Purification of integral membrane proteins

Preparation of the sample for chromatofocusing: buffer exchange and desalting

Chromatofocusing separates proteins on the basis of differences in their isoelectric point (pI). Successful chromatofocusing depends on careful preparation of the protein sample, as described in this protocol. The protein must be transferred to the same buffer that will be used to equilibrate the chromatofocusing column. The buffer pH should be higher than the pI of the protein to keep the proteins negatively charged, which facilitates their binding to the column. A protein sample containing particulate matter will decrease the resolution of the separation and shorten the column lifetime. Therefore, it is necessary to remove all insoluble and immiscible material from the sample prior to loading it on the column. The sample is first clarified by filtration or centrifugation and is then transferred into the start buffer using a high-capacity, fast-flowing, size-exclusion column packed with Sephadex G-25.

High-level production, solubilization and purification of synthetic human GPCR chemokine receptors CCR5, CCR3, CXCR4 and CX3CR1

Chemokine receptors belong to a class of integral membrane G-protein coupled receptors (GPCRs) and are responsible for transmitting signals from the extracellular environment. However, the structural changes in the receptor, connecting ligand binding to G-protein activation, remain elusive for most GPCRs due to the difficulty to produce them for structural and functional studies. We here report high-level production in E.coli of 4 human GPCRs, namely chemokine receptors (hCRs) CCR5, CCR3, CXCR4 and CX3CR1 that are directly involved in HIV-1 infection, asthma and cancer metastasis. The synthetic genes of CCR5, CCR3, CXCR4 and CX3CR1 were synthesized using a two-step assembly/amplification PCR method and inserted into two different kinds of expression systems. After systematic screening of growth conditions and host strains, TB medium was selected for expression of pEXP-hCRs. The low copy number pBAD-DEST49 plasmid, with a moderately strong promoter tightly regulated by L-arabinose, proved helpful for reducing toxicity of expressed membrane proteins. The synthetic Trx-hCR fusion genes in the pBAD-DEST49 vector were expressed at high levels in the Top10 strain. After a systematic screen of 96 detergents, the zwitterionic detergents of the Fos-choline series (FC9-FC16) emerged as the most effective for isolation of the hCRs. The FC14 was selected both for solubilization from bacterial lysates and for stabilization of the Trx-hCRs during purification. Thus, the FC-14 solubilized Trx-hCRs could be purified using size exclusion chromatography as monomers and dimers with the correct apparent MW and their alpha-helical content determined by circular dichroism. The identity of two of the expressed hCRs (CCR3 and CCR5) was confirmed using immunoblots using specific monoclonal antibodies. After optimization of expression systems and detergent-mediated purification procedures, we achieved large-scale, high-level production of 4 human GPCR chemokine receptor in a two-step purification, yielding milligram quantities of CCR5, CCR3, CXCR4 and CX3CR1 for biochemical, biophysical and structural analysis.

Purification and characterization of a 45-kDa concealed antigen from the midgut membranes of Ornithodoros erraticus that induces lethal anti-tick immune responses in pigs

Ornithodoros erraticus is an argasid tick that can transmit severe diseases such as human relapsing fever and African swine fever. In the search for a vaccine against this parasite, a crude extract of tick midgut membranes (GME) was obtained that in pigs and mice induced a protective response able to kill up to 80% of the nymphs in the first 72 h post-feeding and to reduce the fecundity of females by more than 50%. To identify the protective antigens, the GME was subjected to successive biochemical fractionations and the resulting simpler protein fractions were inoculated in pigs. A 45-kDa antigen, the so-called Oe45, was detected, purified and demonstrated to be responsible for the protection induced by the GME. Oe45 seems to be a membrane protein that is presumably expressed on the luminal membrane of midgut epithelial cells. Oe45 consists of at least two differently charged bands (cationic and neutral), which show antigenic cross-reactivity. The possibility that these bands might be different isoforms of the same protein is discussed. Although Oe45 is constitutively expressed at low levels throughout the trophogonic cycle, its expression is up-regulated by the ingestion of blood, as suggested by the higher levels observed between 6 and 72 h post-feeding.

