Probing the structural and functional domains of the CFTR chloride channel

Pinning Down the Mechanism of Transport: Probing the Structure and Function of Transporters Using Cysteine Cross-Linking and Site-Specific Labeling

Transporters are crucial in a number of cellular functions, including nutrient uptake, cell signaling, and toxin removal. As such, transporters are important drug targets and their malfunction is related to several disease states. Treating transporter-related diseases and developing pharmaceuticals targeting transporters require an understanding of their mechanism. Achieving a detailed understanding of transporter mechanism depends on an integrative approach involving structural and computational approaches as well as biochemical and biophysical methodologies. Many of the elements of this toolkit exploit the unique and useful chemistry of the amino acid cysteine. Cysteine offers researchers a specific molecular handle with which to precisely modify the protein, which enables the introduction of biophysical probes to assess ligand binding and the conformational ensemble of the transporter, to topologically map transporters and validate structural models, and to assess essential conformational changes. Here, we summarize several uses for cysteine-based labeling and cross-linking in the pursuit of understanding transporter mechanism, the common cysteine-reactive reagents used to probe transporter mechanism, and strategies that can be used to confirm cysteine cross-link formation. In addition, we provide methodological considerations for each approach and a detailed procedure for the cross-linking of introduced cysteines, and a simple screening method to assess cross-link formation.

CFTR: Ligand exchange between a permeant anion ([Au(CN)2]-) and an engineered cysteine (T338C) blocks the pore

Previous attempts to identify residues that line the pore of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel have utilized cysteine-substituted channels in conjunction with impermeant, thiol-reactive reagents like MTSET+ and MTSES-. We report here that the permeant, pseudohalide anion [Au(CN)2]- can also react with a cysteine engineered into the pore of the CFTR channel. Exposure of Xenopus oocytes expressing the T338C CFTR channel to as little as 100 nM [Au(CN)2]- produced a profound reduction in conductance that was not reversed by washing but was reversed by exposing the oocytes to a competing thiol like DTT (dithiothreitol) and 2-ME (2-mercaptoethanol). In detached, inside out patches single-channel currents were abolished by [Au(CN)2]- and activity was not restored by washing [Au(CN)2]- from the bath. Both single-channel and macroscopic currents were restored, however, by exposing [Au(CN)2]- -blocked channels to excess [CN]-. The results are consistent with the hypothesis that [Au(CN)2]- can participate in a ligand exchange reaction with the cysteine thiolate at 338 such that the mixed-ligand complex, with a charge of -1, blocks the anion conduction pathway.

Potentiation of effect of PKA stimulation of Xenopus CFTR by activation of PKC: role of NBD2

Activity of the human (h) cystic fibrosis transmembrane conductance regulator (CFTR) channel is predominantly regulated by PKA-mediated phosphorylation. In contrast, Xenopus (X)CFTR is more responsive to PKC than PKA stimulation. We investigated the interaction between the two kinases in XCFTR. We expressed XCFTR in Xenopus oocytes and maximally stimulated it with PKA agonists. The magnitude of activation after PKC stimulation was about eightfold that without pretreatment with PKC agonist. hCFTR, expressed in the same system, lacked this response. We name this phenomenon XCFTR-specific PKC potentiation effect. To ascertain its biophysical mechanism, we first tested for XCFTR channel insertion into the plasma membrane by a substituted-cysteine-accessibility method. No insertion was detected during kinase stimulation. Next, we studied single-channel properties and found that the single-channel open probability (Po) with PKA stimulation subsequent to PKC stimulation was 2.8-fold that observed in the absence of PKC preactivation and that single-channel conductance (gamma) was increased by approximately 22%. To ascertain which XCFTR regions are responsible for the potentiation, we constructed several XCFTR-hCFTR chimeras, expressed them in Xenopus oocytes, and tested them electrophysiologically. Two chimeras [hCFTR NH2-terminal region or regulatory (R) domain in XCFTR] showed a significant decrease in potentiation. In the chimera in which XCFTR nucleotide-binding domain (NBD)2 was replaced with the hCFTR sequence there was no potentiation whatsoever. The converse chimera (hCFTR with Xenopus NBD2) did not exhibit potentiation. These results indicate that potentiation by PKC involves a large increase in Po (with a small change in gamma) without CFTR channel insertion into the plasma membrane, that XCFTR NBD2 is necessary but not sufficient for the effect, and that the potentiation effect is likely to involve other CFTR domains.

