Association of L-arginine transporters with fodrin: implications for hypoxic inhibition of arginine uptake

In this study, we investigated the possible interaction between the cationic amino acid transporter (CAT)-1 arginine transporter and ankyrin or fodrin. Because ankyrin and fodrin are substrates for calpain and because hypoxia increases calpain expression and activity in pulmonary artery endothelial cells (PAEC), we also studied the effect of hypoxia on ankyrin, fodrin, and CAT-1 contents in PAEC. Exposure to long-term hypoxia (24 h) inhibited L-arginine uptake by PAEC, and this inhibition was prevented by calpain inhibitor 1. The effects of hypoxia and calpain inhibitor 1 were not associated with changes in CAT-1 transporter content in PAEC plasma membranes. However, hypoxia stimulated the hydrolysis of ankyrin and fodrin in PAEC, and this could be prevented by calpain inhibitor 1. Incubation of solubilized plasma membrane proteins with anti-fodrin antibodies resulted in a 70% depletion of CAT-1 immunoreactivity and in a 60% decrease in L-arginine transport activity in reconstituted proteoliposomes (3,291 +/- 117 vs. 8,101 +/- 481 pmol. mg protein(-1). 3 min(-1) in control). Incubation with anti-ankyrin antibodies had no effect on CAT-1 content or L-arginine transport in reconstituted proteoliposomes. These results demonstrate that CAT-1 arginine transporters in PAEC are associated with fodrin, but not with ankyrin, and that long-term hypoxia decreases L-arginine transport by a calpain-mediated mechanism that may involve fodrin proteolysis.     

A novel protein refolding protocol for the solubilization and purification of recombinant peptidoglycan-associated lipoprotein from Xylella fastidiosa overexpressed in Escherichia coli

Xylella fastidiosa is a Gram-negative xylem-limited plant pathogenic bacterium responsible for several economically important crop diseases. Here, we present a novel and efficient protein refolding protocol for the solubilization and purification of recombinant X. fastidiosa peptidoglycan-associated lipoprotein (XfPal). Pal is an outer membrane protein that plays important roles in maintaining the integrity of the cell envelope and in bacterial pathogenicity. Because Pal has a highly hydrophobic N-terminal domain, the heterologous expression studies necessary for structural and functional protein characterization are laborious once the recombinant protein is present in inclusion bodies. Our protocol based on the denaturation of the XfPal-enriched inclusion bodies with 8M urea followed by buffer-exchange steps via dialysis proved effective for the solubilization and subsequent purification of XfPal, allowing us to obtain a large amount of relatively pure and folded protein. In addition, XfPal was biochemically and functionally characterized. The method for purification reported herein is valuable for further research on the three-dimensional structure and function of Pal and other outer membrane proteins and can contribute to a better understanding of the role of these proteins in bacterial pathogenicity, especially with regard to the plant pathogen X. fastidiosa.     

The microvillus 110K cytoskeletal protein is an integral membrane protein

A protein of 110,000 MW connects actin filaments to the plasma membrane in microvilli of intestinal epithelial cells. In the present study four independent lines of evidence suggest that the 110K protein is directly bound to the lipid bilayer. The solubilization of the 110K protein requires detergents and removal of detergent after solubilization results in aggregation. The 110K protein partitions into the detergent phase in Triton X-114 solutions. It is selectively incorporated into liposomes. It is specifically labeled with the hydrophobic probe ¹⁴C-phenylisothiocyanate. In addition we present a purification scheme for the 110K protein in milligram amounts. This represents the simplest system of membrane to filament attachment, in which an integral membrane protein is also a cytoskeletal protein.     