Effect of adaptation to ethanol on cytoplasmic and membrane protein profiles of Oenococcus oeni

The practical application of commercial malolactic starter cultures of Oenococcus oeni surviving direct inoculation in wine requires insight into mechanisms of ethanol toxicity and of acquired ethanol tolerance in this organism. Therefore, the site-specific location of proteins involved in ethanol adaptation, including cytoplasmic, membrane-associated, and integral membrane proteins, was investigated. Ethanol triggers alterations in protein patterns of O. oeni cells stressed with 12% ethanol for 1 h and those of cells grown in the presence of 8% ethanol. Levels of inosine-5'-monophosphate dehydrogenase and phosphogluconate dehydrogenase, which generate reduced nicotinamide nucleotides, were decreased during growth in the presence of ethanol, while glutathione reductase, which consumes NADPH, was induced, suggesting that maintenance of the redox balance plays an important role in ethanol adaptation. Phosphoenolpyruvate:mannose phosphotransferase system (PTS) components of mannose PTS, including the phosphocarrier protein HPr and EII(Man), were lacking in ethanol-adapted cells, providing strong evidence that mannose PTS is absent in ethanol-adapted cells, and this represents a metabolic advantage to O. oeni cells during malolactic fermentation. In cells grown in the presence of ethanol, a large increase in the number of membrane-associated proteins was observed. Interestingly, two of these proteins, dTDT-glucose-4,6-dehydratase and D-alanine:D-alanine ligase, are known to be involved in cell wall biosynthesis. Using a proteomic approach, we provide evidence for an active ethanol adaptation response of O. oeni at the cytoplasmic and membrane protein levels.

Solubilization of lymphocytes

Purification and study of transmembrane proteins require isolation of these structures from their lipid environment. This isolation is carried out through the use of detergents. In this unit, several approaches to solubilization of membrane proteins are presented. Solubilization of whole lymphocytes, using conditions aimed at minimizing the disruption of protein-protein interactions, is described with an optional step that may be useful when the disruption of protein interactions is desired as part of a purification protocol. In some situations, it may be desirable to purify membranes prior to their solubilization or to determine the physical relationship between proteins, which can be accomplished by a cross-linking.

Two-dimensional maps in soft immobilized pH gradient gels: a new approach to the proteome of the Third Millennium

Same major improvements in proteome analysis of cytosolic and membrane proteins by two-dimensional mapping are here reported. A much improved transfer of proteins from the first to the second dimensional sodium dodecyl sulfate (SDS)-gel is obtained by simply diluting the gel matrix, normally composed of 4%T polyacrylamide in all commercially available Immobiline strips down to as low as 3%T. In the analysis of total lysates of platelets, this augmented transfer has been evaluated as being 2-3 times higher than in standard 4%T gels. A second major improvement, in the case of analysis of membrane protein preparations, has been demonstrated to consist in a delipidation step in a tertiary solvent mixture composed of tri-n-butyl phosphate:acetone:methanol in a 1:12:1 ratio. By adopting this protocol, large amounts of spectrins (240-220 kDa, filamentous proteins of the red blood cell membranes) could be transferred vs. essentially none when delipidation was omitted. The present report also confirms the importance of a reduction and alkylation step of the protein sample prior to all electrophoretic steps, including focusing in the Immobiline gel, as recently reported by Herbert et al.

Ligand-binding characterization of xanthophyll carotenoids to solubilized membrane proteins derived from human retina

The macula of the human retina contains extraordinarily high concentrations of lutein and zeaxanthin, xanthophyll carotenoids that appear to play an important role in protecting against age-related macular degeneration, the leading cause of blindness among the elderly. It is likely that the uptake and stabilization of these carotenoids is mediated by specific xanthophyll-binding proteins. In order to purify and characterize such a binding protein, a carotenoid-rich membrane fraction derived from human macula or peripheral retina was prepared by homogenization, differential centrifugation, and detergent solubilization. Further purification was carried out using ion-exchange chromatography and gel-filtration chromatography coupled with continuous photodiode-array monitoring for endogenously associated xanthophyll carotenoids. The most highly purified preparations contained two major protein bands at 25 and 55 kDa that consistently co-eluted with endogenous lutein and zeaxanthin. The visible absorbance spectrum of the binding protein preparation closely matches the spectral absorbance of the human macular pigment, and it is bathochromically shifted about 10 nm from the spectrum of lutein and zeaxanthin dissolved in organic solvents. Binding of exogenously added lutein and zeaxanthin is saturable and specific with an apparent Kd of approximately 1 microM. Canthaxanthin and beta-carotene exhibit no significant binding activity to solubilized retinal membrane proteins when assayed under identical conditions. Other potential mammalian xanthophyll-binding proteins such as albumin, tubulin, lactoglobulin and serum lipoproteins possess only weak non-specific binding affinity for carotenoids when assayed under the same stringent binding conditions. This investigation provides the first direct evidence for the existence of specific xanthophyll-binding protein(s) in the vertebrate retina and macula. The possible roles of xanthophyll-binding proteins in normal macular function and in the pathogenesis of age-related macular degeneration remain to be elucidated.

