists to cover the entire size range or type of aggregates which may appear. Each analytical method not only shows its specific advantages but also has its limitations. The limits of detection and the possibility of creating artifacts through sample preparation by inducing or destroying aggregates need to be considered with each method used. Therefore, it may also be advisable to carefully compare analytical results of orthogonal methods for similar size ranges to evaluate method performance.

Ethanol consumption alters expression and colocalization of bile salt export pump and multidrug resistance protein 2 in the rat

Chronic ethanol consumption elicits detrimental changes of liver metabolism. By employing a 12-week-long feeding regimen, we investigated the effects of chronic ethanol consumption on the expression and localization of bile salt export pump (Bsep), a major canalicular exporter of bile salts, and multidrug resistance protein 2 (Mrp2), a canalicular organic anion transporter, in the rat liver. RT-PCR, confocal immunofluorescence microscopy, immunoblotting, and quantitative colocalization analysis were used to examine their gene and protein expression, and changes in the distribution of antigenic sites. Bsep mRNA was upregulated, while Mrp2 mRNA responded by downregulation. In agreement with mRNA, the expression of Bsep protein increased, while the expression of Mrp2 protein responded with a decrease, suggesting that the expression of both of them is transcriptionally regulated. Confocal immunofluorescence microscopy showed disruption of the colocalization of Bsep and Mrp2 proteins at the hepatocyte canalicular membrane and their relocation intracellularly. Quantitative colocalization analysis of Bsep and Mrp2 proteins revealed a steady decrease in the degree of colocalization and Mrp2 expression, indicating that although the properties of both transporters are affected, Mrp2 is altered more. These findings provide evidence that ethanol alters Bsep and Mrp2 canalicular transporters in the rat liver, at both the mRNA and protein levels. Mrp2 shows deeper involvement. Eight weeks appears to be a critical time point in this process.

Agarose isoelectric focusing can improve resolution of membrane proteins in the two-dimensional electrophoresis of bacterial proteins

2-D separation of bacterial membrane proteins is still difficult despite using high-resolution IPG-IEF/SDS-PAGE. We were searching for alternative methods to avoid typical problems such as precipitation, low solubility, and aggregation of membrane proteins in the 1-D separation with IPG-IEF. Blue native electrophoresis (BNE) and agarose IEF (A-IEF) were tested for their separation capacity and their capability of replacing IPG-IEF in the first dimension. SDS-PAGE was chosen for the second dimension on account of its outstanding resolution. We could confirm that only A-IEF was a useful replacement for the IPG-IEF in the first dimension resulting in 2-D protein distributions with additional membrane protein spots not being found after IPG-IEF/SDS-PAGE. A second interesting result was that the agarose IEF mediates the possibility of separation of membrane proteins in a partially native state in the first dimension. This native A-IEF resulted in drastically changed spot patterns with an acidic shift of nearly all spots and divergent distribution of proteins compared to non-native A-IEF and IPG-IEF. We found out that native and non-native A-IEF are powerful tools to supplement IPG-IEF/SDS-PAGE.

Angiotensin II-induced changes of calcium sparks and ionic currents in human atrial myocytes: potential role for early remodeling in atrial fibrillation

AIMS

Atrial angiotensin II (ANG II) levels have been shown to be increased in atrial fibrillation (AF). The purpose of the study was to evaluate a potential role of ANG II in the early remodeling and susceptibility to chronicization of AF.

METHODS AND RESULTS

Isolated human atrial myocytes were incubated in ANG II and/or angiotensin type 1 receptor blocker candesartan. ANG II markedly increased the frequency of spontaneous Ca(2+) sparks, spark full duration, time to peak Ca(2+) fluorescence and decay time measured by confocal imaging. Sarcoplasmic reticulum calcium content estimated by caffeine-evoked calcium release did not differ between ANG II-treated cells and controls. Patch-clamp recordings revealed that ANG II significantly decreased I(to) and increased I(Ca,L) current densities. Candesartan blocked these ANG II-mediated alterations. ANG II exhibited no effect on I(K1), I(Kur) and I(f) current size. Expression of connexin 40 and connexin 43 was not significantly changed by ANG II as assessed by immunohistochemistry and Western blot analysis.

CONCLUSION

ANG II-induced alterations of calcium handling and electrophysiological changes in human atrial cells similar to those previously observed in the onset of AF. Prevention of these alterations by candesartan might constitute a useful pharmacological strategy for the treatment of AF.

Membrane Protein Solubilization: Recent Advances and Challenges in Solubilization of Serotonin1A Receptors

Solubilization of integral membrane proteins is a process in which the proteins and lipids that are held together in native membranes are suitably dissociated in a buffered detergent solution. The controlled dissociation of the membrane results in formation of small protein and lipid clusters that remain dissolved in the aqueous solution. Effective solubilization and purification of membrane proteins, especially heterologously-expressed proteins in mammalian cells in culture, in functionally active forms represent important steps in understanding structure-function relationship of membrane proteins. In this review, critical factors determining functional solubilization of membrane proteins are highlighted with the solubilization of the serotonin 1A receptor taken as a specific example.

