A monomer is the minimum functional unit required for channel and ATPase activity of the cystic fibrosis transmembrane conductance regulator

Tandem mass tag-based quantitative proteomic analysis of the liver reveals potential protein targets of Xiaochaihutang in CUMS model of depression

Depression is a global mental disorder disease and greatly threatened human health. Xiaochaihutang (XCHT) has been used successfully in treatment of depression for many years in China, but the mechanism is unclear. Using the chronic unpredictable mild stress (CUMS) mice model of depression, the present study aimed to reveal possible antidepressant mechanisms of XCHT from the perspective of liver by analyzing hepatic proteomics in mice. Bioinformatics analysis identified 31 differentially expressed proteins (DEPs), including 5 upregulated and 26 downregulated proteins, between the CUMS model and XCHT groups. The bile secretion pathway was found by KEGG pathway analysis of these DEPs. Four of the 31 differentially expressed proteins, including 2 active proteins involved in bile secretion, carbonic anhydrase 2 (CA2) and cystic fibrosis transmembrane conductance regulator (CFTR), were selected to verify their genes. Four genes (Cyp7a1, Fxr, Shp and Ntcp) related to bile acid synthesis and transport were further investigated by quantitative real-time polymerase chain reaction (qRT-PCR). Both biochemical tests and gene studies demonstrated that CUMS affected bile acid synthesis and transport, while XCHT regulated this pathway. The results indicated that there may be a potential relationship between CUMS induced depression and hepatic injury caused by increased bile acid, and also provide a novel insight to understand the underlying anti-depression mechanisms of XCHT.

Genome Sequence of the Freshwater Yangtze Finless Porpoise

The Yangtze finless porpoise (Neophocaena asiaeorientalis ssp. asiaeorientalis) is a subspecies of the narrow-ridged finless porpoise (N. asiaeorientalis). In total, 714.28 gigabases (Gb) of raw reads were generated by whole-genome sequencing of the Yangtze finless porpoise, using an Illumina HiSeq 2000 platform. After filtering the low-quality and duplicated reads, we assembled a draft genome of 2.22 Gb, with contig N50 and scaffold N50 values of 46.69 kilobases (kb) and 1.71 megabases (Mb), respectively. We identified 887.63 Mb of repetitive sequences and predicted 18,479 protein-coding genes in the assembled genome. The phylogenetic tree showed a relationship between the Yangtze finless porpoise and the Yangtze River dolphin, which diverged approximately 20.84 million years ago. In comparisons with the genomes of 10 other mammals, we detected 44 species-specific gene families, 164 expanded gene families, and 313 positively selected genes in the Yangtze finless porpoise genome. The assembled genome sequence and underlying sequence data are available at the National Center for Biotechnology Information under BioProject accession number PRJNA433603.

Involvement of the heterodimeric interface region of the nucleotide binding domain-2 (NBD2) in the CFTR quaternary structure and membrane stability

The cystic fibrosis transmembrane conductance regulator (CFTR) is the only member of the ATP-binding cassette (ABC) superfamily that functions as a chloride channel. The predicted structure of CFTR protein contains two membrane-spanning domains (MSDs), each followed by a nucleotide binding domain (NBD1 and NBD2). The opening of the Cl- channel is directly linked to ATP-driven tight dimerization of CFTR's NBD1 and NBD2 domains. The presence of a heterodimeric interfaces (HI) region in NBD1 and NBD2 generated a head to tail orientation necessary for channel activity. This process was also suggested to promote important conformational changes in the associated transmembrane domains of CFTR, which may impact the CFTR plasma membrane stability. To better understand the role of the individual HI region in this process, we generated recombinant CFTR protein with suppressed HI-NBD1 and HI-NBD2. Our results indicate that HI-NBD2 deletion leads to the loss of the dimerization profile of CFTR that affect its plasma membrane stability. We conclude that, in addition to its role in Cl- transport, HI-NBD2 domain confers membrane stability of CFTR by consolidating its quaternary structure through interactions with HI-NBD1 region.

CFTR structure and cystic fibrosis

CFTR (cystic fibrosis transmembrane conductance regulator) is a member of the ATP-binding cassette family of membrane proteins. Although almost all members of this family are transporters, CFTR functions as a channel with specificity for anions, in particular chloride and bicarbonate. In this review we look at what is known about CFTR structure and function within the context of the ATP-binding cassette family. We also review current strategies aimed at obtaining the high resolution structure of the protein.

Cystic fibrosis transmembrane conductance regulator (CFTR) potentiator VX-770 (ivacaftor) opens the defective channel gate of mutant CFTR in a phosphorylation-dependent but ATP-independent manner

The cystic fibrosis transmembrane conductance regulator (CFTR) acts as a channel on the apical membrane of epithelia. Disease-causing mutations in the cystic fibrosis gene can lead to CFTR protein misfolding as in the case of the F508del mutation and/or channel dysfunction. Recently, a small molecule, VX-770 (ivacaftor), has shown efficacy in restoring lung function in patients bearing the G551D mutation, and this has been linked to repair of its channel gating defect. However, these studies did not reveal the mechanism of action of VX-770 in detail. Normally, CFTR channel activity is regulated by phosphorylation, ATP binding, and hydrolysis. Hence, it has been hypothesized that VX-770 modifies one or more of these metabolic events. In this study, we examined VX-770 activity using a reconstitution system for purified CFTR protein, a system that enables control of known regulatory factors. We studied the consequences of VX-770 interaction with CFTR incorporated in planar lipid bilayers and in proteoliposomes, using a novel flux-based assay. We found that purified and phosphorylated CFTR was potentiated in the presence of Mg-ATP, suggesting that VX-770 bound directly to the CFTR protein, rather than associated kinases or phosphatases. Interestingly, we also found that VX-770 enhanced the channel activity of purified and mutant CFTR in the nominal absence of Mg-ATP. These findings suggest that VX-770 can cause CFTR channel opening through a nonconventional ATP-independent mechanism. This work sets the stage for future studies of the structural properties that mediate CFTR gating using VX-770 as a probe.