Hydrophobic patches on the surfaces of protein structures

A survey of hydrophobic patches on the surface of 112 soluble, monomeric proteins is presented. The largest patch on each individual protein averages around 400 A2 but can range from 200 to 1,200 A2. These areas are not correlated to the sizes of the proteins and only weakly to their apolar surface fraction. Ala, Lys, and Pro have dominating contributions to the apolar surface for smaller patches, while those of the hydrophobic amino acids become more important as the patch size increases. The hydrophilic amino acids expose an approximately constant fraction of their apolar area independent of patch size; the hydrophobic residue types reach similar exposure only in the larger patches. Though the mobility of residues on the surface is generally higher, it decreases for hydrophilic residues with increasing patch size. Several characteristics of hydrophobic patches catalogued here should prove useful in the design and engineering of proteins.

beta-Carotene-15,15'-dioxygenase (EC 1.13.11.21) isolation reaction mechanism and an improved assay procedure

beta-Carotene-15,15'-dioxygenase (EC 1.13.11.21; beta-carotene dioxygenase) activity in extracts from guinea-pig intestinal mucosa was assayed by supplying [15,15'-14C2]- or [15,15'-3H2] beta-carotene dissolved in Tween 80. Methods were developed to minimize the breakdown of labelled beta-carotene and beta-carotene cleavage products during the isolation procedure. Antioxidants and unlabelled carriers were added to extracting solvents and C18 Sep-Pak cartridges were used to isolate the remaining beta-carotene and retinaldehyde, which was the only cleavage product detected. The labelled material produced by the enzyme was analysed by either normal-phase TLC or reversed-phase HPLC and characterized chemically as retinaldehyde. The lack of other labelled apo-carotenals isolated in these experiments and the formation of between 1.5 and 2 mol retinaldehyde/mol beta-carotene consumed confirm the central cleavage mechanism for the enzyme's action. More beta-carotene dioxygenase activity was obtained from guinea-pig mucosa than from chicken or pig intestinal mucosa. The beta-carotene dioxygenase was obtained as a soluble enzyme which was partially purified by gel filtration and ion-exchange chromatography to a specific activity of 0.6 nmol retinaldehyde formed/mg protein per h. The formation of a lipid-protein aggregate containing the beta-carotene dioxygenase activity, which has been reported to be present in the exclusion volume of Sephadex columns, was avoided if the mucosal scrapings were homogenized in buffer at a proportion of 1:4 (w/v).

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Bacterial solute transport proteins in their lipid environment

The cytoplasmic membrane of bacteria is a selective barrier that restricts entry and exit of solutes. Transport of solutes across this membrane is catalyzed by specific membrane proteins. Integral membrane proteins usually require specific lipids for optimal activity and are inhibited by other lipid species. Their activities are also sensitive to the lipid bilayer dynamics and physico-chemical state. Bacteria can adapt to changes in the environments (respective temperature, hydrostatic pressure, and pH) by altering the lipid composition of the membrane. Homeoviscous adaptation results in the maintenance of the liquid-crystalline phase through alterations in the degree of acyl chain saturation and branching, acyl chain length and the sterol content of the membrane. Homeophasic adaptation prevents the formation of non-bilayer phases, which would disrupt membrane organization and increase permeability. A balance is maintained between the lamellar phase, preferring lipids, and those that adopt a non-bilayer organization. As a result, the membrane proteins are optimally active under physiological conditions. The molecular basis of lipid-protein interactions is still obscure. Annular lipids stabilize integral membrane proteins. Stabilization occurs through electrostatic and possibly other interactions between the lipid headgroups and the charged amino acid residues close to the phospholipid-water interface, and hydrophobic interactions between the fatty acyl chains and the membrane-spanning segments. Reconstitution techniques allow manipulation of the lipid composition of the membrane in a way that is difficult to achieve in vivo. The physical characteristics of membrane lipids that affect protein-mediated transport functions have been studied in liposomal systems that separate an inner and outer compartment. The activity of most transport proteins is modulated by the bulk physical characteristics of the lipid bilayer, while specific lipid requirements appear rare.