Perisomatic glutamatergic axon terminals: a novel feature of cortical synaptology revealed by vesicular glutamate transporter 1 immunostaining

The cellular localization of the vesicular glutamate transporter 1, VGLUT1, was studied in the rat cerebral cortex with immunocytochemical techniques. VGLUT1 immunoreactivity (ir) was localized to punctate structures dispersed in the neuropil of all cortical layers as well as around the profile of somata and proximal dendritic segments of virtually all pyramidal neurons. Using a correlative light and electron microscopic method, we found that VGLUT1 ir is expressed in axon terminals forming synapses exclusively with dendritic shafts and spines. Perisomatic VGLUT1-positive terminals never formed synapses with the pyramidal cell bodies to which they were in apposition, but formed asymmetric synapses with adjacent neuropilar dendritic elements. The high probability of a close spatial relationship between glutamatergic and GABAergic terminals in perisomatic regions suggests that spilled-out glutamate may act on inhibitory axon terminals innervating the soma of cortical pyramidal neurons.

Biosynthesis and biophysical analysis of domains of a yeast G protein-coupled receptor

The alpha-factor receptor(Ste2p) is required for the sexual conjugation of the yeast Saccharomyces cerevisiae. Ste2p belongs to the G protein-coupled receptor (GPCR) family sharing a common heptahelical transmembrane structure. Biological synthesis of regions of Ste2p fused to a leader protein in Escherichia coli yielded milligram quantities of polypeptides that corresponded to one or two transmembrane domains. Fusion proteins were characterized by polyacrylamide gel electrophoresis, high performance liquid chromatography, and mass spectrometry. CD studies on the fusion proteins in trifluoroethanol:water mixtures indicated the existence of alpha-helical structures in the single- and the double-transmembrane domains. NMR experiments were performed in CDCl(3):CD(3)OH:H(2)O (4:4:1) on the (15)N-labeled single-transmembrane peptide showing a clear dispersion of the nitrogen-amide proton correlation cross peaks indicative of a high-purity, uniformly labeled molecule. The results indicate that single- and double-transmembrane domains of a GPCR can be produced by biosynthetic methods in quantities and purity sufficient for biophysical studies.

Identification of paraoxonase 3 in rat liver microsomes: purification and biochemical properties

Three paraoxonase genes (PON1, PON2 and PON3) have been described so far in mammals. Although considerable information is available regarding PON1, little is known about PON2 and PON3. PON3 has been isolated recently from rabbit serum [Draganov, Stetson, Watson, Billecke and La Du (2000) J. Biol. Chem. 275, 33435-33442] and liver [Ozols (1999) Biochem. J. 338, 265-275]. In the present study, we have identified the presence of PON3 in rat liver microsomes and a method for the purification to homogeneity is presented. PON3 has been purified 177-fold to apparent homogeneity with a final specific activity of 461 units/mg using a method consisting of seven steps: solubilization of the microsomal fraction, hydroxyapatite adsorption, chromatography on DEAE-Sepharose CL-6B, non-specific affinity chromatography on Cibacron Blue 3GA, two DEAE-cellulose steps and a final affinity chromatography on concanavalin A-Sepharose. SDS/PAGE of the final preparation indicated a single protein-staining band with an apparent molecular mass of 43 kDa. The isolated protein was identified by nanoelectrospray MS. Internal amino acid sequences of several peptides were determined and compared with those of human, rabbit and mouse PON3, showing a high similarity. Some biochemical properties of PON3 were also studied, including optimum pH, K(m) and heat and pH stability.

Postnatal development of the vesicular GABA transporter in rat cerebral cortex

Light and electron microscopic immunocytochemical techniques and Western blotting were used to investigate the postnatal development of the vesicular GABA transporter (VGAT) in the rat somatic sensory cortex. VGAT immunoreactivity was low at birth, it increased gradually through the first and second weeks of life and achieved the adult pattern during the third week. At postnatal day (P)0-P5, VGAT immunoreactivity was associated exclusively to fibers and puncta. Electron microscopic studies performed at P5 showed that all identified synaptic contacts formed by VGAT-positive axonal swellings were of the symmetric type and that a substantial proportion of the boutons appeared not to have formed synapses. From P10 onward, labeled puncta were both scattered in the neuropil and in apposition to unstained cellular profiles; VGAT was also expressed in few GABAergic cell bodies. Western blottings at the same postnatal ages revealed a 55-kDa band whose intensity was weak at P0 (17% of adult), it increased constantly until P15 (P2: 35%; P5: 44%; P10: 68%; P15: 97%), and then leveled off. Overall, the present results show that during neocortical development the expression of VGAT slightly precedes the complete maturation of inhibitory synaptogenesis and suggest that it may contribute to the formation of neocortical GABAergic circuitry.