Expression and purification of the cystic fibrosis transmembrane conductance regulator protein in Saccharomyces cerevisiae

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel, that when mutated, can give rise to cystic fibrosis in humans.There is therefore considerable interest in this protein, but efforts to study its structure and activity have been hampered by the difficulty of expressing and purifying sufficient amounts of the protein^1-3. Like many 'difficult' eukaryotic membrane proteins, expression in a fast-growing organism is desirable, but challenging, and in the yeast S. cerevisiae, so far low amounts were obtained and rapid degradation of the recombinant protein was observed ^4-9. Proteins involved in the processing of recombinant CFTR in yeast have been described^6-9 .In this report we describe a methodology for expression of CFTR in yeast and its purification in significant amounts. The protocol describes how the earlier proteolysis problems can be overcome and how expression levels of CFTR can be greatly improved by modifying the cell growth conditions and by controlling the induction conditions, in particular the time period prior to cell harvesting. The reagants associated with this protocol (murine CFTR-expressing yeast cells or yeast plasmids) will be distributed via the US Cystic Fibrosis Foundation, which has sponsored the research. An article describing the design and synthesis of the CFTR construct employed in this report will be published separately (Urbatsch, I.; Thibodeau, P. et al., unpublished). In this article we will explain our method beginning with the transformation of the yeast cells with the CFTR construct - containing yeast plasmid (Fig. 1). The construct has a green fluorescent protein (GFP) sequence fused to CFTR at its C-terminus and follows the system developed by Drew et al. (2008)¹⁰. The GFP allows the expression and purification of CFTR to be followed relatively easily. The JoVE visualized protocol finishes after the preparation of microsomes from the yeast cells, although we include some suggestions for purification of the protein from the microsomes. Readers may wish to add their own modifications to the microsome purification procedure, dependent on the final experiments to be carried out with the protein and the local equipment available to them. The yeast-expressed CFTR protein can be partially purified using metal ion affinity chromatography, using an intrinsic polyhistidine purification tag. Subsequent size-exclusion chromatography yields a protein that appears to be >90% pure, as judged by SDS-PAGE and Coomassie-staining of the gel.

Optimized purification of a heterodimeric ABC transporter in a highly stable form amenable to 2-D crystallization

Optimized protocols for achieving high-yield expression, purification and reconstitution of membrane proteins are required to study their structure and function. We previously reported high-level expression in Escherichia coli of active BmrC and BmrD proteins from Bacillus subtilis, previously named YheI and YheH. These proteins are half-transporters which belong to the ABC (ATP-Binding Cassette) superfamily and associate in vivo to form a functional transporter able to efflux drugs. In this report, high-yield purification and functional reconstitution were achieved for the heterodimer BmrC/BmrD. In contrast to other detergents more efficient for solubilizing the transporter, dodecyl-ß-D-maltoside (DDM) maintained it in a drug-sensitive and vanadate-sensitive ATPase-competent state after purification by affinity chromatography. High amounts of pure proteins were obtained which were shown either by analytical ultracentrifugation or gel filtration to form a monodisperse heterodimer in solution, which was notably stable for more than one month at 4°C. Functional reconstitution using different lipid compositions induced an 8-fold increase of the ATPase activity (k_cat∼5 s⁻¹). We further validated that the quality of the purified BmrC/BmrD heterodimer is suitable for structural analyses, as its reconstitution at high protein densities led to the formation of 2-D crystals. Electron microscopy of negatively stained crystals allowed the calculation of a projection map at 20 Å resolution revealing that BmrC/BmrD might assemble into oligomers in a lipidic environment.

Domain location within the cystic fibrosis transmembrane conductance regulator protein investigated by electron microscopy and gold labelling

The domain organisation of the cystic fibrosis transmembrane conductance regulator (CFTR) protein was studied using electron microscopy of detergent-solubilised dimeric complexes. Ni-NTA nanogold labelling data suggest that in the nonphosphorylated, nucleotide-free state, the C-terminus is intimately associated with the cytoplasmic ATP-binding regions, whilst part of the regulatory domain occupies a position close to the cytoplasmic surface of the lipid membrane. Removal of the entire second nucleotide binding domain (NBD2) results in a deficit in the CFTR structure that is consistent with the size and shape of a single NBD. The data suggest that NBD2 lies closer to the C2 symmetry axis than the first nucleotide binding domain (NBD1) and that NBD2 from one CFTR monomer also contacts NBD1 from the opposing one. These data suggest that current homology models for CFTR based on other ATP-binding cassette proteins appear to be reasonable, at least to low resolution. We also find that Ni-NTA nanogold labelling of an internal hexa-Histidine sequence is a valuable approach to locate individual protein domains.

Architecture of the cystic fibrosis transmembrane conductance regulator protein and structural changes associated with phosphorylation and nucleotide binding

We describe biochemical and structural studies of the isolated cystic fibrosis transmembrane conductance regulator (CFTR) protein. Using electron cryomicroscopy, low resolution three-dimensional structures have been obtained for the non-phosphorylated protein in the absence of nucleotide and for the phosphorylated protein with ATP. In the latter state, the cytosolic nucleotide-binding domains move closer together, forming a more compact packing arrangement. Associated with this is a reorganization within the cylindrical transmembrane domains, consistent with a shift from an inward-facing to outward-facing configuration. A region of density in the non-phosphorylated protein that extends from the bottom of the cytosolic regions up to the transmembrane domains is hypothesised to represent the unique regulatory region of CFTR. These data offer insights into the architecture of this ATP-binding cassette protein, and shed light on the global motions associated with nucleotide binding and priming of the chloride channel via phosphorylation of the regulatory region.

