Sequence and structural homology among membrane-associated domains of CFTR and certain transporter proteins

Allosteric coupling between the intracellular coupling helix 4 and regulatory sites of the first nucleotide-binding domain of CFTR

Cystic fibrosis is caused by mutations in CFTR (cystic fibrosis transmembrane conductance regulator), leading to folding and processing defects and to chloride channel gating misfunction. CFTR is regulated by ATP binding to its cytoplasmic nucleotide-binding domains, NBD1 and NBD2, and by phosphorylation of the NBD1 regulatory insert (RI) and the regulatory extension (RE)/R region. These regulatory effects are transmitted to the rest of the channel via NBD interactions with intracellular domain coupling helices (CL), particularly CL4. Using a sensitive method for detecting inter-residue correlations between chemical shift changes in NMR spectra, an allosteric network was revealed within NBD1, with a construct lacking RI. The CL4-binding site couples to the RI-deletion site and the C-terminal residues of NBD1 that precede the R region in full-length CFTR. Titration of CL4 peptide into NBD1 perturbs the conformational ensemble in these sites with similar titration patterns observed in F508del, the major CF-causing mutant, and in suppressor mutants F494N, V510D and Q637R NBD1, as well as in a CL4-NBD1 fusion construct. Reciprocally, the C-terminal mutation, Q637R, perturbs dynamics in these three sites. This allosteric network suggests a mechanism synthesizing diverse regulatory NBD1 interactions and provides biophysical evidence for the allosteric coupling required for CFTR function.

Tumor eradication in rat glioma and bypass of immunosuppressive barriers using internal radiation with (188)Re-lipid nanocapsules

To date, glioblastoma treatments have only been palliative. In this context, locoregional drug delivery strategies, which allow for blood--brain barrier bypass and reduced systemic toxicity, are of major significance. Recent progress in nanotechnology has led to the development of colloidal carriers of radiopharmaceutics, such as lipid nanocapsules loaded with rhenium-188 (LNC(188)Re-SSS) that are implanted in the brain. In our study, we demonstrated that fractionated internal radiation using LNC(188)Re-SSS triggered remarkable survival responses in a rat orthotopic glioma model (cure rates of 83%). We also highlighted the importance of the radioactivity activity gradient obtained by combining a simple stereotactic injection (SI) with convection-enhanced delivery (CED).We assumed that the immune system played a role in the treatment's efficacy on account of the overproduction of peripheral cytokines, recruitment of immune cells to the tumor site, and memory response in long-term survivor animals. Hence, nanovectorized internal radiation therapy with activity gradients stimulating immune responses may represent a new and interesting alternative for the treatment of solid tumors such as glioblastomas.

CSN5 binds to misfolded CFTR and promotes its degradation

Cystic fibrosis is mainly caused by mutations that interfere with the biosynthetic folding of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. The aim of this study was to find cellular proteins interacting with CFTR and regulating its processing. We have used a genetic screen in yeast to identify such proteins and identified CSN5 that interacted with the third cytoplasmic loop of CFTR. CSN5 is the 5th component of the COP9 signalosome, a complex of eight subunits that shares significant homologies to the lid subcomplex of the 26S proteasome and controls the stability of many proteins. The present study shows that CSN5 associates with the core-glycosylated form of CFTR and suggests that this association targets misfolded CFTR to the degradative pathway. Identifying CSN5 as a new component of the degradative pathway is an important step towards the goal of unraveling the sorting between misfolded and correctly folded CFTR proteins.

Lung infections associated with cystic fibrosis

While originally characterized as a collection of related syndromes, cystic fibrosis (CF) is now recognized as a single disease whose diverse symptoms stem from the wide tissue distribution of the gene product that is defective in CF, the ion channel and regulator, cystic fibrosis transmembrane conductance regulator (CFTR). Defective CFTR protein impacts the function of the pancreas and alters the consistency of mucosal secretions. The latter of these effects probably plays an important role in the defective resistance of CF patients to many pathogens. As the modalities of CF research have changed over the decades from empirical histological studies to include biophysical measurements of CFTR function, the clinical management of this disease has similarly evolved to effectively address the ever-changing spectrum of CF-related infectious diseases. These factors have led to the successful management of many CF-related infections with the notable exception of chronic lung infection with the gram-negative bacterium Pseudomonas aeruginosa. The virulence of P. aeruginosa stems from multiple bacterial attributes, including antibiotic resistance, the ability to utilize quorum-sensing signals to form biofilms, the destructive potential of a multitude of its microbial toxins, and the ability to acquire a mucoid phenotype, which renders this microbe resistant to both the innate and acquired immunologic defenses of the host.