Asynchronous expression and colocalization of Bsep and Mrp2 during development of rat liver

In the liver, function of the bile salt export pump (Bsep), a major canalicular exporter of bile salts, is complemented by activity of the multidrug resistance protein 2 (Mrp2), a canalicular organic anions transporter. Mrp2 was found capable of transporting various anticancer drugs out of cells, eventually undermining their therapeutic potential and contributing to multidrug resistance. We employed a RT-PCR, immunoblotting, and immunofluorescence to examine their gene, protein expression, and distribution of antigenic sites in the rat liver during development from 16-day-old fetus to adult animal. Bsep mRNA was almost undetectable before birth. It was first clearly expressed in the liver of newborn rats. On the contrary, Mrp2 mRNA was seen before birth, although at low levels. In concert with mRNA expression, Bsep protein was undetectable before birth, while Mrp2 protein was already expressed. Both proteins were clearly detectable in the postnatal period. Confocal immunofluorescent microscopy showed asynchronous appearance of Bsep and Mrp2 proteins during development but their colocalization in the bile canaliculi once each one is expressed. During the gestational period, a weak immunofluorescence for Mrp2 was observed only in livers of 16-day-old embryos. No fluorescence for Bsep was seen. Both proteins were clearly visualizable after birth, although the pattern of immunostaining varied. These findings provide molecular evidence that expression of both Bsep and Mrp2 during development is transcriptionally regulated. They also point out the differences in relevance to the liver function of the systems responsible for canalicular transport of bile salts versus organic anions.

Protein kinase C epsilon mediates the induction of P-glycoprotein in LNCaP prostate carcinoma cells

P-glycoprotein (P-gp) mediates drug resistance. Protein kinase C (PKC) expression correlates with drug resistance in several types of cancer. We determined whether PKC signals the induction of P-gp in LNCaP human prostate cancer cells, and identified a specific isozyme involved, in a model of aspirin-induced P-glycoprotein expression. An inhibitor of PKC activity, and a specific peptide inhibitor of PKC epsilon translocation, suppressed the induction of P-gp. The PKC activator ingenol, but not OAG, induced P-gp expression in a dose-dependent manner. Based on our results, we conclude that PKC epsilon mediates the induction of P-gp. Accordingly, PKC epsilon is activated and translocates from the membrane fraction to the cytoskeleton fraction in aspirin-treated cells. The findings of this study point to PKC epsilon as a signalling molecule for the induction of P-gp in LNCaP prostate cancer cells.

L-DOPA decarboxylase association with membranes in mouse brain

This work presents evidence on the association of active DDC molecules with membranes in mammalian brain. L-DOPA decarboxylase (DDC) is generally considered to be a cytosolic enzyme. Membrane-associated DDC was detected by immunoblotting and enzymatic assay experiments. DDC activity and immunoreactivity could be partially extracted from mammalian brain membranes by detergent. Fractionation of membranes by temperature-induced phase separation in Triton X-114, resulted in the recovery of membrane-associated DDC in separation phases where integral and hydrophobic membrane proteins separate. Treatment of membranes with phosphatidylinositol-specific phospholipase C or proteinase K, did not elute membrane-associated DDC activity, suggesting that a population of DDC molecules exist embedded within membranes. The elucidation of the functional significance of the enzyme's association with membranes could provide us with new information leading to the better understanding of the biological pathways that DDC is involved in.

Hydrolysis of platelet-activating factor by human serum paraoxonase

Human serum paraoxonase (human PON1) has been shown to be important in the metabolism of phospholipid and cholesteryl ester hydroperoxides, thereby preventing the oxidation of low-density lipoprotein (LDL) and retarding atherogenesis. However, the exact substrate specificity of PON1 has not been established. In the present study we show that purified PON1 hydrolyses platelet-activating factor (PAF). We could find no evidence for contamination of our preparation with authentic platelet-activating-factor acetylhydrolase (PAFAH) by immunoblotting with a PAFAH monoclonal antibody or by sequencing the purified protein. In addition the specific PAFAH inhibitor SB-222657 did not affect the ability of PON1 to hydrolyse PAF (30.1+/-2.8 micromol/min per mg of protein with no inhibitor; 31.4+/-2.2 micromol/min per mg of protein with 100 nM inhibitor) or phenyl acetate (242.6+/-30.8 versus 240.8+/-31.5 micromol/min per mg of protein with and without inhibitor respectively). SB-222657 was also unable to inhibit PAF hydrolysis by isolated human high-density lipoprotein (HDL), but completely abolished the activity of human LDL. Ostrich (Struthio camelus) HDL, which does not contain PON1, was unable to hydrolyse PAF. These data provide evidence that PON1 may limit the action of this bioactive pro-inflammatory phospholipid.

Human placental NAD(P)H oxidase: solubilization and properties

The NAD(P)H oxidase in the syncytiotrophoblast of the term human placenta, which had previously been identified only by cytochemical methods, has been solubilized and a number of its biochemical properties have been defined. It is a protein composed of a 58 kDa and a 33 kDa subunit, with properties that clearly distinguish it from the respiratory burst oxidase of transient neutrophils and macrophages. The enzyme activity is constitutive in the trophoblast and is a source of reactive oxygen species (ROS) at the fetal-maternal interface.