The primary structure of bovine brain myelin lipophilin (proteolipid apoprotein)

The amino-acid sequence of bovine myelin lipophilin (proteolipid apoprotein, Folch-protein) has been completed. Lipophilin is a 276 amino acid residues containing, extremely hydrophobic membrane protein with molecular mass 30,000 Da. The sequence determination was based on automated Edman degradation of four tryptophan and four cyanogen bromide fragments and of proteolytic peptides of complete lipophilin as well as the fragments obtained by chemical cleavage. Four additional sequences were determined which led to the completion of the primary structure. Lipophilin is esterified at threonine-198 by long chain fatty acids (palmitic, stearic and oleic acid). The attachment site has been established at the same threonine residue in three different peptides isolated from thermolysinolytic, papainolytic and chymotrypsinolytic hydrolysates. This threonine residue is part of a hydrophilic segment of lipophilin. The covalent fatty acyl bond is being discussed together with important structural and functional properties of this membrane protein which can be derived from sequence information. New separation and purification methods of hydrophobic and hydrophilic polypeptides for this sequence determination (fractional solubilization, silica gel exclusion, high-performance liquid chromatography) had to be elaborated as indispensable tools. They are generally applicable to the structural analysis of hydrophobic membrane proteins. Four long (26, 29, 40 and 36 residues) and one medium long (12 residues) hydrophobic segments are separated by four predominantly positively and one negatively charged hydrophilic segments. On the basis of structural data a model for the membrane integration of lipophilin is proposed.     

Site-specific mutations in the COOH-terminus of placental alkaline phosphatase: a single amino acid change converts a phosphatidylinositol- glycan-anchored protein to a secreted protein

Placental alkaline phosphatase (PLAP) is anchored in the plasma membrane by a phosphatidylinositol-glycan moiety (PI-glycan). PI-glycan is added posttranslationally to the nascent peptide chain after the removal of 29 amino acids from the COOH-terminus. The contribution of selected COOH-terminal amino acids to the signal for PI-glycan addition was tested by creating a fusion protein with the COOH-terminus of PLAP and a secreted protein and by mutagenesis of specific PLAP COOH-terminal amino acids. The cDNA encoding the COOH-terminus of PLAP was fused in frame to the cDNA for human clotting Factor X and expressed in transfected COS-1 cells. Fusion proteins containing 32 amino acids of the PLAP COOH-terminus were modified by PI-glycan addition. Thus, the signal for PI-glycan modification must reside in these amino acids. Next, the region between the hydrophobic domain and the cleavage site was examined for additional determinants. Mutations of the hydrophilic residues in the spacer region demonstrated that these amino acids do not contribute to the signal for PI-glycan addition. Deletion of amino acids in the spacer region prevented the addition of PI-glycan suggesting that the length of the spacer domain or the amino acids around the cleavage site are important determinants. Finally, we demonstrated that interruption of the hydrophobic domain by a charged residue prevents PI-glycan addition and results in a protein that is secreted into the medium. The finding that a single Leu to Arg substitution in the hydrophobic domain converts a PI-glycan anchored, membrane protein to a secreted protein suggests that an essential signal for the correct sorting of PI-glycan anchored proteins versus secreted proteins resides in the hydrophobic domain. Substitution of a charged amino acid for a hydrophobic amino acid may be a mechanism for producing membrane bound and secreted forms of the same protein.     

Isolation of an integral membrane glycoprotein by chloroform-methanol extraction and C3-reversed-phase high-performance liquid chromatography

Methodology is presented for the isolation of integral membrane proteins and applied to the purification of the major myelin glycoprotein, P0. This isolation scheme depends on the detergent solubilization of an isoosmotically extracted membrane fraction from sciatic nerve endoneurium, followed by the removal of lipids and detergent by chloroform/methanol extraction. The resulting membrane proteins are readily dissolved in acetic acid/water (1/1) and directly analyzed by reversed-phase high-performance liquid chromatography. The hydrophobic nature of the intrinsic membrane protein mixture results in strong binding to a C8 stationary phase, leading to poor resolution and yields. These problems can be eliminated by employing a C3 alkylsilane column, thereby allowing separation of the protein components and the isolation of P0. The purified P0 has an amino-terminal sequence that matches that predicted from nucleotide sequencing, and the glycoprotein contains the expected amount of sialic acid. This latter finding indicates that the isolation procedure is not detrimental to the complex-type oligosaccharide structure of P0 and should make the methodology readily applicable to the purification of other integral membrane proteins and glycoproteins.     