Chemical crosslinking studies with the mouse Kcc1 K-Cl cotransporter

Oligomerization, function, and regulation of unmodified mouse Kcc1 K-Cl cotransporter were studied by chemical crosslinking. Treatment of Xenopus oocytes and 293T cells expressing K-Cl cotransporter Kcc1 with several types of chemical cross-linkers shifted Kcc1 polypeptide to higher molecular weight forms. More extensive studies were performed with the amine-reactive disuccinyl suberate (DSS) and with the sulfhydryl-reactive bis-maleimidohexane (BMH). Kcc1 cross-linking was time-dependent in intact oocytes, and was independent of protein concentration in detergent lysates from oocytes or 293T cells. Kcc1 cross-linking by the cleavable cross-linker DTME was reversible. The N-terminal and C-terminal cytoplasmic tails of Kcc1 were not essential for Kcc1 crosslinking. PFO-PAGE and gel filtration revealed oligomeric states of uncrosslinked KCC1 corresponding in mobility to that of cross-linked protein. DSS and BMH each inhibited KCC1-mediated (86)Rb(+) uptake stimulated by hypotonicity or by N-ethylmaleimide (NEM) without reduction in nominal surface abundance of KCC1. These data add to evidence supporting the oligomeric state of KCC polypeptides.

Direct interaction of a small-molecule modulator with G551D-CFTR, a cystic fibrosis-causing mutation associated with severe disease

CF (cystic fibrosis) is caused by mutations in CFTR (CF transmembrane conductance regulator), which cause its mistrafficking and/or dysfunction as a regulated chloride channel on the apical surface of epithelia. CFTR is a member of the ABC (ATP-binding-cassette) superfamily of membrane proteins and a disease-causing missense mutation within the ABC signature sequence; G551D-CFTR exhibits defective phosphorylation and ATP-dependent channel gating. Studies of the purified and reconstituted G551D-CFTR protein revealed that faulty gating is associated with defective ATP binding and ATPase activity, reflecting the key role of G551 in these functions. Recently, high-throughput screens of chemical libraries led to identification of modulators that enhance channel activity of G551D-CFTR. However, the molecular target(s) for these modulators and their mechanism of action remain unclear. In the present study, we evaluated the mechanism of action of one small-molecule modulator, VRT-532, identified as a specific modulator of CF-causing mutants. First, we confirmed that VRT-532 causes a significant increase in channel activity of G551D-CFTR using a novel assay of CFTR function in inside-out membrane vesicles. Biochemical studies of purified and reconstituted G551D-CFTR revealed that potentiation of the ATPase activity of VRT-532 is mediated by enhancing the affinity of the mutant for ATP. Interestingly, VRT-532 did not affect the ATPase activity of the Wt (wild-type) CFTR, supporting the idea that this compound corrects the specific molecular defect in this mutant. To summarize, these studies provide direct evidence that this compound binds to G551D-CFTR to rescue its specific defect in ATP binding and hydrolysis.

Monomeric CFTR in plasma membranes in live cells revealed by single molecule fluorescence imaging

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel. There is indirect and conflicting evidence about whether CFTR exists in cell membranes as monomers, dimers, or higher order oligomers. We measured fluorescence intensities and photobleaching dynamics of distinct fluorescent spots in cells expressing functional CFTR-green fluorescent protein (GFP) chimeras. Intensity analysis of GFP-labeled CFTR in live cells showed single-component distributions with mean intensity equal to that of purified monomeric GFP, indicating monomeric CFTR in cell membranes. Fluorescent spots showed single-step photobleaching, independently verifying that CFTR is monomeric. Results did not depend on whether GFP was added to the CFTR N terminus or fourth extracellular loop or on whether CFTR chloride conductance was stimulated by cAMP agonists. Control measurements with a CFTR chimera containing two GFPs showed two-step photobleaching and a single-component intensity distribution with mean intensity twice that of monomeric GFP. These results provide direct evidence for monomeric CFTR in live cells.

Three-dimensional reconstruction of human cystic fibrosis transmembrane conductance regulator chloride channel revealed an ellipsoidal structure with orifices beneath the putative transmembrane domain

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a membrane-integral protein that belongs to an ATP-binding cassette superfamily. Mutations in the CFTR gene cause cystic fibrosis in which salt, water, and protein transports are defective in various tissues. Here we expressed wild-type human CFTR as a FLAG-fused protein in HEK293 cells heterologously and purified it in three steps: anti-FLAG and wheat germ agglutinin affinity chromatographies and size exclusion chromatography. The stoichiometry of the protein was analyzed using various biochemical approaches, including chemical cross-linking, blue-native PAGE, size exclusion chromatography, and electron microscopy (EM) observation of antibody-decorated CFTR. All these data support a dimeric assembly of CFTR. Using 5,039 automatically selected particles from negatively stained EM images, the three-dimensional structure of CFTR was reconstructed at 2-nm resolution assuming a 2-fold symmetry. CFTR, presumably in a closed state, was shown to be an ellipsoidal particle with dimensions of 120 x 106 x 162 A. It comprises a small dome-shaped extracellular and membrane-spanning domain and a large cytoplasmic domain with orifices beneath the putative transmembrane domain. EM observation of CFTR.anti-regulatory domain antibody complex confirmed that two regulatory domains are located around the bottom end of the larger oval cytoplasmic domain.