Quantitative determination of zwitterionic detergents using salt-induced phase separation of Triton X-100

Zwitterionic detergents interfere with the salt-induced phase separation for nonionic detergents in a concentration-dependent manner by shifting the normal cloud point of nonionic detergents to a higher ionic strength at room temperature. This phenomenon was used to determine the concentration of the zwitterionic detergents CHAPS, CHAPSO, and sulfobetaine SB-12 in solution by titration with ammonium sulfate in the presence of Triton X-100. Among the ionic detergents tested, the method was only applicable to sodium cholate. The assay can be used to control the removal of zwitterionic detergents during the reconstitution of membrane proteins in liposomes. However, it cannot be used to determine the specific binding of zwitterionic detergents to highly diluted, pure membrane proteins because of the limited sensitivity. Neither proteins nor phospholipids interfered with this method at concentrations up to 20 mg/ml of test solution (human serum albumin) or 10 mg/ml (phospholipids), respectively. Since the assay is based on the competition between salts and nonionic detergents for water molecules, it is important to equalize the ionic strength of samples and calibration standards.

Association of the cytoplasmic membrane protein XpsN with the outer membrane protein XpsD in the type II protein secretion apparatus of Xanthomonas campestris pv. campestris

An xps gene cluster composed of 11 open reading frames is required for the type II protein secretion in Xanthomonas campestris pv. campestris. Immediately upstream of the xpsD gene, which encodes an outer membrane protein that serves as the secretion channel by forming multimers, there exists an open reading frame (previously designated ORF2) that could encode a protein of 261 amino acid residues. Its N-terminal hydrophobic region is a likely membrane-anchoring sequence. Antibody raised against this protein could detect in the wild-type strain of X. campestris pv. campestris a protein band with an apparent molecular mass of 36 kDa by Western blotting. Its aberrant slow migration in sodium dodecyl sulfate-polyacrylamide gels might be due to its high proline content. We designated this protein XpsN. By constructing a mutant strain with an in-frame deletion of the chromosomal xpsN gene, we demonstrated that it is required for the secretion of extracellular enzyme by X. campestris pv. campestris. Subcellular fractionation studies indicated that the XpsN protein was tightly associated with the membrane. Sucrose gradient sedimentation followed by immunoblot analysis revealed that it primarily appeared in the cytoplasmic membrane fractions. Immune precipitation experiments indicated that the XpsN protein was coprecipitated with the XpsD protein. In addition, the XpsN protein was co-eluted with the (His)(6)-tagged XpsD protein from the metal affinity chromatography column. All observations suggested that the XpsN protein forms a stable complex with the XpsD protein. In addition, immune precipitation analysis of the XpsN protein with various truncated XpsD proteins revealed that the C-terminal region of the XpsD protein between residues 650 and 759 was likely to be involved in complex formation between the two.

Cloning and preliminary characterization of a calcium-binding protein closely related to nucleolin on the apical surface of inner medullary collecting duct cells

Calcium stone crystal attachment to the urinary epithelium plays an essential role in the development of kidney stones by allowing small crystals to be retained in the kidney until they become macroscopic. We among others have described attachment of stone crystals to cultured renal epithelia (Wiessner, J. H., Kleinman, J. G., Blumenthal, S. S., Garancis, J. C., and Mandel, G. S. (1987) J. Urol. 138, 640-643). To isolate protein(s) that may participate in crystal attachment, apical membranes of cultured renal inner medullary collecting duct were biotinylated, the cells were lysed with detergent, the lysate was subjected to hydroxyapatite chromatography, and fractions were incubated with calcium oxalate monohydrate. Electrophoresis of material solubilized from the crystals showed several selectively adsorbed protein bands. A 110-kDa band stained positively for biotin and for glycosides and bound (45)Ca. The amino acid sequence of this band was determined to be that of a protein closely related to rat nucleolin (nucleolin-related protein; NRP). NRP was cloned and sequenced and was 83% homologous with the previously sequenced nucleolar protein nucleolin. Using temperature-induced phase partitioning with Triton X-114, NRP was associated with both the insoluble membrane skeleton pellet and the soluble aqueous phase but not the soluble detergent phase. This association with the membrane skeleton was increased in the presence of calcium. Thus, NRP is associated with the apical membranes of cultured renal tubular cells and is bound to membrane skeletal elements in a calcium-dependent fashion. The physiological role of NRP remains to be determined; however, a pathophysiological role may be that of mediating the attachment to the renal tubular epithelium of calcium stone crystals.

Glucose consumption by rats decreases cytochrome P450 enzyme activity by altering hepatic lipids

Although glucose is a ubiquitous nutrient, increased consumption of glucose decreases the metabolism of numerous drugs in humans and animals. To understand the mechanisms involved that cause decreased drug metabolism in rats that consume glucose in their water, enzyme activity and expression as well as determining the contribution of the lipids toward decreasing in vitro metabolic activity were investigated. Enzyme assays of hepatic CYP1A2, 2C6, 2C11 and 3A2 showed significant decreases in activity from glucose-treated rats compared to control. While immunodetection of CYP1A1, 2B1/2, 2C11, and 3A1/2 showed no significant difference in protein expression. Hepatic fatty acid synthase activity increased in the glucose-treated rats compared to controls. Studies with glucose-treated microsomal lipids reconstituted with microsomal proteins from control rats caused a significant decrease in benzyloxyresorufin O-dealkylase activity. The results presented here support the hypothesis that the activities of cytochrome P450 proteins are altered by modulating their catalytic activity as a result of the lipid environment rather than changing the level of expression of the individual enzymes.