Two mild cystic fibrosis-associated mutations result in severe cystic fibrosis when combined in cis and reveal a residue important for cystic fibrosis transmembrane conductance regulator processing and function

The number of complex cystic fibrosis transmembrane conductance regulator (CFTR) genotypes identified as having double-mutant alleles with two mutations inherited in cis has been growing. We investigated the structure-function relationships of a severe cystic fibrosis (CF)-associated double mutant (R347H-D979A) to evaluate the contribution of each mild mutation to the phenotype. CFTR mutants expressed in HeLa cells were analyzed for protein biosynthesis and Cl(-) channel activity. Our data show that R347H is associated with mild defective Cl(-) channel activity and that the D979A defect leads to misprocessing. The mutant R347H-D979A combines both defects for a dramatic decrease in Cl(-) current. To decipher the molecular mechanism of this phenotype, single and double mutants with different charge combinations at residues 347 and 979 were constructed as charged residues were involved in this complex genotype. These studies revealed that residue 979, located in the third cytoplasmic loop, is critical for CFTR processing and Cl(-) channel activity highlighting the role of charged residues. These results have also important implications for CF, as they show that two mutations in cis can act in concert to alter dramatically CFTR function contributing to the wide phenotypic variability of CF disease.

Functional Analysis by Site-Directed Mutagenesis of the Complex Polymorphism in Rat Transporter Associated with Antigen Processing

The transporter associated with Ag processing, TAP, is an endoplasmic reticulum resident heterodimeric member of the ATP-binding cassette transporter family. TAP transports short peptides from cytosol to the endoplasmic reticulum lumen for loading into recently synthesized class I MHC molecules. In the rat, two alleles of the TAP2 chain differ in their permissiveness to the transport of peptides with small hydrophobic, polar, or charged amino acids at the C terminus, and this correlates with differences between the peptide sets loaded into certain class I molecules in vivo. We have used segmental exchanges and site-directed mutagenesis to identify the residues in rat TAP2 responsible for differential transport between the two alleles of peptides terminating above all in the positively charged residue, arginine. Of the 25 residues by which the two functional TAP2 alleles differ, we have localized differential transport of peptides with a C-terminal arginine to two adjacent clusters of exchanges in the membrane domain involving a total of five amino acids. Each cluster, transferred by site-directed mutagenesis from the permissive to the restrictive sequence, can independently confer on TAP a partial ability to transport peptides with arginine at the C terminus. The results suggest that the permissive TAP2-A allele evolved in at least two steps, each partially permissive for peptides with charged C termini.

Overexpression of the ABC transporter TAP in multidrug-resistant human cancer cell lines

Multidrug resistance (MDR) to anti-cancer drugs has been associated with the overexpression of P-glycoprotein (P-gp) and the multidrug resistance-associated protein (MRP), both being members of the ATP-binding cassette (ABC) superfamily of transporters. We investigated whether in addition to P-gp and MRP, another ABC transporter, the transporter associated with antigen processing (TAP), is associated with MDR. TAP plays a major role in MHC class I-restricted antigen presentation by mediating peptide translocation over the endoplasmic reticulum membrane. TAP1 and P-gp share a significant degree of homology among their transmembrane domains, which are thought to be the primary determinants of substrate specificity, and both can apparently mediate the translocation of peptides. Using immunocytochemistry and Western blot, TAP was overexpressed in parallel with MHC class I in several MDR human cancer cell lines. TAP was overexpressed more frequently in MRP-positive MDR cell lines (three out of three) than in P-gp positive MDR cells (two out of five). Reversal of resistance resulted in a decrease in TAP levels. Transfection of the TAP genes into TAP-deficient lymphoblastoid T2 cells conferred mild resistance to etoposide, vincristine and doxorubicin (2- to 2.5-fold). Furthermore, etoposide and vincristine inhibited TAP-dependent peptide translocation to the endoplasmic reticulum. Collectively, our results suggest that TAP may modestly contribute to the MDR phenotype, in particular in MRP- overexpressing MDR cells. Further insight into the role of TAP in MDR will require the study of other transfectants, as well as the investigation of TAP expression in P-gp and MRP-negative MDR cancer cell lines.