The intact CFTR protein mediates ATPase rather than adenylate kinase activity

The two NBDs (nucleotide-binding domains) of ABC (ATP-binding-cassette) proteins function in a complex to mediate ATPase activity and this activity has been linked to their regulated transport activity. A similar model has been proposed for CFTR (cystic fibrosis transmembrane conductance regulator), the chloride channel defective in cystic fibrosis, wherein ATP binding and hydrolysis regulate the channel gate. Recently, it was shown that the individual NBDs isolated from CFTR primarily mediate adenylate kinase activity, raising the possibility that this activity may also contribute to gating of the CFTR channel. However, this present study shows that whereas the isolated NBDs exhibit adenylate kinase activity, the full-length purified and reconstituted CFTR protein functions as an ATPase, arguing that the enzymatic activity of the NBDs is dependent on their molecular context and appropriate domain-domain assembly. As expected, the disease-causing mutant bearing a mutation in the ABC signature motif, CFTR-G551D, exhibited a markedly reduced ATPase activity. Furthermore, mutation of the putative catalytic base in CFTR caused a reduction in ATPase activity, with the CFTR-E1371Q mutant supporting a low level of residual activity. Neither of these mutants exhibited detectable adenylate kinase activity. Together, these findings support the concept that the molecular mechanism of action of CFTR is dependent on ATP binding and hydrolysis, and that the structure of prokaryotic ABC ATPases provide a useful template for understanding their mechanism of action.

Mutations at arginine 352 alter the pore architecture of CFTR

Arginine 352 (R352) in the sixth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) previously was reported to form an anion/cation selectivity filter and to provide positive charge in the intracellular vestibule. However, mutations at this site have nonspecific effects, such as inducing susceptibility of endogenous cysteines to chemical modification. We hypothesized that R352 stabilizes channel structure and that charge-destroying mutations at this site disrupt pore architecture, with multiple consequences. We tested the effects of mutations at R352 on conductance, anion selectivity and block by the sulfonylurea drug glipizide, using recordings of wild-type and mutant channels. Charge-altering mutations at R352 destabilized the open state and altered both selectivity and block. In contrast, R352K-CFTR was similar to wild-type. Full conductance state amplitude was similar to that of wild-type CFTR in all mutants except R352E, suggesting that R352 does not itself form an anion coordination site. In an attempt to identify an acidic residue that may interact with R352, we found that permeation properties were similarly affected by charge-reversing mutations at D993. Wild-type-like properties were rescued in R352E/D993R-CFTR, suggesting that R352 and D993 in the wild-type channel may interact to stabilize pore architecture. Finally, R352A-CFTR was sensitive to modification by externally applied MTSEA+, while wild-type and R352E/D993R-CFTR were not. These data suggest that R352 plays an important structural role in CFTR, perhaps reflecting its involvement in forming a salt bridge with residue D993.

Molecular basis for the ATPase activity of CFTR

CFTR is a member of the ABC (ATP binding cassette) superfamily of transporters. It is a multidomain membrane protein, which utilizes ATP to regulate the flux of its substrate through the membrane. CFTR is distinct in that it functions as a channel and it possesses a unique regulatory R domain. There has been significant progress in understanding the molecular basis for CFTR activity as an ATPase. The dimeric complex of NBD structures seen in prokaryotic ABC transporters, together with the structure of an isolated CF-NBD1, provide a unifying molecular template to model the structural basis for the ATPase activity of CFTR. The dynamic nature of the interaction between the NBDs and the R domain has been revealed in NMR studies. On the other hand, understanding the mechanisms mediating the transmission of information from the cytosolic domains to the membrane and the channel gate of CFTR remains a central challenge.

Oligomerization domain of the multidrug resistance-associated transporter ABCG2 and its dominant inhibitory activity

Overexpression of human ATP-binding cassette transporter ABCG2 in cancer cells causes multidrug resistance by effluxing anticancer drugs. ABCG2 is considered as a half transporter and is thought to function as a homodimer. However, recent evidence suggests that it may exist as a higher form of oligomer consisting of 12 subunits. In this study, we mapped the oligomerization domain of human ABCG2 to its transmembrane domain consisting of TM5-loop-TM6. This oligomerization domain, when expressed alone in HEK293 cells, also forms a homododecamer. Furthermore, this domain has activity that inhibits drug efflux and resistance function of the full-length ABCG2 likely by disrupting the formation of the homo-oligomeric full-length ABCG2. These findings suggest that human ABCG2 may exist and work as a homo-oligomer by interactions located in TM5-loop-TM6, and that ABCG2 oligomerization may be used as a target for therapeutic development to circumvent ABCG2-mediated drug resistance in cancer treatment.

Regulation of function by dimerization through the amino-terminal membrane-spanning domain of human ABCC1/MRP1

Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCB1/P-glycoprotein/MDR1 and ABCC1/MRP1 causes multidrug resistance in cancer chemotherapy. It has been thought that half-ABC transporters with one nucleotide-binding domain and one membrane-spanning domain (MSD) likely work as dimers, whereas full-length transporters with two nucleotide-binding domains and two or three MSDs function as monomers. In this study, we examined the oligomeric status of the human full-length ABC transporter ABCC1/MRP1 using several biochemical approaches. We found 1) that it is a homodimer, 2) that the dimerization domain is located in the amino-terminal MSD0L0 (where L0 is loop 0) region, and 3) that MSD0L0 has a dominant-negative function when coexpressed with wild-type ABCC1/MRP1. These findings suggest that ABCC1/MRP1 may exist and function as a dimer and that MSD0L0 likely plays some structural and regulatory functions. It is also tempting to propose that the MSD0L0-mediated dimerization may be targeted for therapeutic development to sensitize ABCC1/MRP1-mediated drug resistance in cancer chemotherapy.

