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Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis. The most common mutation, a deletion of the phenylalanine at position 508 ([DELTA]F508), disrupts processing of the protein. Nearly all human CFTR-[DELTA]F508 is retained in the endoplasmic reticulum and degraded, preventing maturation to the plasma membrane. In addition, the F508 deletion reduces the activity of single CFTR channels. Human CFTR-[DELTA]F508 has been extensively studied to better understand its defects. Here, we adopted a cross-species comparative approach, examining human, pig, and mouse CFTR-[DELTA]F508. As with human CFTR-[DELTA]F508, the [DELTA]F508 mutation reduced the single-channel activity of the pig and mouse channels. However, the mutant pig and mouse proteins were at least partially processed like their wild-type counterparts. Moreover, pig and mouse CFTR-[DELTA]F508 partially restored transepithelial [Cl.sup.-] transport to CF airway epithelia. Our data, combined with earlier work, suggest that there is a gradient in the severity of the CFTR-[DELTA]F508 processing defect, with human more severe than pig or mouse. These findings may explain some previously puzzling observations in CF mice, they have important implications for evaluation of potential therapeutics, and they suggest new strategies for discovering the mechanisms that disrupt processing of human CFTR-[DELTA]F508. airway epithelia | chloride transport | cystic fibrosis | mouse models

ADP interacts with the nucleotide-binding domains (NBDs) of the cystic fibrosis transmembrane conductance regulator (CFTR) to inhibit its [Cl.sup-] channel activity. Because CFTR NBD2 has reversible adenylate kinase activity (ATP + AMP [??] ADP + ADP) that gates the channel, we asked whether ADP might inhibit current through this enzymatic activity. In adenylate kinases, binding of the two ADP molecules is cooperative. Consistent with this hypothesis, CFTR current inhibition showed positive cooperativity for ADP. We also found that ADP inhibition of current was attenuated when we prevented adenylate kinase activity with [P.sup.1],[P.sup.5-di(adenosine-5') pentaphosphate. Additional studies suggested that adenylate kinase-dependent inhibition involved phosphotransfer between two nucleotide diphosphates. These data indicate that the adenylate kinase reaction at NBD2 contributed to the inhibitory effect of ADP. Finding that ADP inhibits function via an adenylate kinase activity also helps explain the earlier observation that mutations that disrupt adenylate kinase activity also disrupt ADP inhibition. Thus, the results reveal a previously unrecognized mechanism by which ADP inhibits an ABC transporter. ATPase | chloride channel| nucleotide-binding domain | phosphotransfer

To better understand the function of the conserved C terminus of the cystic fibrosis (CF) transmembrane conductance regulator, we studied constructs containing deletions in the C-terminal tail. When expressed in well differentiated CF airway epithelia, each construct localized predominantly to the apical membrane and generated transepithelial [CI.sup.-] current. The results suggested that neither the C-terminal PSD-95/Discs-large/ZO-1 (PDZ)-interacting motif nor other C-terminal sequences were absolutely required for apical expression in airway epithelia. Surprisingly, deleting an acidic cluster near the C terminus reduced both channel opening rate and transepithelial [CI.sup.-] transport, indicating that it influences channel gating. These results may help explain the relative paucity of CF-associated mutations in the C terminus.

In developing gene therapy for cystic fibrosis (CF) airways disease, a transgene encoding a partially deleted CF transmembrane conductance regulator (CFTR) [Cl.sup.-] channel could be of value for vectors such as adeno-associated virus that have a limited packaging capacity. Earlier studies in heterologous cells indicated that the CFTR R (regulatory) domain is predominantly random coil and that parts of the R domain can be deleted without abolishing channel function. Therefore, we designed a series of CFTR variants with shortened R domains (between residues 708 and 835) and expressed them in well-differentiated cultures of CF airway epithelia. All of the variants showed normal targeting to the apical membrane, and for the constructs we tested, biosynthesis was like wild type. Moreover, all constructs generated transepithelial [Cl.sup.-] current in CF epithelia. Comparison of the [Cl.sup.-] transport suggested that the length of the R domain, the presence of phosphorylation sites, and other factors contribute to channel activity. A variant deleting residues 708-759 complemented CF airway epithelia to the same extent as wild-type CFTR and showed no current in the absence of cAMP stimulation. In addition, expression in nasal mucosa of CF mice corrected the [Cl.sup.-] transport defect. These data provide insight into the structure and function of the R domain and identify regions that can be deleted with retention of function. Thus they suggest a strategy for shortening the transgene used in CF gene therapy.