The effects of arginine on protein binding and elution in hydrophobic interaction and ion-exchange chromatography

Arginine is effective in suppressing aggregation of proteins and may be beneficial to be included during purification processes. We have shown that arginine reduces non-specific protein binding in gel permeation chromatography and facilitates elution of antibodies from Protein-A columns. Here we have examined the effects of arginine on binding and elution of the proteins during hydrophobic interaction (HIC) and ion- exchange chromatographies (IEC) using recombinant monoclonal antibodies (mAbs) and human interleukin-6. In the case of HIC, the proteins were bound to a phenyl-Sepharose column in the presence of ammonium sulfate (AS) with or without arginine and eluted with a descending concentration of AS. While use of 1 M AS in the loading buffer resulted in complete binding of the mAb, inclusion of 1 M arginine in loading and equilibration buffer, only when using low-substituted phenyl-Sepharose, resulted in weaker binding of the proteins. While decreasing AS concentration to 0.75 M resulted in partial elution of the mAB, elution was facilitated with inclusion of 0.5-1 M arginine. In the case of IEC, arginine was included in the loading samples. Inclusion of arginine during binding to the IEC columns resulted in a greater recovery and less aggregation even when elution was done in the absence of arginine. These results indicate that arginine enhances elution of proteins bound to the resin, suggesting its effectiveness as a solvent for elution in HIC and IEC.     

Endogenous ADP-ribosylation in skeletal muscle membranes

The characteristics of ADP-ribosyltransferase activity in skeletal muscle membranes have been studied. The membrane enzymes can ADP-ribosylate exogenous substrates such as guanylhydrazones, polyarginine, lysozyme, and histones. The properties of the enzyme are investigated by using diethylaminobenzylidineaminoguanidine as a model substrate. Incubation of the membranes with [32P]adenylate-labeled NAD results in the labeling of a number of cellular proteins. Magnesium ions, detergents, and diethylaminobenzylidineaminoguanidine stimulated the ADP-ribosylation of membrane proteins, whereas L-arginine methyl ester and arginine inhibited ADP-ribosylation. The labeling of specific proteins in the sarcoplasmic reticulum and glycogen pellet is influenced significantly by detergents, nucleotides, and thiols. The hydroxylamine sensitivity of the ADP-ribose linkage in the membrane proteins is similar to that reported for (ADP-ribose)-arginine linkage. Snake venom phosphodiesterase digestion of the ADP-ribosylated membranes produces 5'-AMP as the major acid-soluble digestion product. The results suggest that the primary mode of modification is mono(ADP-ribosyl)ation. The ADP-ribosyltransferase activity in the membrane preparations is not extracted under conditions used for solubilization of extrinsic proteins, suggesting that the activity is associated with some integral membrane protein.     

Variation of the Detergent-Binding Capacity and Phospholipid Content of Membrane Proteins When Purified in Different Detergents

Purified membrane proteins are ternary complexes consisting of protein, lipid, and detergent. Information about the amounts of detergent and endogenous phospholipid molecules bound to purified membrane proteins is largely lacking. In this systematic study, three model membrane proteins of different oligomeric states were purified in nine different detergents at commonly used concentrations and characterized biochemically and biophysically. Detergent-binding capacities and phospholipid contents of the model proteins were determined and compared. The insights on ternary complexes obtained from the experimental results, when put into a general context, are summarized as follows. 1), The amount of detergent and 2) the amount of endogenous phospholipids bound to purified membrane proteins are dependent on the size of the hydrophobic lipid-accessible protein surface areas and the physicochemical properties of the detergents used. 3), The size of the detergent and lipid belt surrounding the hydrophobic lipid-accessible surface of purified membrane proteins can be tuned by the appropriate choice of detergent. 4), The detergents n-nonyl-β-D-glucopyranoside and Cymal-5 have exceptional delipidating effects on ternary complexes. 5), The types of endogenous phospholipids bound to membrane proteins can vary depending on the detergent used for solubilization and purification. 6), Furthermore, we demonstrate that size-exclusion chromatography can be a suitable method for estimating the molecular mass of ternary complexes. The findings presented suggest a strategy to control and tune the numbers of detergent and endogenous phospholipid molecules bound to membrane proteins. These two parameters are potentially important for the successul crystallization of membrane proteins for structure determination by crystallographic approaches.     