Transition from dimers to higher oligomeric forms occurs during the ATPase cycle of the ABCA1 transporter

Fluorescence resonance energy transfer and native PAGE analytical techniques were employed to assess the quaternary structure of ABCA1, an ATP binding cassette transporter playing a crucial role in cellular lipid handling. These experimental approaches support the conclusion that ABCA1 is associated in dimeric structures that undergo transition into higher order structures, i.e. tetramers, during the ATP catalytic cycle. Our data hence underline molecular assembly as a crucial parameter in ABCA1 function and the advantage of native PAGE as analytical tool for intractable membrane proteins.

Recent progress in understanding the mechanism of P-glycoprotein-mediated drug efflux

P-glycoprotein (P-gp) is an ATP-dependent drug pump that can transport a broad range of hydrophobic compounds out of the cell. The protein is clinically important because of its contribution to the phenomenon of multidrug resistance during AIDS/HIV and cancer chemotherapy. P-gp is a member of the ATP-binding cassette (ABC) family of proteins. It is a single polypeptide that contains two repeats joined by a linker region. Each repeat has a transmembrane domain consisting of six transmembrane segments followed by a hydrophilic domain containing the nucleotide-binding domain. In this mini-review, we discuss recent progress in determining the structure and mechanism of human P-glycoprotein.

Nucleotide-binding domains of cystic fibrosis transmembrane conductance regulator, an ABC transporter, catalyze adenylate kinase activity but not ATP hydrolysis

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel in the ATP-binding cassette (ABC) transporter family. CFTR consists of two transmembrane domains, two nucleotide-binding domains (NBD1 and NBD2), and a regulatory domain. Previous biochemical reports suggest NBD1 is a site of stable nucleotide interaction with low ATPase activity, whereas NBD2 is the site of active ATP hydrolysis. It has also been reported that NBD2 additionally possessed adenylate kinase (AK) activity. Knowledge about the intrinsic biochemical activities of the NBDs is essential to understanding the Cl(-) ion gating mechanism. We find that purified mouse NBD1, human NBD1, and human NBD2 function as adenylate kinases but not as ATPases. AK activity is strictly dependent on the addition of the adenosine monophosphate (AMP) substrate. No liberation of [(33)P]phosphate is observed from the gamma-(33)P-labeled ATP substrate in the presence or absence of AMP. AK activity is intrinsic to both human NBDs, as the Walker A box lysine mutations abolish this activity. At low protein concentration, the NBDs display an initial slower nonlinear phase in AK activity, suggesting that the activity results from homodimerization. Interestingly, the G551D gating mutation has an exaggerated nonlinear phase compared with the wild type and may indicate this mutation affects the ability of NBD1 to dimerize. hNBD1 and hNBD2 mixing experiments resulted in an 8-57-fold synergistic enhancement in AK activity suggesting heterodimer formation, which supports a common theme in ABC transporter models. A CFTR gating mechanism model based on adenylate kinase activity is proposed.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Human organic anion transporter hOAT1 forms homooligomers

Human organic anion transporter hOAT1 belongs to a superfamily of organic anion transporters, which play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. To gain insight into the regulation of hOAT1, detailed information on its structural assembly is essential. In the present study, we investigate the quaternary structure of hOAT1 using combined approaches of chemical cross-linking, gel filtration chromatography, co-immunoprecipitation, cell surface biotinylation, and metabolic labeling. Chemical cross-linking of intact membrane proteins from LLC-PK1 cells stably expressing hOAT1 converted quantitatively hOAT1 monomer to putative trimer and higher order of oligomer, indicating that hOAT1 is present in the membrane as multimeric complexes. When co-expressed in LLC-PK1 cells, FLAG-tagged hOAT1 co-immunoprecipitated with myc-tagged hOAT1. The hOAT1 oligomer was also detected in gel filtration chromatography of total membranes from hOAT1-expressing LLC-PK1 cells. Cell surface biotinylation with membrane-impermeable reagents and metabolic labeling with [(35)S]methionine followed by immunoprecipitation showed that the oligomeric hOAT1 did not contain any other proteins. Taken together, this is the first study demonstrating that hOAT1 exists in the plasma membrane as a homooligomer, possibly trimer, and higher order of oligomer.

Macromolecular complexes of cystic fibrosis transmembrane conductance regulator and its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is the product of the gene mutated in patients with cystic fibrosis (CF). CFTR is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells lining the airway, gut, exocrine glands, etc., where it is responsible for transepithelial salt and water transport. CFTR chloride channel belongs to the superfamily of the ATP-binding cassette (ABC) transporters, which bind ATP and use the energy to drive the transport of a wide variety of substrates across extra- and intracellular membranes. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might regulate the activities of other ion channels, receptors, or transporters, in addition to its role as a chloride conductor. The molecular assembly of CFTR with these interacting proteins is of great interest and importance because several human diseases are attributed to altered regulation of CFTR, among which cystic fibrosis is the most serious one. Most interactions primarily occur between the opposing terminal tails (N- or C-) of CFTR and its binding partners, either directly or mediated through various PDZ domain-containing proteins. These dynamic interactions impact the channel function as well as the localization and processing of CFTR protein within cells. This review focuses on the recent developments in defining the assembly of CFTR-containing complexes in the plasma membrane and its interacting proteins.