Non-Native Interhelical Hydrogen Bonds in the Cystic Fibrosis Transmembrane Conductance Regulator Domain Modulated by Polar Mutations

Polar residues comprise about 15% of the transmembrane (TM) domains of proteins, where they can stabilize structure via native side chain-side chain interhelical hydrogen bonds between TM helices. However, non-native H-bonds may be implicated in disease states, through limiting protein dynamics during transport and/or misfolding the protein by inducing non-native rotational positions about TM helical axes. Here we have undertaken an investigation of the presence and strength of H-bond interactions within a series of helix-loop-helix ("hairpin") constructs derived from TM helices 3 and 4 (italic) of the cystic fibrosis transmembrane conductance regulator (CFTR) (prototypic sequence G(194)LALAHFVWIAPLQ(207)VALLMGLIWELLQASAFAGLGFLIV(232)LALFQ(237)AGLG(241)) in which wild-type Q207 in TM3 forms an interhelical H-bond with CF-phenotypic mutant V232D in TM4 [Therien, A. G., Grant, F. E., and Deber, C. M. (2001) Nat. Struct. Biol 8, 597-601]. In the present work, a library of 21 TM3/4 constructs was prepared, where Asp residues were placed individually at TM4 positions 221-241. Using gel shift assays-in which H-bond-linked hairpins (closed conformation) migrate faster than the elongated forms (open conformation)-we found that Q207 in TM3 is able to "capture" all 21 TM4 D mutations into measurable populations of interhelical H-bonds. A similar library of TM4 D mutants-but also containing Q207L-reverted to wild-type migration rates, confirming Q207 as the polar partner for TM4 D residues. In view of the broad capture range of Q207, these results emphasize the potential consequences to folding and dynamics of introducing polar mutations into the TM domains of membrane proteins in the vicinity of a native polar TM residue.

Interhelical packing in detergent micelles. Folding of a cystic fibrosis transmembrane conductance regulator construct

Using a helix-loop-helix construct consisting of the adjacent transmembrane segments 3 and 4 of the cystic fibrosis transmembrane conductance regulator (CFTR) labeled with pyrene at both N and C termini, we describe a system for the study of intramolecular helix-helix interactions within a polytopic membrane protein. Through measurement of pyrene excimer band intensity as a determinant of helix-helix proximity, we show that the helices retain tertiary contacts in detergent micelles. Notably, the nature of the micellar detergent can alter the stability of these contacts, with perfluorooctanoate highly supportive, lysophosphatidylcholine and lysophosphatidylglycerol somewhat less tolerant, and SDS largely intolerant of such interactions. This construct is further employed to study the role of the acyl chain length of micellar detergents in modulating interhelical packing; detergents having acyl chains of 9 carbons display the greatest extent of helical packing. These results provide important information regarding the role of lipids on membrane protein folding and conformation as well as demonstrate the usefulness of a pyrene-based system in studying the forces that govern interhelical packing.

Cystic fibrosis: a brief look at some highlights of a decade of research focused on elucidating and correcting the molecular basis of the disease

The disease Cystic Fibrosis (CF) is caused by mutations in the protein called CFTR, cystic fibrosis transmembrane conductance regulator, an ABC-transporter-like protein found in the plasma membrane of animal cells. CFTR is believed to function primarily as a Cl- channel, but evidence is mounting that this protein has other roles as well. Structurally, CFTR consists of a single polypeptide chain (1480 amino acids) that folds into 5 distinct domains. These include 2 transmembrane domains that are involved in channel formation; 2 nucleotide-binding domains (NBF1 and NBF2), the first of which clearly binds and hydrolyzes ATP; and 1 regulatory domain (R) that is phosphorylated in a cAMP-dependent process. Currently, the 3D structure of neither CFTR nor its domains has been elucidated, although both nucleotide domains have been modeled in 3D, and solution structures in 3D have been obtained for peptide segments of NBF1. The most common mutation causing CF is the deletion (delta) of a single phenylalanine (F) in position 508 within a putative helix located in NBF1. CF patients bearing this deltaF508 mutation frequently experience chronic lung infections, particularly by Pseudomonas aeruginosa, and have a life span that rarely exceeds the age of 30. Since the CFTR gene was cloned and sequenced in 1989, there has been over a decade of research focused on understanding the molecular basis of CF caused by the deltaF508 mutation, with the ultimate objective of using the knowledge gained to carry out additional research designed to correct the underlying defect. In general, this pioneering or "ground roots" research has succeeded according to plan. This brief review summarizes some of the highlights with a focus on those studies conducted in the authors' laboratory. For us, this research has been both exciting and rewarding mainly because the results obtained, despite very limited funding, have provided considerable insight, not only into the chemical, molecular, and pathogenic basis of CF, but have made it possible for us and others to now develop novel, chemically rational, and "cost effective" strategies to identify agents that correct the structural defect in the deltaF508 CFTR protein causing most cases of CF.