Detection of membrane-bound cytokinin-binding proteins in Arabidopsis thaliana cells

In order to isolate cytokinin-binding proteins (CBPs), we have developed new affinity probes constituted of a cytokinin such as zeatin riboside ([9R]Z) conjugated to a carrier protein. These probes were used for detecting CBPs in an ELISA procedure. The efficiency of the cytokinin conjugate in detecting CBPs was controlled with protein model: proteins having an affinity for cytokinin such as the monoclonal anti-[9R]Z antibodies did bind the cytokinin conjugate whereas proteins unable to bind cytokinin such as bovine serum albumin did not. Using these new affinity probes, we showed that CBPs are present in the membrane fraction of in vitro cultured Arabidopsis thaliana cells. The nature of the protein at the detected binding sites was demonstrated by submitting the microsomal proteins to a proteolytic treatment, which was found to eradicate the binding. Free biologically active cytokinins or monoclonal anti-[9R]Z antibodies inhibited the binding, thus showing the specificity of the interaction. The detected CBPs were partially solubilized from the membranes with potassium chloride, indicating their peripheral membrane location. The separation by anion exchange chromatography of solubilized microsomal proteins revealed the existence of two different CBPs. They were present at higher levels in cells during the exponential growth phase.

Zinc-induced inactivation of the yeast ZRT1 zinc transporter occurs through endocytosis and vacuolar degradation

The ZRT1 gene encodes the transporter responsible for high affinity zinc uptake in yeast. ZRT1 is transcribed in zinc-limited cells and its transcription is repressed in zinc-replete cells. In this report, we describe a second, post-translational mechanism that regulates ZRT1 activity. In zinc-limited cells, ZRT1 is a stable, N-glycosylated plasma membrane protein. Exposure to high levels of extracellular zinc triggers a rapid loss of ZRT1 uptake activity. Our results demonstrate that this inactivation occurs through zinc-induced endocytosis of the protein and its subsequent degradation in the vacuole. Mutations that inhibit the internalization step of endocytosis also inhibited zinc-induced ZRT1 inactivation and the major vacuolar proteases were required to degrade ZRT1 in response to zinc. Furthermore, immunofluorescence microscopy showed that ZRT1 is localized to the plasma membrane in zinc-limited cells and that the protein is transferred to the vacuole via an endosome-like compartment upon exposure to zinc. ZRT1 inactivation is a relatively specific response to zinc; cadmium and cobalt ions trigger the response but less effectively than zinc. Moreover, zinc does not alter the stability of several other plasma membrane proteins. Therefore, zinc-induced ZRT1 inactivation is a specific regulatory system to shut off zinc uptake activity in cells exposed to high extracellular zinc levels thereby preventing overaccumulation of this potentially toxic metal.

Characterization of spermidine binding to solubilized plasma membrane proteins from zucchini hypocotyls

In this work [14C]spermidine binding to total proteins solubilized from plasma membrane purified from zucchini (Cucurbita pepo L.) hypocotyls was investigated. Proteins were solubilized using octyl glucoside as a detergent. Specific polyamine binding was thermolabile, reversible, pH dependent with an optimum at pH 8.0, and had a Kd value of 5 &mgr;M, as determined by glass-fiber-filter assays. Sephadex G-25 M gel-filtration assays confirmed the presence of a spermidine-protein(s) complex with a specific binding activity. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and native polyacrylamide gel electrophoresis of collected fractions having the highest specific spermidine-binding activity, several protein bands (113, 75, 66, and 44 kD) were identified. The specificity of spermidine binding was examined by gel-filtration competition experiments performed using other polyamines and compounds structurally related to spermidine. Partial purification on Sephadex G-200 led to the identification of 66- and 44-kD protein bands, which may represent the putative spermidine-binding protein(s) on the plasmalemma.

Oligoglucoside elicitor-mediated activation of plant defense

Plants have acquired defense mechanisms to counteract potential pathogens. One such strategy involves inducible defense reactions that are activated by elicitors, signaling compounds of diverse nature. For one class of elicitors, oligoglucosides, recent developments in the characterization and isolation of an oligoclucan-binding protein, a putative elicitor receptor, and isolation of a cDNA that encodes the binding protein are discussed. Furthermore, the discovery of a role for calcium in the elicitation process is described. Finally, the identification of polymerase chain reaction products whose sequences indicate that they encode cytochrome P-450-dependent enzymes with possible roles in the formation of phytoalexins, antimicrobial plant defense compounds, is reported. These advances may lay the foundation for the first characterization of a receptor and subsequent signaling events in oligoglucan elicitor perception by higher plants.