A systematic assessment of mature MBP in membrane protein production: overexpression, membrane targeting and purification

Obtaining enough membrane protein in native or native-like status is still a challenge in membrane protein structure biology. Maltose binding protein (MBP) has been widely used as a fusion partner in improving membrane protein production. In the present work, a systematic assessment on the application of mature MBP (mMBP) for membrane protein overexpression and purification was performed on 42 membrane proteins, most of which showed no or poor expression level in membrane fraction fused with an N-terminal Histag. It was found that most of the small membrane proteins were overexpressed in the native membrane of Escherichia coli when using mMBP. In addition, the proteolysis of the fusions were performed on the membrane without solubilization with detergents, leading to the development of an efficient protocol to directly purify the target membrane proteins from the membrane fraction through a one-step affinity chromatography. Our results indicated that mMBP is an excellent fusion partner for overexpression, membrane targeting and purification of small membrane proteins. The present expression and purification method may be a good solution for the large scale preparation of small membrane proteins in structural and functional studies.     

Is arginine a protein-denaturant?

Arginine is a useful solvent additive for many applications, including refolding and solubilization of proteins from insoluble pellets, and suppression of protein aggregation and non-specific adsorption during formulation and purification. However, there is a concern that arginine may be a protein-denaturant, which may limit the expansion of its applications. Such concern arises from the facts that arginine decreases melting temperature and perturbs the spectroscopic properties of certain proteins and contains a guanidinium group, which is a critical chemical structure for denaturing activity of guanidine hydrochloride. Here, we show that although arginine does lower the melting temperatures of certain proteins, the extent is insufficient to cause denaturation of proteins at or below room temperature. The proteins described here show enzymatic activity and folded structure in the presence of arginine, although the local structure around aromatic amino acids is perturbed by arginine. Arginine differs from guandinine hydrochloride in the mode of interactions with proteins, which may be a primary reason why arginine is not a protein-denaturant.     

Towards crystallization of hydrophobic myelin glycoproteins: P0 and PASII/PMP22

The preparation of a pure and homogeneous protein sample at proper concentration is a prerequisite for success when attempting their crystallization for structural determination. The detergents suitable for solubilization particularly of membrane proteins are not always the best for crystallization. Myelin of the peripheral nervous system of vertebrates is the example of a membrane for which neutral or "gentle" detergents are not even strong enough to solubilize its proteins. In contrast, sodium- or lithium-dodecyl sulfate is very effective. We solubilized myelin membrane in 2%(w/v) sodium dodecyl sulfate, followed by chromatographic purification of the hydrophobic myelin glycoproteins P0 and PASII/PMP22, and finally, we have exchanged the sodium dodecyl sulfate bound to protein for other neutral detergents using ceramic hydroxyapatite column. Theoretically, we should easily exchange sodium dodecyl sulfate for any neutral detergent, but for some of them, the solubility of myelin glycoproteins is low. To monitor the potential variability in the secondary structure of glycoproteins, we have used circular dichroism. Sodium dodecyl sulfate seems to be the appropriate detergent for the purpose of purification of very hydrophobic glycoproteins, since it can be easily exchanged for another neutral detergent.     

Effects of arginine on heat-induced aggregation of concentrated protein solutions