The glial and the neuronal glycine transporters differ in their reactivity to sulfhydryl reagents

The neuronal (GlyT2) and glial (GlyT1) glycine transporters, two members of the Na(+)/Cl(-)-dependent neurotransmitter transporter superfamily, differ by many aspects, such as substrate specificity and Na(+) coupling. We have characterized under voltage clamp their reactivity toward the membrane impermeant sulfhydryl reagent [2-(trimethylammonium)-ethyl]-methanethiosulfonate (MTSET). In Xenopus oocytes expressing GlyT1b, application of MTSET reduced to the same extent the Na(+)-dependent charge movement, the glycine-evoked current, and the glycine uptake, indicating a complete inactivation of the transporters following cysteine modification. In contrast, this compound had no detectable effect on the glycine uptake and the glycine-evoked current of GlyT2a. The sensitivities to MTSET of the two transporters can be permutated by suppressing a cysteine (C62A) in the first extracellular loop (EL1) of GlyT1b and introducing one at the equivalent position in GlyT2a, either by point mutation (A223C) or by swapping the EL1 sequence (GlyT1b-EL1 and GlyT2a-EL1) resulting in AFQ <--> CYR modification. Inactivation by MTSET was five times faster in GlyT2a-A223C than in GlyT2a-EL1 or GlyT1b, suggesting that the arginine in position +2 reduced the cysteine reactivity. Protection assays indicate that EL1 cysteines are less accessible in the presence of all co-transported substrates: Na(+), Cl(-), and glycine. Application of dithioerythritol reverses the inactivation by MTSET of the sensitive transporters. Together, these results indicate that EL1 conformation differs between GlyT1b and GlyT2a and is modified by substrate binding and translocation.

Pores formed by single subunits in mixed dimers of different CLC chloride channels

CLC chloride channels comprise a gene family with nine mammalian members. Probably all CLC channels form homodimers, and some CLC proteins may also associate to heterodimers. ClC-0 and ClC-1, the only CLC channels investigated at the single-channel level, display two conductances of equal size which are thought to result from two separate pores, formed individually by the two monomers. We generated concatemeric channels containing one subunit of ClC-0 together with one subunit of ClC-1 or ClC-2. They should display two different conductances if one monomer were sufficient to form one pore. Indeed, we found a 8-picosiemens (pS) conductance (corresponding to ClC-0) that was associated with either a 1.8-pS (ClC-1) or a 2.8-pS (ClC-2) conductance. These conductances retained their typical gating, but the slow gating of ClC-0 that affects both pores simultaneously was lost. ClC-2 and ClC-0 current components were modified by point mutations in the corresponding subunit. The ClC-2 single pore of the mixed dimer was compared with the pores in the ClC-2 homodimer and found to be unaltered. We conclude that each monomer individually forms a gated pore. CLC dimers in general must be imagined as having two pores, as shown previously for ClC-0.

Structural, mechanistic and clinical aspects of MRP1

The cDNA encoding ATP-binding cassette (ABC) multidrug resistance protein MRP1 was originally cloned from a drug-selected lung cancer cell line resistant to multiple natural product chemotherapeutic agents. MRP1 is the founder of a branch of the ABC superfamily whose members (from species as diverse as plants and yeast to mammals) share several distinguishing structural features that may contribute to functional and mechanistic similarities among this subgroup of transport proteins. In addition to its role in resistance to natural product drugs, MRP1 (and related proteins) functions as a primary active transporter of structurally diverse organic anions, many of which are formed by the biotransformation of various endo- and xenobiotics by Phase II conjugating enzymes, such as the glutathione S-transferases. MRP1 is involved in a number of glutathione-related cellular processes. Glutathione also appears to play a key role in MRP1-mediated drug resistance. This article reviews the discovery of MRP1 and its relationships with other ABC superfamily members, and summarizes current knowledge of the structure, transport functions and relevance of this protein to in vitro and clinical multidrug resistance.

Conjugation of polyethylene glycol via a disulfide bond confers water solubility upon a peptide model of a protein transmembrane segment

The aqueous insolubility of hydrophobic peptides has presented a barrier to the structural characterization of membrane protein transmembrane domains. Since the conjugation of polyethylene glycol is known to modulate the solubility of certain proteins and peptides, we have prepared PEG-a-Cys reagent, a polyethylene glycol derivative which reacts spontaneously with Cys residues to attach polyethylene glycol to polypeptides via a mixed disulfide bond. When desired, the PEG moiety can be readily removed by reduction with tricarboxyethylphosphine. The aqueous solubilizing power of PEG-a-Cys reagent is confirmed with a synthetic hydrophobic peptide model of a generic transmembrane segment-soluble carrier fusion protein.

CpABC, a Cryptosporidium parvum ATP-binding cassette protein at the host-parasite boundary in intracellular stages

The intracellular parasite Cryptosporidium parvum develops inside a vacuole at the apex of its epithelial host cell. The developing parasite is separated from the host cell cytoplasm by a zone of attachment that consists of an extensively folded membranous structure known as the feeder organelle. It has been proposed that the feeder organelle is the site of regulation of transport of nutrients and drugs into the parasite. In this report, we localize an approximately 200-kDa integral membrane protein, CpABC, from Cryptosporidium parvum to the host-parasite boundary, possibly the feeder organelle. The predicted amino acid sequence of CpABC has significant structural similarity with the cystic fibrosis conductance regulator and the multidrug resistance protein subfamily of ATP-binding cassette proteins. This is an example of a parasite-encoded transport protein localized at the parasite-host interface of an intracellular protozoan.