Immunoreactivity for the GABA transporter-1 and GABA transporter-3 is restricted to astrocytes in the rat thalamus. A light and electron-microscopic immunolocalization

GABA plasma membrane transporters mediate GABA uptake into presynaptic terminals and surrounding glial processes and thus play a key role in shaping the time course and spatial extent of GABA's action. In the present study we have investigated the cellular and subcellular localization of two GABA transporters (1 and 3) in the rat thalamus using affinity-purified polyclonal antibodies. GABA transporter-1 and -3 immunoreactivity, detected with immunoperoxidase and immunofluorescence methods, is present throughout the thalamus in small punctate structures scattered in the neuropil among unlabelled neuronal perikarya. Labelling for GABA transporter-3 is always more intense than that for GABA transporter-1. Astrocytic processes, identified by their immunoreactivity for glial fibrillary acidic protein, express both GABA transporters. Ultrastructural investigations confirm that GABA transporter-1 and -3 labelling is restricted to astrocytes. Labelled astrocytes are adjacent to terminals making either symmetric or asymmetric synaptic contacts, and are close to neuronal profiles that do not form synaptic contacts in the plane of the section. In double-labelled thin sections some GABA transporter-1- or -3-positive astrocytic processes, detected with immunoperoxidase labelling, surround GABA-positive terminals, detected with antibodies to GABA and immunogold labelling. These findings demonstrate that in rat thalamus the GABA uptake system mediated by GABA transporter-1 and -3 is localized exclusively to astrocytes near the synapses and in the neuropil, and absent from GABAergic terminals. Astrocytes play therefore an important role in mediating GABA transmission in the thalamus, compared to cortical regions.

Biochemical characterization of a delta12 acyl-lipid desaturase after overexpression of the enzyme in Escherichia coli

The Delta12 acyl-lipid desaturase of Synechocystis sp. PCC 6803 was overexpressed in Escherichia coli as an active enzyme. The overexpressed protein was associated with cell membranes; it represented about 10% of the total cellular protein and 25% of the total membrane protein. The enzyme in the membrane fraction exhibited strong fatty-acid desaturase activity. The desaturase in salt-washed membranes was stabilized by the presence of sorbitol. Storage of salt-washed membranes in 2 M sorbitol at 4 degrees C and at pH 7-8 for six days resulted in the loss of less than 10% of the desaturase activity. The desaturase activity had a positive temperature coefficient, a result that suggests that the increase in the desaturation of fatty acids at low temperature might not be caused by the activation of desaturases at low temperature but, rather, by the increased synthesis of desaturases de novo.

Characterization of endogenous APP processing in a cell-free system

We have developed a simple in vitro assay using tissue homogenates that allows detection and characterization of several endogenous proteolytic activities which convert Alzheimer's amyloid precursor protein (APP) to the smaller, carboxy-terminal fragments, postulated to be intermediates in the formation of β-amyloid peptide (Aβ). Incubation at 37°C results in the degradation of transmembrane APP and formation of a mixture of carboxy-terminal containing peptides with mass values of 9-12 kDa. Epitope mapping and electrophoretic comparison with a truncated APP standard showed one of these peptides to contain the entire Aβ sequence. Analysis of pH dependence shows that formation of this carboxy-terminal product as well as another fragment, that is the likely product of 'secretase' activity, requires acidic pH. This suggests that cleavage of full-length APP to secreted forms may take place in an acidic intracellular compartment.

The functional purification of P-glycoprotein is dependent on maintenance of a lipid-protein interface

P-Glycoprotein (P-gp) is a 180-kDa membrane-bound transporter which can confer the multi-drug resistance phenotype on tumor cells. We have examined the factors required to preserve activity of P-gp during its purification. The starting material for purification was plasma membranes from Chinese hamster ovary (CHrB30) cells, overexpressing P-glycoprotein. These membranes displayed drug stimulated ATPase activity (Vm = 897 +/- 55 nmol min(-1) mg(-1); Km = 1.8 +/- 0.4 mM) and high affinity binding of [3H]vinblastine (Kd = 36 +/- 5 nM; Bm = 161 +/- 11 pmol/mg). Several non-ionic detergents which readily solubilized P-glycoprotein significantly inhibited ATPase activity and drug binding at concentrations well below their respective CMC values. This inactivation was prevented by excess crude lipid mixtures, with the greatest protection afforded against dodecyl-maltoside. Furthermore, the significantly reduced binding affinity and capacity of solubilized P-gp was partly reversed by the addition of lipids. A combination of anion-exchange and hydroxyapatite chromatography were used to purify P-gp with high yield to greater than 90%. The purified, reconstituted P-gp displayed high ATPase activity (Vm = 2137 +/- 309; Km = 2.9 +/- 0.9 mM) which was stimulated by verapamil (EC50 = 3.8 +/- 0.6 microM) and inhibited by orthovanadate (3.1 +/- 0.8 microM). Pure P-gp also displayed high affinity vinblastine binding (Kd = 64 +/- 9 nM) with a capacity of 2320 +/- 192 pmol/mg. This purification scheme yields the highest P-gp activity reported to date, and indicates a dependence of function on maintaining a lipid-protein interface.

Characterization of the FET4 protein of yeast. Evidence for a direct role in the transport of iron

The low affinity Fe2+ uptake system of Saccharomyces cerevisiae requires the FET4 gene. In this report, we present evidence that FET4 encodes the Fe2+ transporter protein of this system. Antibodies prepared against FET4 detected two distinct proteins with molecular masses of 63 and 68 kDa. In vitro synthesis of FET4 suggested that the 68-kDa form is the primary translation product, and the 63-kDa form may be generated by proteolytic cleavage of the full-length protein. Consistent with its role as an Fe2+ transporter, FET4 is an integral membrane protein present in the plasma membrane. The level of FET4 closely correlated with uptake activity over a broad range of expression levels and is itself regulated by iron. Furthermore, mutations in FET4 can alter the kinetic properties of the low affinity uptake system, suggesting a direct interaction between FET4 and its Fe2+ substrate. Mutations affecting potential Fe2+ ligands located in the predicted transmembrane domains of FET4 significantly altered the apparent Km and/or Vmax of the low affinity system. These mutations may identify residues involved in Fe2+ binding during transport.

