No abstract

Maturation of wild-type CFTR nascent chains at the endoplasmic reticulum (ER) occurs inefficiently; many disease-associated mutant forms do not mature but instead are eliminated by proteolysis involving the cytosolic proteasome. Although calnexin binds nascent CFTR via its oligosaccharide chains in the ER lumen and Hsp70 binds CFTR cytoplasmic domains, perturbation of these interactions alone is without major influence on maturation or degradation. We show that the ansamysin drugs, geldanamycin and herbimycin A, which inhibit the assembly of some signaling molecules by binding to specific sites on Hsp90 in the cytosol or Grp94 in the ER lumen, block the maturation of nascent CFTR and accelerate its degradation. The immature CFTR molecule was detected in association with Hsp90 but not with Grp94, and geldanamycin prevented the Hsp90 association. The drug-enhanced degradation was decreased by lactacystin and other proteasome inhibitors. Therefore, consistent with other examples of countervailing effects of Hsp90 and the proteasome, it would seem that this chaperone may normally contribute to CFTR folding and, when this function is interfered with by an ansamycin, there is a further shift to proteolytic degradation. This is the first direct evidence of a role for Hsp90 in the maturation of a newly synthesized integral membrane protein by interaction with its cytoplasmic domains on the ER surface.

Cystic fibrosis transmembrane conductance regulator (CFTR) is a non-rectifying, low-conductance channel regulated by ATP and phosphorylation, which mediates apical chloride conductance in secretory epithelia and malfunctions in cystic fibrosis (CF). Mutations at Lys 335 and Arg 347 in the sixth predicted transmembrane helix of CFTR alter its halide selectivity in whole-cell studies and its single channel conductance, but the physical basis of these alterations is unknown and permeation in CFTR is poorly understood. Here we present evidence that wild-type CFTR can contain more than one anion simultaneously. The conductance of CFTR passes through a minimum when channels are bathed in mixtures of two permeant anions. This anomalous mole fraction effect can be abolished by replacing Arg 347 with an aspartate and can be toggled on or off by varying the pH after the same residue is replaced with a histidine. Thus the CFTR channel should provide a convenient model in which to study multi-ion pore behaviour and conduction. The loss of multiple occupancy may explain how naturally occurring CF mutations at this site cause disease.

Membrane transporters of the adenine nucleotide binding cassette (ABC) superfamily utilize two either identical or homologous nucleotide binding domains (NBDs). Although the hydrolysis of ATP by these domains is believed to drive transport of solute, it is unknown why two rather than a single NBD is required. In the well studied P-glycoprotein multidrug transporter, the two appear to be functionally equivalent, and a strongly supported model proposes that ATP hydrolysis occurs alternately at each NBD (Senior, A. E., al-Shawi, M. K., and Urbatsch, I. L. (1995) FEBS Lett 377, 285-289). To assess how applicable this model may be to other ABC transporters, we have examined adenine nucleotide interactions with the multidrug resistance protein, MRP1, a member of a different ABC family that transports conjugated organic anions and in which sequences of the two NBDs are much less similar than in P-glycoprotein. Photoaffinity labeling experiments with 8-azido-ATP, which strongly supports transport revealed ATP binding exclusively at NBD1 and ADP trapping predominantly at NBD2. Despite this apparent asymmetry in the two domains, they are entirely interdependent as substitution of key lysine residues in the Walker A motif of either impaired both ATP binding and ADP trapping. Furthermore, the interaction of ADP at NBD2 appears to allosterically enhance the binding of ATP at NBD1. Glutathione, which supports drug transport by the protein, does not enhance ATP binding but stimulates the trapping of ADP. Thus MRP1 may employ a more complex mechanism of coupling ATP utilization to the export of agents from cells than P-glycoprotein.The multidrug resistance protein, MRP1, 1 is believed to function as an active exporter of many conjugated organic anions from cells (1-7). Among agents that are transported are also some unconjugated compounds, including certain cancer drugs, provided that glutathione is also present (8, 9). In this case, the hydrophilic conjugating compound such as glutathione may be co-transported along with the hydrophobic drug (8). As a member of the ABC super family of membrane transporters (10), MRP1 is an ATPase that is stimulated by agents that it transports (11, 12). However, as yet there is little further information about how the two nucleotide binding domains of the protein act to bring about the transport event. In the case of the better studied P-glycoprotein, which belongs to a different family of this super family (13), the two NBDs have been shown to be functionally equivalent with identical ATP hydrolysis steps occurring alternatively at each domain (14, 15). According to an insightful model based largely on this information, binding and translocation of the hydrophobic compound exported is controlled in an ordered fashion by the two hydrolysis steps (15). The objective of the present study was to determine whether MRP1 performs a similar symmetrical cycle of partial reactions of ATP hydrolysis to accomplish the export of conjugated organic anions. To follow the interaction of the nucleotide substr...