ASSEMBLY OF FUNCTIONAL CFTR CHLORIDE CHANNELS

The assembly of the cystic fibrosis transmembrane regulator (CFTR) chloride channel is of interest from the broad perspective of understanding how ion channels and ABC transporters are formed as well as dealing with the mis-assembly of CFTR in cystic fibrosis. CFTR is functionally distinct from other ABC transporters because it permits bidirectional permeation of anions rather than vectorial transport of solutes. This adaptation of the ABC transporter structure can be rationalized by considering CFTR as a hydrolyzable-ligand-gated channel with cytoplasmic ATP as ligand. Channel gating is initiated by ligand binding when the protein is also phosphorylated by protein kinase A and made reversible by ligand hydrolysis. The two nucleotide-binding sites play different roles in channel activation. CFTR self-associates, possibly as a function of its activation, but most evidence, including the low-resolution three-dimensional structure, indicates that the channel is monomeric. Domain assembly and interaction within the monomer is critical in maturation, stability, and function of the protein. Disease-associated mutations, including the most common, DeltaF508, interfere with domain folding and association, which occur both co- and post-translationally. Intermolecular interactions of mature CFTR have been detected primarily with the N- and C-terminal tails, and these interactions have some impact not only on channel function but also on localization and processing within the cell. The biosynthetic processing of the nascent polypeptide leading to channel assembly involves transient interactions with numerous chaperones and enzymes on both sides of the endoplasmic reticulum membrane.

Determination of the functional unit of the cystic fibrosis transmembrane conductance regulator chloride channel. One polypeptide forms one pore

The magnitudes and distributions of subconductance states were studied in chloride channels formed by the wild-type cystic fibrosis transmembrane conductance regulator (CFTR) and in CFTRs bearing amino acid substitutions in transmembrane segment 6. Within an open burst, it was possible to distinguish three distinct conductance states referred to as the full conductance, subconductance 1, and subconductance 2 states. Amino acid substitutions in transmembrane segment 6 altered the duration and probability of occurrence of these subconductance states but did not greatly alter their relative amplitudes. Results from real time measurements indicated that covalent modification of single R334C-CFTR channels by [2-(trimethylammonium)ethyl]methanethiosulfonate resulted in the simultaneous modification of all three conductance levels in what appeared to be a single step, without changing the proportion of time spent in each state. This behavior suggests that at least a portion of the conduction path is common to all three conducting states. The time course for the modification of R334C-CFTR, measured in outside-out macropatches using a rapid perfusion system, was also consistent with a single modification step as if each pore contained only a single copy of the cysteine at position 334. These results are consistent with a model for the CFTR conduction pathway in which a single anion-conducting pore is formed by a single CFTR polypeptide.

Characterization of oligomeric human ATP binding cassette transporter A1. Potential implications for determining the structure of nascent high density lipoprotein particles

The oligomeric structure of ABCA1 transporter and its function related to the biogenesis of nascent apoA-I-containing particles (LpA-I) were investigated. Using n-dodecylmaltoside and perfluoro-octanoic acid combined with non-denaturing gel, the majority of ABCA1 was found as a tetramer in ABCA1-induced human fibroblasts. Furthermore, using chemical cross-linking and SDS-PAGE, ABCA1 dimers but not the tetramers were found covalently linked. Oligomeric ABCA1 was present in isolated plasma membranes as well as in intracellular compartments. Interestingly, apoA-I was found to be associated with both dimeric and tetrameric, but not monomeric, forms of ABCA1. Neither apoA-I nor lipid molecules did affect ABCA1 oligomerization. Immunoprecipitation analysis showed that oligomeric ABCA1 did not contain other associated proteins. We next investigated the relationship between the oligomeric ABCA1 complex and the structure of LpA-I. Lipid-free apoA-I incubated with normal cells generated LpA-I with diameters between 9.5 and 20 nm. Subsequent isolation of LpA-I followed by cross-linking revealed the presence of four and eight apoA-I molecules per particle, whereas apoA-I incubated with ABCA1 mutant (Q597R) cells was unable to form such particles and remained in the monomeric form. These results demonstrate that: 1) ABCA1 exists as an oligomeric complex; and 2) ABCA1 oligomerization was independent of apoA-I binding and lipid molecules. The findings that the majority of ABCA1 exists as a tetramer that binds apoA-I, together with the observation that LpA-I contains at least four molecules of apoA-I per particle, support the concept that the homotetrameric ABCA1 complex constitutes the minimum functional unit required for the biogenesis of high density lipoprotein particles.

Protein kinase A regulates ATP hydrolysis and dimerization by a CFTR (cystic fibrosis transmembrane conductance regulator) domain

Gating of the CFTR Cl- channel is associated with ATP hydrolysis at the nucleotide-binding domains (NBD1, NBD2) and requires PKA (protein kinase A) phosphorylation of the R domain. The manner in which the NBD1, NBD2 and R domains of CFTR (cystic fibrosis transmembrane conductance regulator) interact to achieve a properly regulated ion channel is largely unknown. In this study we used bacterially expressed recombinant proteins to examine interactions between these soluble domains of CFTR in vitro. PKA phosphorylated a fusion protein containing NBD1 and R (NBD1-R-GST) on CFTR residues Ser-660, Ser-700, Ser-712, Ser-737, Ser-768, Ser-795 and Ser-813. Phosphorylation of these serine residues regulated ATP hydrolysis by NBD1-R-GST by increasing the apparent K(m) for ATP (from 70 to 250 microM) and the Hill coefficient (from 1 to 1.7) without changing the V(max). When fusion proteins were photolabelled with 8-azido-[alpha-32P]ATP, PKA phosphorylation increased the apparent k(d) for nucleotide binding and it caused binding to become co-operative. PKA phosphorylation also resulted in dimerization of NBD1-R-GST but not of R-GST, a related fusion protein lacking the NBD1 domain. Finally, an MBP (maltose-binding protein) fusion protein containing the NBD2 domain (NBD2-MBP) associated with and regulated the ATPase activity of PKA-phosphorylated NBD1-R-GST. Thus when the R domain in NBD1-R-GST is phosphorylated by PKA, ATP binding and hydrolysis becomes co-operative and NBD dimerization occurs. These findings suggest that during the activation of native CFTR, phosphorylation of the R domain by PKA can control the ability of the NBD1 domain to hydrolyse ATP and to interact with other NBD domains.