Role of the ABC transporter Ste6 in cell fusion during yeast conjugation

Though early stages of yeast conjugation are well-mimicked by treatment with pheromones, the final degradation of the cell wall and membrane fusion of mating that leads to cytoplasmic mixing may require separate signals. Mutations that blocked cell fusion during mating in Saccharomyces cerevisiae were identified in a multipartite screen. The three tightest mutations proved to be partial-function alleles of the ABC-transporter gene STE6 required for transport of a-factor. The ste6(cefl-1) allele was recovered and sequenced. The ste6(cefl-1) allele contained a stop codon predicted to truncate Ste6 at amino acid residue 862 (of 1290). The ste6(cef) mutations reduced, but did not eliminate, expression of a-factor. Light and electron microscopy revealed that unlike ste6 null mutations which block mating before the formation of mating pairs, the ste6(cef) (cell fusion) alleles permitted early steps in mating to proceed normally but blocked at a late stage in conjugation where mating partners were encased by a single cell wall and separated by only a thin layer of cell wall material we term the fusion wall. Morphologically the prezygotes appeared symmetrical with successful cell wall fusion at the periphery of the region of cell contact. Responses to a-factor were efficiently induced in partner cells under mating conditions as expected given the symmetric appearance of the prezygotes. A strain expressing a ste6(K1093A) mutation that conferred export of a twofold to fourfold higher level of a-factor than ste6(cef) did not accumulate prezygotes during mating which could indicate a tight threshold of a-factor signaling required for mating. However, mating to an sst2 partner which has a greatly increased sensitivity to a-factor did not suppress the fusion defect of a ste6(cef) strain. Overexpression of the structural gene for a-factor also did not suppress the fusion defect. It is possible that a-factor or STE6 play more complex roles in cell fusion.

Effect of cystic fibrosis-associated mutations in the fourth intracellular loop of cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) contains multiple membrane spanning sequences that form a Cl- channel pore and cytosolic domains that control the opening and closing of the channel. The fourth intracellular loop (ICL4), which connects the tenth and eleventh transmembrane spans, has a primary sequence that is highly conserved across species, is the site of a preserved sequence motif in the ABC transporter family, and contains a relatively large number of missense mutations associated with cystic fibrosis (CF). To investigate the role of ICL4 in CFTR function and to learn how CF mutations in this region disrupt function, we studied several CF-associated ICL4 mutants. We found that most ICL4 mutants disrupted the biosynthetic processing of CFTR, although not as severely as the most common DeltaF508 mutation. The mutations had no discernible effect on the channel's pore properties; but some altered gating behavior, the response to increasing concentrations of ATP, and stimulation in response to pyrophosphate. These effects on activity were similar to those observed with mutations in the nucleotide-binding domains, suggesting that ICL4 might help couple activity of the nucleotide-binding domains to gating of the Cl- channel pore. The data also explain how these mutations cause a loss of CFTR function and suggest that some patients with mutations in ICL4 may have a milder clinical phenotype because they retain partial activity of CFTR at the cell membrane.