Permeability of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel to polyatomic anions of known dimensions was studied in stably transfected Chinese hamster ovary cells by using the patch clamp technique. Biionic reversal potentials measured with external polyatomic anions gave the permeability ratio (PX/PCl) sequence NO3 − > Cl− > HCO3 − > formate > acetate. The same selectivity sequence but somewhat higher permeability ratios were obtained when anions were tested from the cytoplasmic side. Pyruvate, propanoate, methane sulfonate, ethane sulfonate, and gluconate were not measurably permeant (PX/PCl < 0.06) from either side of the membrane. The relationship between permeability ratios from the outside and ionic diameters suggests a minimum functional pore diameter of ∼5.3 Å. Permeability ratios also followed a lyotropic sequence, suggesting that permeability is dependent on ionic hydration energies. Site-directed mutagenesis of two adjacent threonines in TM6 to smaller, less polar alanines led to a significant (24%) increase in single channel conductance and elevated permeability to several large anions, suggesting that these residues do not strongly bind permeating anions, but may contribute to the narrowest part of the pore.

Human multidrug resistance protein (MRP) was expressed at high levels in stably transfected baby hamster kidney (BHK-21) cells. These cells exhibited a pattern of cross-resistance to several different drugs typical of an MRP-mediated phenotype despite the addition of 10 histidine residues at the C terminus to facilitate purification. Consistent with this functional evidence of the presence of MRP at the surface of these transfectants, strong signals were detected by immunoblotting and immunofluorescence using a specific monoclonal antibody to MRP. There was intense uniform staining of the cell surface as well as weaker staining of intracellular membranes. MRP-containing membranes were solubilized in 1% N-dodecyl-␤-D-maltoside in the presence of 0.4% sheep brain phospholipids. Two sequential affinity purification steps on Ni-NTA agarose and wheat germ agglutinin agarose provided substantial enrichment, and contaminating bands were not detected. ATPase activity of the purified protein was assayed in the presence of the phospholipids, which had been maintained throughout all purification steps. ATP was hydrolyzed in proportion to the amount of purified protein assayed, and typical Michaelis-Menten behavior was exhibited, yielding estimations of K m of ϳ3.0 mM and V max of 0.46 mol mg ؊1 min ؊1 . This activity was moderately stimulated by the drugs that others have shown to be transported by MRP-containing membrane vesicles. This stimulation was enhanced by reduced glutathione as is its drug transport, and oxidized glutathione, itself a substrate for transport, caused a strong stimulation. These data describe the first purification of MRP and provide the first direct evidence that the molecule possesses drug-stimulated ATPase activity.The multidrug resistance-associated protein (MRP) 1 was discovered in multidrug-resistant tumor cells that did not express any of the structurally related P-glycoproteins (1). Several such cell lines have been described (2-7), although drug-resistant tumor cells frequently express both MRP and P-glycoprotein. MRP function has since been characterized using cells in which it is either endogenously or heterologously expressed and in membrane vesicles containing the protein, isolated from these cells (8 -18). Results of these studies are consistent with the notion that the protein brings about the active export of a number of anti-tumor drugs provided that intracellular glutathione is present (12,19). Significantly, the protein has also been found to transport conjugated natural substrates of physiological importance, including glutathione-conjugated leukotrienes (9, 11, 13, 19 -21) and glucuronide-conjugated steroids (14). More recently, cDNAs have been cloned for several socalled multispecific organic anion transporters, and their sequences indicate that they are homologs of MRP (22,23). The sequences of these molecules indicate that they all contain two nucleotide binding domains, and their transport functions require ATP. Therefore, they may all be transport ATPases. However, altho...