Molecular assembly of cystic fibrosis transmembrane conductance regulator in plasma membrane

Based on electrophysiological measurements, it has been argued that the active form of cystic fibrosis trans-membrane conductance regulator (CFTR) Cl(-) channel is a multimer. It has also been demonstrated that this multimerization is likely due to PDZ domain-interacting partners. Here we demonstrate that although CFTR in vitro can self-associate into multimers, which depends on PDZ-based interactions, this may not be the case in cell membrane. Using chemical cross-linking, we demonstrated that CFTR exists as a higher order complex in cell membrane. However, this higher order complex is predominantly CFTR dimers, and the PDZ-interacting partners (Na(+)/H(+) exchanger regulatory factor-1 (NHERF1) and NHERF2) constitute approximately 2% of this complex. Interestingly solubilizing membrane expressing CFTR in detergents such as Triton X-100, Nonidet P-40, deoxycholate, and SDS tended to destabilize the CFTR dimers and dissociate them into monomeric form. The dimerization of CFTR was tightly regulated by cAMP-dependent protein kinase-dependent phosphorylation and did not depend on the active form of the channel. In addition, the dimerization was not influenced by either the PDZ motif or its interacting partners (NHERF1 and NHERF2). We also demonstrated that other signaling-related proteins such as Gbeta and syntaxin 1A can be in this higher order complex of CFTR as well. Our studies provide a deeper understanding of how the CFTR assembly takes place in native cell membrane.

Characterization of oligomeric human half-ABC transporter ATP-binding cassette G2

Human ATP-binding cassette G2 (ABCG2, also known as mitoxantrone resistance protein, breast cancer-resistance protein, ABC placenta) is a member of the superfamily of ATP-binding cassette (ABC) transporters that have a wide variety of substrates. Overexpression of human ABCG2 in model cancer cell lines causes multidrug resistance by actively effluxing anticancer drugs. Unlike most of the other ABC transporters which usually have two nucleotide-binding domains and two transmembrane domains, ABCG2 consists of only one nucleotide-binding domain followed by one transmembrane domain. Thus, ABCG2 has been thought to be a half-transporter that may function as a homodimer. In this study, we characterized the oligomeric feature of human ABCG2 using non-denaturing detergent perfluoro-octanoic acid and Triton X-100 in combination with gel filtration, sucrose density gradient sedimentation, and gel electrophoresis. Unexpectedly, we found that human ABCG2 exists mainly as a tetramer, with a possibility of a higher form of oligomerization. Monomeric and dimeric ABCG2 did not appear to be the major form of the protein. Further immunoprecipitation analysis showed that the oligomeric ABCG2 did not contain any other proteins. Taken together, we conclude that human ABCG2 likely exists and functions as a homotetramer.

Intermolecular interaction between R domains of cystic fibrosis transmembrane conductance regulator

The function of the R domain of cystic fibrosis transmembrane conductance regulator (CFTR) has not yet been fully established. The cis-trans proline isomerase cyclophilin A stimulates channel activity, and stimulation depends on the presence of highly conserved prolines at positions 740, 750, and 759. When the prolines at these positions, which normally exist in the cis conformation, are locked into the trans conformation by mutation to alanine (the P3A mutant), the open probability of P3A is high and is not further increased by cyclophilin A. We speculated that one mechanism by which this could occur was by promoting CFTR dimerization, which has been shown to increase open probability, and that the P3A-CFTR might favor dimerization more strongly than the native sequence. To test the hypothesis that R-R interaction occurs and is stronger in the P3A-R mutants, we investigated R-R interactions. GST-R and StrepII-R proteins expressed in Escherichia coli could interact with R domain protein translated in vitro as well as with full-length CFTR. In similar assays, the P3A mutant of R domain also interacts with R domain and P3A-R. The P3A-R-P3A-R interaction is stronger than the R-R interaction, which corroborates our data from the channel study and supports our hypothesis. Studies of deletion constructs of the isolated R domain and of full-length CFTR localize the region of interaction to the C-terminal portion of R (after amino acid 708). Particularly, the last 22 a.a. residues (838-859) of R are essential for this binding. R-R interaction possibly plays a role in channel gating.

Stable dimeric assembly of the second membrane-spanning domain of CFTR (cystic fibrosis transmembrane conductance regulator) reconstitutes a chloride-selective pore

Structural information is required to define the molecular basis for chloride conduction through CFTR (cystic fibrosis transmembrane conductance regulator). Towards this goal, we expressed MSD2, the second of the two MSDs (membrane-spanning domains) of CFTR, encompassing residues 857-1158 in Sf9 cells using the baculovirus system. In Sf9 plasma membranes, MSD2 migrates as expected for a dimer in non-dissociative PAGE, and confers the appearance of an anion permeation pathway suggesting that dimeric MSD2 mediates anion flux. To assess directly the function and quaternary structure of MSD2, we purified it from Sf9 cells by virtue of its polyhistidine tag and nickel affinity. Reconstitution of MSD2 into liposomes conferred a 4,4'-di-isothiocyanostilbene-2,2'-disulphonate-inhibitable, chloride-selective electrodiffusion pathway. Further, this activity is probably mediated directly by MSD2 as reaction of its single cysteine residue (Cys866) with the thiol modifying reagent, N(alpha)(3-maleimidylpropionyl)biocytin, inhibited chloride flux. Only MSD2 dimers were labelled by N(alpha)(3-maleimidylpropionyl)biocytin, supporting the idea that only dimeric MSD2 can mediate anion flux. As a further test of this hypothesis, we conducted a second purification procedure, wherein purified dimeric and monomeric MSD2 proteins were reconstituted separately. Only proteoliposomes containing stable MSD2 dimers mediated chloride electrodiffusion, providing direct evidence that dimeric MSD2 mediates chloride channel function. In summary, we have shown that the second membrane domain of CFTR can be purified and functionally reconstituted as a chloride channel, providing a tool for probing the structural basis of chloride conduction through CFTR.