NADH-ascorbate free radical and -ferricyanide reductase activities represent different levels of plasma membrane electron transport

Plasma membranes isolated from rat liver by two-phase partition exhibited dehydrogenase activities for ascorbate free radical (AFR) and ferricyanide reduction in a ratio of specific activities of 1:40. NADH-AFR reductase could not be solubilized by detergents from plasma membrane fractions. NADH-AFR reductase was inhibited in both clathrin-depleted membrane and membranes incubated with anti-clathrin antiserum. This activity was reconstituted in plasma membranes in proportion to the amount of clathrin-enriched supernatant added. NADH ferricyanide reductase was unaffected by both clathrin-depletion and antibody incubation and was fully solubilized by detergents. Also, wheat germ agglutinin only inhibited NADH-AFR reductase. The findings suggest that NADH-AFR reductase and NADH-ferricyanide reductase activities of plasma membrane represent different levels of the electron transport chain. The inability of the NADH-AFR reductase to survive detergent solubilization might indicate the involvement of more than one protein in the electron transport from NADH to the AFR but not to ferricyanide.

L-pipecolic acid oxidation in rat: subcellular localization and developmental study

By using a sensitive radioactive assay method, we present here evidence that L-pipecolic acid oxidase is localized in both mitochondria and peroxisomes of rat liver. Brain white matter contained a more than 2-fold higher activity of L-pipecolic acid oxidation than the brain cortex. Suborganellar fractionation studies indicate that while the enzyme is a matrix protein in mitochondria, it is membrane-associated in peroxisomes. Both rotenone and antimycin A completely inhibited the enzyme activity in mitochondria but not in peroxisomes. The enzyme was shown to be inducible in mitochondria and peroxisomes of rat liver and brain tissues by glucagon and di-(2-ethylhexyl)phthalate, respectively. We report here for the first time the developmental aspects of L-pipecolic acid oxidation activity in rat liver and brain tissues. L-Pipecolic acid oxidase activity was detectable in whole rat embryo at 10 days of gestation, suggesting active L-pipecolic acid metabolism early during development. In both liver and brain tissues L-pipecolic acid oxidation activity was highest at 15 days of gestation and decreased with age in prenatal and postnatal conditions.

Hydrophobic interaction chromatography of fibroblast proteoglycans

We have investigated the hydrophobic properties of human skin fibroblast proteoglycans and related material by affinity chromatography on Octyl-Sepharose CL-4B in 4 M guanidinium hydrochloride (GdnHCl). Proteoglycans and related material could be separated into non-, medium and highly hydrophobic forms by elution with gradients of Triton X-100 in 4 M Gdn HCl. The non-hydrophobic material included endogenously produced glycosaminoglycan chains and oligosaccharides as well as an HS-proteoglycan with a 35 kDa core. The 65-70 kDa core (glypican-related) proteoglycans appeared among the highly hydrophobic ones, but variable proportions were seen both in the medium and the non-hydrophobic material. Other membrane-bound proteoglycans, like fibroglycan (45 kDa core) and the HS-proteoglycans with 90 and 130 kDa cores, as well as the CS/DS-proteoglycan with a 90 kDa core, were all of high hydrophobicity. There were also indications of a highly hydrophobic CS/DS-proteoglycan with a 45 kDa core. The extracellular proteoglycans, PG-L, PG-S1 and PG-S2, and the HS-proteoglycans with 350 and 250 kDa cores were all of medium hydrophobicity. These proteoglycans emerged in distinct positions when the column was eluted with a gradient of 3-[(3-cholamidopropyl)dimethylammonio]propanesulphonate.