Developmental expression pattern of Stra6, a retinoic acid-responsive gene encoding a new type of membrane protein

Retinoic acid plays important roles in development, growth and differentiation by regulating the expression of target genes. A new retinoic acid-inducible gene, Stra6, has been identified in P19 embryonal carcinoma cells using a subtractive hybridization cDNA cloning technique. Stra6 codes for a very hydrophobic membrane protein of a new type, which does not display similarities with previously characterized integral membrane proteins. Stra6, which exhibits a specific pattern of expression during development and in the adult, is strongly expressed at the level of blood-organ barriers. Interestingly, in testis Sertoli cells, Stra6 has a spermatogenic cycle-dependent expression which is lost in testes of RAR alpha null mutants where Stra6 is expressed in all tubules. We suggest that the Stra6 protein may be a component of an as yet unidentified transport machinery.

Adherence of Borrelia burgdorferi to granulocytes of different animal species

Adherence of 4 Borrelia (B.) burgdorferi strains (z7/22, z7/27, z7/41, PBi) to polymorphonuclear granulocytes from different domestic animals (horses, cattle, sheep, dogs) was investigated. All 4 strains adhered to the granulocytes. Binding assays indicated that the adherence occurred between structures on the surface of the borreliae ("binding-sites") and on the membranes of the granulocytes ("receptors"). The "receptors" consisted of 4 fractions (A, B, C, and D) with components differing in molecular weight (MW) and binding activity for proteins on the surface of B. burgdorferi. Fraction A (MW 80000) had the highest binding activity for B. burgdorferi.

Purification of the blood vessel ATP diphosphohydrolase, identification and localisation by immunological techniques

ATP diphosphohydrolase (ATPDase) or apyrase (EC 3.6.1.5), an enzyme that hydrolyses the gamma and beta phosphate residues of triphospho- and diphosphonucleosides, has been purified from the bovine aorta media. A particulate fraction was isolated by differential, and sucrose cushion centrifugations, producing a 33-fold enrichment in ADPase activity. Solubilization of the enzyme from the particulate fraction with Triton X-100 caused a partial loss of activity. The solubilized enzyme was purified by DEAE-agarose, Affi-Gel blue and Concanavalin A column chromatographies yielding an additional 138-fold enrichment of the enzyme. The enzyme preparation was further purified by PAGE under non-denaturing conditions, followed by its detection on the gel. The active band was cut out and separated by SDS/PAGE. Overstaining with silver nitrate revealed a single band corresponding to a molecular mass of 78000. Presence of an ATP binding site on the latter protein was demonstrated by labelling with 5'-p-fluorosulfonylbenzoyladenosine (FSBA), an analogue of ATP, followed by its detection by a Western blot technique. Labelling specificity was demonstrated by competition experiments with Ca-ATP and Ca-ADP. An antiserum directed against the N-terminal sequence of the pig pancreas ATPDase (54 kDa) cross-reacted with the bovine aorta ATPDase at 78 kDa. Digestion of the ATPDase with N-glycosidase F caused a marked shift of the molecular mass, thereby showing multiple N-oligosaccharide chains. Immunohistochemical localisation confirmed the presence of ATPDase on both endothelial and smooth muscle cells.

The early stage of chloroplast protein import involves Com70

The chloroplast envelope protein Com70 is a hsp70 homolog identified recently as a component of the protein translocation apparatus. The stage of protein import involving Com70 was determined by examining the nature of the association of Com70 with the envelope and its interaction with translocating proteins. Com70 is accessible to thermolysin, but its association with the envelope could not be disrupted by stringent washes. In light of the external membrane-bound location, the involvement of Com70 at the early stage of protein translocation was investigated using a combination of in vitro binding assays, chemical cross-linking, and coimmunoprecipitation. The results provide evidence that Com70 is in close physical proximity to different types of chloroplast protein precursors under conditions supporting binding rather than complete translocation. The formation of cross-linked complexes is dependent on the presence of a typical plastid transit signal and protease-accessible outer envelope components. The close proximity of Com70 and the translocating protein occurs while the protein is still exposed to the cytosol.