Arginine is one of the commonly used additives to enhance refolding yield of proteins, to suppress aggregation of proteins, and to increase solubility of proteins, and yet the molecular interactions that contribute to the role of arginine are unclear. Here, we present experiments, using bovine serum albumin (BSA), lysozyme (LYZ), and β-lactoglobulin (BLG) as model proteins, to show that arginine can enhance heat-induced aggregation of concentrated protein solutions, contrary to the conventional belief that arginine is a universal suppressor of aggregation. Results show that the enhancement in aggregation is caused only for BSA and BLG, but not for LYZ, indicating that arginine's preferential interactions with certain residues over others could determine the effect of the additive on aggregation. We use this previously unrecognized behavior of arginine, in combination with density functional theory calculations, to identify the molecular-level interactions of arginine with various residues that determine arginine's role as an enhancer or suppressor of aggregation of proteins. The experimental and computational results suggest that the guanidinium group of arginine promotes aggregation through the hydrogen-bond-based bridging interactions with the acidic residues of a protein, whereas the binding of the guanidinium group to aromatic residues (aggregation-prone) contributes to the stability and solubilization of the proteins. The approach, we describe here, can be used to select suitable additives to stabilize a protein solution at high concentrations based on an analysis of the amino acid content of the protein.     

Surfactant-free purification of membrane proteins with intact native membrane environment

In order to study the structure and function of a protein, it is generally required that the protein in question is purified away from all others. For soluble proteins, this process is greatly aided by the lack of any restriction on the free and independent diffusion of individual protein particles in three dimensions. This is not the case for membrane proteins, as the membrane itself forms a continuum that joins the proteins within the membrane with one another. It is therefore essential that the membrane is disrupted in order to allow separation and hence purification of membrane proteins. In the present review, we examine recent advances in the methods employed to separate membrane proteins before purification. These approaches move away from solubilization methods based on the use of small surfactants, which have been shown to suffer from significant practical problems. Instead, the present review focuses on methods that stem from the field of nanotechnology and use a range of reagents that fragment the membrane into nanometre-scale particles containing the protein complete with the local membrane environment. In particular, we examine a method employing the amphipathic polymer poly(styrene-co-maleic acid), which is able to reversibly encapsulate the membrane protein in a 10?nm disc-like structure ideally suited to purification and further biochemical study.     

Purification of membrane proteins in SDS and subsequent renaturation

A prerequisite for the purification of any protein to homogeneity is that the protein is not non-specifically associated with other proteins especially during the final stage(s) of the fractionation procedure. This requirement is not so often fulfilled when nonionic detergents (for instance Triton X-100) are used for solubilization of membrane proteins. The reason is that these detergents are not efficient enough to prevent the protein of interest from forming aggregates with other proteins upon contact with chromatographic or electrophoretic supporting media, which, due to their polymeric nature, have a tendency to induce aggregation of other polymers, for instance, hydrophobic proteins. The aggregation can be avoided if sodium dodecyl sulfate (SDS) is employed as detergent. We therefore suggest that membrane proteins should be purified by conventional methods in the presence of SDS and that the purified proteins, which are in a denatured state, are allowed to renature. There is good change to renature internal membrane proteins since they should not be so susceptible to denaturation by detergents as are water-soluble proteins because the natural milieu of the former proteins is lipids which in fact are detergents. In this paper we present a renaturation method based on the removal of SDS by addition of a large excess of G 3707, a nonionic detergent. By this technique we have renatured a 5'-nucleotidase from Acholeplasma laidlawii and a neuraminidase from influenza virus. The enzyme activities were higher (up to 6-fold) after the removal of SDS than prior to the addition of SDS.     

Molecular Model for the Solubilization of Membranes into Nanodisks by Styrene Maleic Acid Copolymers

A recent discovery in membrane research is the ability of styrene-maleic acid (SMA) copolymers to solubilize membranes in the form of nanodisks allowing extraction and purification of membrane proteins from their native environment in a single detergent-free step. This has important implications for membrane research because it allows isolation as well as characterization of proteins and lipids in a near-native environment. Here, we aimed to unravel the molecular mode of action of SMA copolymers by performing systematic studies using model membranes of varying compositions and employing complementary biophysical approaches. We found that the SMA copolymer is a highly efficient membrane-solubilizing agent and that lipid bilayer properties such as fluidity, thickness, lateral pressure profile, and charge density all play distinct roles in the kinetics of solubilization. More specifically, relatively thin membranes, decreased lateral chain pressure, low charge density at the membrane surface, and increased salt concentration promote the speed and yield of vesicle solubilization. Experiments using a native membrane lipid extract showed that the SMA copolymer does not discriminate between different lipids and thus retains the native lipid composition in the solubilized particles. A model is proposed for the mode of action of SMA copolymers in which membrane solubilization is mainly driven by the hydrophobic effect and is further favored by physical properties of the polymer such as its relatively small cross-sectional area and rigid pendant groups. These results may be helpful for development of novel applications for this new type of solubilizing agent, and for optimization of the SMA technology for solubilization of the wide variety of cell membranes found in nature.     