Purification and characterization of a novel sialidase found in procyclic culture forms of Trypanosoma brucei

A membrane-bound sialidase (EC 3.2.1.18) was found in procyclic trypomastigotes of Trypanosoma brucei. The mammalian stage bloodstream form, however, displayed no sialidase activity. This sialidase is an integral surface protein, linked to the membrane via a glycosylphosphatidylinositol anchor. After osmotic lysis and solubilization with Triton CF-54, the enzyme was purified 1900-fold by gel filtration and ion exchange chromatography. Its size, as determined by conventional and high-performance liquid gel chromatography, is 67 kDa. The sialidase is active over a broad pH and temperature range with optima at pH 6.9 and 35 degrees C, respectively. No loss of activity is observed after 4 freeze-thaw cycles. T. brucei sialidase activity is inhibited by N-(4-nitrophenyl)oxamic acid and 2-deoxy-2,3-didehydro-N-acetylneuraminic acid, the latter, however, being less effective. N-Acetylneuraminic acid shows no inhibitory effect, whereas a variety of metal ions are potent inhibitors. The sialidase is activated by di- and tricarboxylic acids, but inhibited by chloride. Relative hydrolysis rates of various sialic acid-containing compounds reveal that de-O-acetylated bovine submandibular gland mucin is the preferred substrate and that alpha(2-3)-linkages are hydrolyzed faster than alpha(2-6)-linkages.

Differential distribution of a glucuronyltransferase, involved in glucuronoxylan synthesis, within the Golgi apparatus of pea (Pisum sativum var. Alaska)

The subcellular location of a glucuronyltransferase (GT) involved in glucuronoxylan synthesis in pea (Pisum sativum) has been investigated. Most of the GT activity was found in the Golgi fraction, but activity was also detected in the plasma-membrane fraction. Separation of Golgi membranes on a shallow continuous sucrose density gradient resulted in three distinct subfractions, with GT activity being confined to Golgi membranes of a density similar to that of smooth endoplasmic reticulum. The differential distribution of GT within the Golgi stack indicates that glucuronoxylan synthesis occurs in specific cisternae and that there is functional compartmentalization of the Golgi with respect to hemicellulose biosynthesis.

Protein purification

This monograph summarizes recent developments in the purification and analysis of natural and recombinant proteins. The basic strategies employed in protein purification are reviewed with regards to the characteristics of the protein of interest that may aid its isolation, choice of the starting material, and use of denaturants. Preparation of cell-free extracts followed by bulk precipitation and/or phase partition constitute the initial steps of many purification schemes. Chromatographic methods (size exclusion, ion exchange, hydroxylapatite, reversed phase, hydrophobic interaction and affinity based) utilizing either traditional, low pressure or high-performance liquid chromatography instrumentation are discussed. Electrophoretic techniques used to analyze the homogeneity of the protein product include SDS-PAGE, isotachophoresis, IEF and two dimensional gel electrophoresis.

Effect of detergents on the structure of integral membrane proteins of Sendai virus studied with size-exclusion high-performance liquid chromatography and monoclonal antibodies

The integral membrane proteins of Sendai virus, the fusion protein F (Mr = 65,000) and the haemagglutinin-neuraminidase protein HN (Mr = 68,000), were used as a model protein mixture. They were subjected to size-exclusion high-performance liquid chromatography on Superose 6HR columns with eluents containing various additives in order to solubilize the proteins. The effect of the additives on the structure of the membrane proteins was investigated with conformation-dependent monoclonal antibodies, either directed against F or HN protein, and by determination of the haemagglutinating capacity of the HN protein. The results show that the structure of the HN protein is more easily disturbed by eluents than that of the F protein. When the elution conditions are mild, e.g., 0.1% octylglucoside, the structure of both proteins is conserved but no separation is obtained. Elution with a buffer containing 0.05% sarkosyl (dodecyl methylglycine sodium salt) did not affect the structure and resulted in pure F protein. Pretreatment of the Amberlite XAD-2-treated Sendai virus envelope extract with 4% sodium dodecyl sulphate (SDS) and elution with 0.1% SDS in 50 mM sodium phosphate (pH 6.5) altered the structure of the HN protein but resulted in purification of the tetramer and the dimer of the HN protein, and the monomer of the F protein.