Many cystic fibrosis transmembrane conductance regulator (CFTR) mutants are recognized as aberrant by the quality control apparatus at the endoplasmic reticulum (ER) and are targeted for degradation. The mechanism whereby nascent chains are distinguished as either competent or incompetent for ER export has not been elucidated. Here we show that export-incompetent chains display multiple arginine-framed tripeptide sequences like the one recently identified in ATP-sensitive K+ channels. Replacement of arginine residues at positions R29, R516, R555, and R766 with lysine residues to inactivate four of these motifs simultaneously causes delta F508 CFTR, present in approximately 90% of CF patients, to escape ER quality control and function at the cell surface. Interference with recognition of these signals may be helpful in the management of CF.

Multidrug resistance protein (MRP1) utilizes two nonequivalent nucleotide-binding domains (NBDsOverexpression of either multidrug resistance P-glycoprotein (P-gp) 1 or multidrug resistance protein (MRP1) or both confers resistance to a broad range of anti-cancer drugs (3, 4). Both P-gp and MRP1 are members of the ATP-binding cassette (ABC) transporter superfamily containing membrane spanning domains and two nucleotide-binding domains (NBDs) (5-7). Although they pump somewhat different solutes out of cells, i.e. P-gp hydrophobic compounds (8 -10), and MRP1, anionic conjugates, such as glutathione-, glucuronide-, or sulfate-conjugated aliphatic, prostanoid and heterocyclic compounds (11-19), they both couple ATP binding/hydrolysis to transport of solutes (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). However, whether they share the same mechanism of this coupling is not clear. In the case of P-gp, the two NBDs have been shown to be functionally equivalent with identical ATP hydrolysis steps occurring alternatively at each NBD (21,22). When one site enters the transition-state conformation the other site is prohibited from doing so (21, 23) and trapping of nucleoside diphosphate by vanadate in either site blocks catalysis at both sites (21,23,24). Covalent inactivation of either site completely blocks turnover of ATP hydrolysis (21,25). A working model of coupling ATP hydrolysis alternatively at each NBD and solute transport in each cycle (one solute transport per ATP hydrolysis) has been proposed (26). In one other report, the two NBDs of P-gp were found to be essential for its function but not entirely symmetric (27). Ambudkar's group (28) reported recently that there are two independent ATP hydrolysis events in a single catalytic cycle of P-gp, one associated with efflux of drug and the other to bring about conformational changes that "reset" the molecule (29). In this interpretation, the ATP-binding sites of P-gp are recruited in a random manner during hydrolysis events and only one is utilized at any given time because of the conformational change in the catalytic site that drastically reduces the affinity of the second site for nucleotide binding (29). In the case of MRP1, however, the two NBDs definitely do not seem to be functionally equivalent. Sequence dissimilarities (7) between the two NBDs may be a clue to unequal functions. Mutations of the consensus Walker motifs in the two NBDs have different effects on solute transport. For example, mutation of the Walker A lysine residue in NBD2 (Lys 1333 ) abolished leukotriene C4 transport almost completely (1, 2), whereas the corresponding mutant in NBD1 (Lys 684 ) retained some activity (1, 2). Photoaffinity labeling experiments with 8-azido-ATP also revealed an asymmetry between NBD1 and NBD2, with [␥-32 P]8-N 3 ATP preferentially labeling NBD1 (1, 2), with trapping of the ATP