2D NMR and structural model for a mitochondrial signal peptide bound to a micelle

The 19 amino acid signal peptide of rat liver aldehyde dehydrogenase, possessing a lysine substitution for an arginine and containing 3 extra amino acid residues at the C terminus, was studied by two-dimensional NMR in a dodecylphosphocholine micelle. In this membrane-like environment, the peptide contains two alpha-helical regions, both of which are amphiphilic, separated by a hinge region. The helix located closer to the C terminus is more stable than is the helix located near the N terminus. This suggests that the hydrophobic face of the C-terminal helix is buried within the hydrophobic region of the micelle. On the basis of these results a general model for protein translocation is presented in which the C-terminal amphiphilic helix of the signal region in the preprotein first binds to the mitochondrial membrane and then diffuses to the translocation receptor. The receptor then recognizes the N-terminal helix of the signal region, which is not anchored to the membrane. To explain how this signal peptide was imported into isolated mitochondria in the absence of energy or receptor protein [Pak, Y. K., & Weiner, H. (1990) J. Biol. Chem. 265, 14298-14307], a model for signal peptide translocation across a membrane barrier without the need for auxiliary membrane proteins is proposed. In this model the faces of the two helices fold upon each other, resulting in the mutual shielding of positively charged residues by the complementary hydrophilic face of the other amphiphilic helix.     

Truncated forms of Escherichia coli lactose permease: models for study of biosynthesis and membrane insertion.

Using in vitro DNA manipulations, we constructed different lacY alleles encoding mutant proteins of the Escherichia coli lactose carrier. With respect to structural models developed for lactose permease, the truncated polypeptides represent model systems containing approximately one, two, four, and five of the N-terminal membrane-spanning alpha-helices. In addition, a protein carrying a deletion of predicted helices 3 and 4 was obtained. The different proteins were radiolabeled in plasmid-bearing E. coli minicells and were found to be stably integrated into the lipid bilayer. The truncated polypeptides of 50, 71, 143, and 174 N-terminal amino acid residues resembled the wild-type protein in their solubilization characteristics, whereas the mutant protein carrying an internal deletion of amino acid residues 72 to 142 of the lactose carrier behaved differently. Minicell membrane vesicles containing truncated proteins comprising amino acid residues 1 to 143 or 1 to 174 were subjected to limited proteolysis. Upon digestion with proteases of different specificities, the same characteristic fragment that was also produced from the membrane-associated wild-type protein was found to accumulate under these conditions. It has previously been shown to contain the intact N terminus of lactose permease. This supports the idea of an independent folding and membrane insertion of this segment even in the absence of the C-terminal part of the molecule. The results suggest that the N-terminal region of the lactose permease represents a well-defined structural domain.     

Mutations in a 19-amino-acid hydrophobic region of the yeast cytochrome c1 presequence prevent sorting to the mitochondrial intermembrane space.

Most mitochondrial proteins destined for the intermembrane space (IMS) carry in their presequence information for localization to the IMS in addition to information for their import. By selecting for mutants in the yeast Saccharomyces cerevisiae that mislocalize an IMS-targeted fusion protein, we identified mutations in the IMS sorting signal of the cytochrome c1 protein. Amino acid substitutions or deletions in a stretch of 19 hydrophobic amino acids of the cytochrome c1 presequence resulted in accumulation of the intermediate form of the cytochrome c1 protein in the matrix. In some cases, the accumulated intermediate appeared to be slowly exported from the matrix, across the inner membrane to the IMS. Our results support the hypothesis that the cytochrome c1 precursor is normally imported completely into the matrix and then exported to the IMS.