Characterization of ZO-1, a protein component of the tight junction from mouse liver and Madin-Darby canine kidney cells

ZO-1, originally identified by mAb techniques, is the first protein shown to be specifically associated with the tight junction. Here we describe and compare the physical characteristics of ZO-1 from mouse liver and the Madin-Darby canine kidney (MDCK) epithelial cell line. The ZO-1 polypeptide has an apparent size of 225 kD in mouse tissues and 210 kD in canine-derived MDCK cells as determined by SDS-PAGE/immunoblot analysis. ZO-1 from both sources is optimally solubilized from isolated plasma membranes by either 6 M urea or high pH conditions; partial solubilization occurs with 0.3 M KCl. The nonionic detergents, Triton X-100 and octyl-beta-D-glucopyranoside, do not solubilize ZO-1. These solubility properties indicate that ZO-1 is a peripherally associated membrane protein. ZO-1 was purified to electrophoretic homogeneity from [35S]methionine metabolically labeled MDCK cells by a combination of gel filtration and immunoaffinity chromatography. Purified ZO-1 has an s20,w of 5.3 and Stokes radius of 8.6 nm. These values suggest that purified ZO-1 is an asymmetric monomeric molecule. Corresponding values for mouse liver ZO-1, characterized in impure protein extracts, were 6 s20,w and 9 nm. ZO-1 was shown to be a phosphoprotein in MDCK cells metabolically labeled with [32P]orthophosphate; analysis of phosphoamino acids from purified ZO-1 revealed only phosphoserine. ZO-1 epitope number was determined by Scatchard analysis of competitive and saturable binding of two different 125I-mAbs to SDS-solubilized proteins from liver and MDCK cells immobilized on nitrocellulose. Saturation binding occurs at 26 ng mAb/mg liver and 63 ng/mg of MDCK cell protein. This is equivalent to 30,000 ZO-1 molecules per MDCK cell assuming a single epitope/ZO-1 molecule.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Column liquid chromatography of integral membrane proteins

Biological membranes have as a major function the compartmentation of biological processes in cells and organelles. They consist of a bilayer of phospholipid molecules in which proteins are embedded. These integral membrane proteins, which cross the bilayer once or several times, generally have a higher than average hydrophobicity and tend to aggregate. Detergents are needed to remove integral membrane proteins from the lipid bilayer and they have to be present during further chromatographic purification. Predominantly, four modes of HPLC have been used alone or in combination for the purification of integral membrane proteins. These are based on differences of proteins in size (size-exclusion chromatography, SEC), electrostatic interaction (ion-exchange chromatography, IEC), bioaffinity (bioaffinity chromatography, BAC) and hydrophobic interaction (reversed-phase chromatography, RPC, and hydrophobic-interaction chromatography, HIC). SEC, IEC, BAC and HIC are used under relatively mild conditions, and buffer systems generally contain a non-ionic detergent. RPC generally has a denaturing effect on the protein and should preferably be used for the purification of integral membrane proteins smaller than 50 kD.

Rapid microprocedure for isolating detergent-insoluble outer membrane proteins from Haemophilus species

A rapid microprocedure for isolating detergent (sodium N-lauroyl sarcosinate)-insoluble major outer membrane proteins from Haemophilus species produced results qualitatively identical to those obtained with a commonly used preparative isolation procedure. Proteins isolated by both procedures were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after staining with Coomassie brilliant blue R-250. The time for outer membrane protein isolation was substantially reduced with the rapid procedure, allowing a larger number of membrane preparations to be obtained rapidly for routine analysis.

Involvement of plasma membrane glycoproteins in the contact-dependent inhibition of growth of human fibroblasts

The human embryonal lung fibroblasts used in this study showed a pronounced inhibition of growth when reaching a critical cell density. This effect has been mimicked by the addition of glutaraldehyde-fixed human fibroblasts to sparsely seeded growing cells. Inhibition of growth was not observed when glutaraldehyde-fixed cells were pretreated with galactosidase or with galactose-specific lectins, or when glutaraldehyde-fixed human or rabbit erythrocytes were added to the proliferating fibroblasts. In addition, glutaraldehyde-fixed mitotic cells were without effect on the proliferation, while cells prepared from sparse culture had lesser potency than cells prepared from confluent cultures. Plasma membranes, isolated from cells of confluent cultures, when added to growing cultures of human fibroblasts inhibited DNA synthesis in a concentration-dependent manner. On the other hand, plasma membranes isolated from sparsely seeded cells had only minor inhibitory potency. When the plasma membranes were isolated from cells treated previously with tunicamycin, an antibiotic which inhibits the synthesis of the oligosaccharide portion of asparagine-linked glycoproteins, the inhibitory effect was abolished. The same effect was observed when plasma membranes were pretreated with galactosidase. These data indicate that the growth of cells in vitro is regulated by specific cell-cell contacts. They also show that one of the molecular reactants in this process are membrane glycoproteins with asparagine-linked oligosaccharides.