Dimeric cystic fibrosis transmembrane conductance regulator exists in the plasma membrane

CFTR (cystic fibrosis transmembrane conductance regulator) mediates chloride conduction across the apical membrane of epithelia, and mutations in CFTR lead to defective epithelial fluid transport. Recently, there has been considerable interest in determining the quaternary structure of CFTR at the cell surface, as such information is a key to understand the molecular basis for pathogenesis in patients harbouring disease-causing mutations. In our previous work [Ramjeesingh, Li, Kogan, Wang, Huan and Bear (2001) Biochemistry 40, 10700-10706], we showed that monomeric CFTR is the minimal functional form of the protein, yet when expressed in Sf 9 cells using the baculovirus system, it also exists as dimers. The purpose of the present study was to determine if dimeric CFTR exists at the surface of mammalian cells, and particularly in epithelial cells. CFTR solubilized from membranes prepared from Chinese-hamster ovary cells stably expressing CFTR and from T84 epithelial cells migrates as predicted for monomeric, dimeric and larger complexes when subjected to sizing by gel filtration and analysis by non-dissociative electrophoresis. Purification of plasma membranes led to the enrichment of CFTR dimers and this structure exists as the complex glycosylated form of the protein, supporting the concept that dimeric CFTR is physiologically relevant. Consistent with its localization in plasma membranes, dimeric CFTR was labelled by surface biotinylation. Furthermore, dimeric CFTR was captured at the apical surface of intact epithelial cells by application of a membrane-impermeable chemical cross-linker. Therefore it follows from the present study that CFTR dimers exist at the surface of epithelial cells. Further studies are necessary to understand the impact of dimerization on the cell biology of wild-type and mutant CFTR proteins.

Expression of human electron transfer flavoprotein-ubiquinone oxidoreductase from a baculovirus vector: kinetic and spectral characterization of the human protein

Electron transfer flavoprotein-ubiquinone oxidoreductase (ETF-QO) is an iron-sulphur flavoprotein and a component of an electron-transfer system that links 10 different mitochondrial flavoprotein dehydrogenases to the mitochondrial bc1 complex via electron transfer flavoprotein (ETF) and ubiquinone. ETF-QO is an integral membrane protein, and the primary sequences of human and porcine ETF-QO were deduced from the sequences of the cloned cDNAs. We have expressed human ETF-QO in Sf9 insect cells using a baculovirus vector. The cDNA encoding the entire protein, including the mitochondrial targeting sequence, was present in the vector. We isolated a membrane-bound form of the enzyme that has a molecular mass identical with that of the mature porcine protein as determined by SDS/PAGE and has an N-terminal sequence that is identical with that predicted for the mature holoenzyme. These data suggest that the heterologously expressed ETF-QO is targeted to mitochondria and processed to the mature, catalytically active form. The detergent-solubilized protein was purified by ion-exchange and hydroxyapatite chromatography. Absorption and EPR spectroscopy and redox titrations are consistent with the presence of flavin and iron-sulphur centres that are very similar to those in the equivalent porcine and bovine proteins. Additionally, the redox potentials of the two prosthetic groups appear similar to those of the other eukaryotic ETF-QO proteins. The steady-state kinetic constants of human ETF-QO were determined with ubiquinone homologues, a ubiquinone analogue, and with human wild-type ETF and a Paracoccus-human chimaeric ETF as varied substrates. The results demonstrate that this expression system provides sufficient amounts of human ETF-QO to enable crystallization and mechanistic investigations of the iron-sulphur flavoprotein.

Distinct Mg(2+)-dependent steps rate limit opening and closing of a single CFTR Cl(-) channel

The roles played by ATP binding and hydrolysis in the complex mechanisms that open and close cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels remain controversial. In this work, the contributions made by ATP and Mg(2+) ions to the gating of phosphorylated cardiac CFTR channels were evaluated separately by measuring the rates of opening and closing of single channels in excised patches exposed to solutions in which [ATP] and [Mg(2+)] were varied independently. Channel opening was found to be rate-limited not by the binding of ATP alone, but by a Mg(2+)-dependent step that followed binding of both ATP and Mg(2+). Once a channel had opened, sudden withdrawal of all Mg(2+) and ATP could prevent it from closing for tens of seconds. But subsequent exposure of such an open channel to Mg(2+) ions alone could close it, and the closing rate increased with [Mg(2+)] over the micromolar range (half maximal at approximately 50 microM [Mg(2+)]). A simple interpretation is that channel closing is stoichiometrically coupled to hydrolysis of an ATP molecule that remains tightly associated with the open CFTR channel despite continuous washing. If correct, that ATP molecule appears able to reside for over a minute in the catalytic site that controls channel closing, implying that the site must entrap, or have an intrinsically high apparent affinity for, ATP, even without a Mg(2+) ion. Such stabilization of the open-channel conformation of CFTR by tight binding, or occlusion, of an ATP molecule echoes the stabilization of the active conformation of a G protein by GTP.