GAT-3, a high-affinity GABA plasma membrane transporter, is localized to astrocytic processes, and it is not confined to the vicinity of GABAergic synapses in the cerebral cortex

The termination of GABA synaptic action by high-affinity, Na(+)-dependent, neuronal, and glial plasma membrane transporters plays an important role in regulating neuronal activity in physiological and pathological conditions. We have investigated the cellular localization and distribution in the cerebral cortex of adult rats of one GABA transporter (GAT), GAT-3, by immunocytochemistry with affinity-purified polyclonal antibodies directed to its predicted C terminus that react monospecifically with a protein of approximately 70 kDa. Light microscopic studies revealed specific GAT-3 immunoreactivity (ir) in small punctate structures, and it was never observed in fibers or cell bodies. No changes in immunostaining were observed in sections incubated with GAT-3 antibodies preadsorbed with the related rat GAT-1 or mouse GAT-2/ BGT-1 C-terminal peptides, whereas in sections incubated with GAT-3 antibodies preadsorbed with rat GAT-3 C-terminal peptide, ir was not present. The highest number of GAT-3-positive puncta was in layer IV and in a narrow band corresponding to layer Vb, followed by layers II and III. Many GAT-3-positive puncta were in close association with pyramidal and nonpyramidal neuron cell bodies. Ultrastructural studies showed that GAT-3 ir was localized exclusively to astrocytic processes, which were found in the neuropil and adjacent to axon terminals having either symmetric or asymmetric specializations. In sections processed by both preembedding labeling for GAT-3 and postembedding immunogold labeling for GABA, only some of the GAT-3-positive astrocytic processes were found close to GABAergic profiles. These findings on the localization of GAT-3 in the cerebral cortex indicate that this transporter mediates GABA uptake into glial cells, and suggest that glial GABA uptake may function to limit the spread of GABA from the synapse, as well as to regulate overall GABA levels in the neuropil.

Chaperonin-promoted post-translational membrane insertion of a multispanning membrane protein lactose permease

Using an in vitro membrane-free translation system from Escherichia coli, it is shown that chaperonin GroEL added cotranslationally interacts with newly synthesized lactose permease (LacY), a polytopic membrane protein, thereby preventing aggregation. Subsequently, when the isolated GroEL-LacY complex is incubated with inverted membrane vesicles, the permease is inserted into the membrane in a MgATP-dependent manner. Post-translational membrane insertion is also observed when aggregation of newly synthesized LacY is prevented by addition of the nonionic detergent n-dodecyl-beta,D-maltoside during translation in place of GroEL. No membrane integration occurs with right-side-out vesicles, indicating that LacY interacts specifically only with the cytosolic face of the membrane. Ligand thiodigalactoside protection against alkylation of the Cys-148 residue in the permease shows proper post-translational insertion. Moreover, limited proteolysis of soluble LacY either complexed with GroEL or in detergent indicates that the newly synthesized protein assumes a conformation that is comparable to that of native, membrane-embedded permease prior to insertion into the membrane.

Purification and stabilization of transcriptionally active glucocorticoid receptor

A major obstacle to the purification of glucocorticoid receptor (GR) is the very high nonspecific surface adsorption of this protein. This phenomenon is a property of the GR itself and does not reflect overall protein concentration or buffer conditions. We have observed that the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS) is unique in its ability to stabilize the receptor and largely eliminate loss to nonspecific adsorption. We have coupled this observation with a two-step purification method that allows efficient purification and stabilization of transcriptionally active glucocorticoid receptor. For this procedure, the GR first undergoes a major purification by anion exchange chromatography following hormone binding and on-column receptor transformation. Second, the GR is resolved to homogeneity utilizing a hydrophobic interaction chromatography step which consists of a 2.5 M to 0 M NaCl gradient elution of contaminating proteins followed by displacement of GR by CHAPS. GR at both stages of purification was able to activate transcription from the glucocorticoid response element containing the promoter region of the long terminal repeat of the mouse mammary tumor virus. This simple and efficient methodology should be of a considerable advantage for studies of the biology of the active, full-length GR.

Downregulation of mdr-1 expression by 8-Cl-cAMP in multidrug resistant MCF-7 human breast cancer cells

8-Cl-cAMP, a site-selective analogue of cAMP, decreased mdr-1 expression in multidrug-resistant human breast cancer cells. A sixfold reduction of mdr-1 mRNA expression by 8-Cl-cAMP began within 8 h of treatment and was associated with a decrease in the synthesis of P-glycoprotein and with an increase in vinblastine accumulation. A reduction in mdr-1 expression after 8-Cl-cAMP treatment was also observed in multidrug-resistant human ovarian cancer cell lines. 8-Cl-cAMP is known to change the ratio between the two regulatory subunits, RI and RII, of protein kinase A (PKA). We observed that RI alpha decreased within 24 h of 8-Cl-cAMP treatment, that RII beta increased after as few as 3 h of treatment, and that PKA catalytic activity remained unchanged during 48 h of 8-Cl-cAMP treatment. The results are consistent with the hypothesis that mdr-1 expression is regulated in part by changes in PKA isoenzyme levels. Although 8-Cl-cAMP has been used to differentiate cells in other model systems, the only differentiating effect that could be detected after 8-Cl-cAMP treatment in the MCF-7TH cells was an increase in cytokeratin expression. Evidence that the reduction of mdr-1 mRNA occurred at the level of gene transcription was obtained by measuring chloramphenicol acetyltransferase (CAT) mRNA in MCF-7TH cells transfected with an mdr-1 promoter-CAT construct prior to 8-Cl-cAMP treatment. Thus, 8-Cl-cAMP is able to downregulate mdr-1 expression and suggests a new approach to reversal of drug resistance in human breast cancer.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].