Disease-associated mutations in the fourth cytoplasmic loop of cystic fibrosis transmembrane conductance regulator compromise biosynthetic processing and chloride channel activity

A cluster of 18 point mutations in exon 17b of the cystic fibrosis transmembrane conductance regulator (CFTR) gene has been detected in patients with cystic fibrosis. These mutations cause single amino acid substitutions in the most C-terminal cytoplasmic loop (CL4, residues 1035-1102) of the CFTR chloride channel. Heterologous expression of the mutants showed that 12 produced only core-glycosylated CFTR, which was retained in the endoplasmic reticulum; the other six mutants matured and reached the cell surface. In some cases substitution of one member of pairs of adjacent residues resulted in misprocessing, whereas the other did not. Thus, the secondary structure of CL4 may contribute crucially to the proper folding of the entire CFTR molecule. Cyclic AMP-stimulated iodide efflux was not detected from cells expressing the misprocessed variants but was from the other six, indicating that their mutations cause relatively subtle channel defects. Consistent with this, these latter mutations generally are present in patients who are pancreatic-sufficient, while the processing mutants are mostly from patients who are pancreatic-insufficient. Single-channel patch-clamp analysis demonstrated that the processed mutants had the same ohmic conductance as wild-type CFTR, but a lower open probability, generally due to an increase in channel mean closed time and a reduction in mean open time. This suggests that mutations in CL4 do not affect pore properties of CFTR, but disrupt the mechanism of channel gating.

Comparative topology studies in Saccharomyces cerevisiae and in Escherichia coli. The N-terminal half of the yeast ABC protein Ste6

Gene fusions have provided a strategy for determining the topology of polytopic membrane proteins in Escherichia coli. To evaluate whether this highly effective approach is applicable to heterologously expressed eukaryotic integral membrane proteins, we have carried out a comparative topological study of the eukaryotic membrane protein Ste6 both in bacteria and in yeast. Ste6, is an ATP binding cassette (ABC) protein, essential for export of the a-factor mating pheromone in Saccharomyces cerevisiae. The topogenic reporters, invertase in S. cerevisiae and alkaline phosphatase in E. coli, were fused to Ste6 at identical sites and the fusions were expressed in yeast and bacteria, respectively. The results obtained in both systems are similar, although more definitive in E. coli, and support the predicted six-transmembrane spans organization of the N-terminal half of Ste6. Thus, the topological determinants for membrane insertion of polytopic proteins in prokaryotic and in eukaryotic systems appear to be highly similar. In this study we also demonstrate that Ste6 does not contain a cleaved signal sequence.

Sequence homologies between nucleotide binding regions of CFTR and G-proteins suggest structural and functional similarities

Sequence homology between the alpha-subunits of G-proteins and other GTP-binding proteins and certain regions within the nucleotide binding domains (NBDs) of cystic fibrosis transmembrane conductance regulator (CFTR) indicates that these protein structures may be similar. A sequence alignment of the NBDs of CFTR and NBDs from other membrane transporters, forms the basis of a structural model. This model predicts that one of the conserved sequences GGQR, within which a number of CF mutations occur, forms part of the nucleotide binding pocket and serves as an ON/OFF conformational switch as observed in GTP binding proteins. Furthermore, based on subtle sequence differences between the first and second NBDs of CFTR and from structure-activity data, we suggest that the nucleotide binding site environments of the two NBDs are different.

Molecular analysis of the multidrug transporter

The multidrug resistance gene product, P-glycoprotein or the multidrug transporter, confers multidrug resistance to cancer cells by maintaining intracellular levels of cytotoxic agents below a killing threshold. P-glycoprotein is located within the plasma membrane and is thought to act as an energy-dependent drug efflux pump. The multidrug transporter represents a member of the ATP-binding cassette superfamily of transporters (or traffic ATPases) and is composed of two highly homologous halves, each of which harbors a hydrophobic transmembrane domain and a hydrophilic ATP-binding fold. This review focuses on various biochemical and molecular genetic approaches used to analyze the structure, function, and mechanism of action of the multidrug transporter, whose most intriguing feature is its ability to interact with a large number of structurally and functionally different amphiphilic compounds. These studies have underscored the complexity of this membrane protein which has recently been suggested to assume alternative topological and quaternary structures, and to serve multiple functions both as a transporter and as a channel. With respect to the multidrug transporter activity of P-glycoprotein, progress has been made towards the elucidation of essential amino acid residues and/or polypeptide regions. Furthermore, the drug-stimulatable ATPase activity of P-glycoprotein has been established. The mechanism of drug transport by P-glycoprotein, however, is still unknown and its physiological role remains a matter of speculation.