Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-binding cassette (ABC) transporter that functions as a chloride channel. Nucleotide-binding domain 1 (NBD1), one of two ABC domains in CFTR, also contains sites for the predominant CF-causing mutation and, potentially, for regulatory phosphorylation. We have determined crystal structures for mouse NBD1 in unliganded, ADP-and ATP-bound states, with and without phosphorylation. This NBD1 differs from typical ABC domains in having added regulatory segments, a foreshortened subdomain interconnection, and an unusual nucleotide conformation. Moreover, isolated NBD1 has undetectable ATPase activity and its structure is essentially the same independent of ligand state. Phe508, which is commonly deleted in CF, is exposed at a putative NBD1-transmembrane interface. Our results are consistent with a CFTR mechanism, whereby channel gating occurs through ATP binding in an NBD1-NBD2 nucleotide sandwich that forms upon displacement of NBD1 regulatory segments.

The major facilitator superfamily (MFS) represents the largest group of secondary active membrane transporters, and its members transport a diverse range of substrates. Recent work shows that MFS antiporters, and perhaps all members of the MFS, share the same three-dimensional structure, consisting of two domains that surround a substrate-translocation pore. The advent of crystal structures of three MFS antiporters sheds light on their fundamental mechanism; they operate via a single-binding site, alternating-access mechanism, involving a rocker-switch type movement of the two halves of the protein. In the sn-glycerol-3-phosphate transporter (GlpT) from E. coli, the substrate-binding site is formed by several charged residues and a histidine that can be protonated. Salt bridge formation and breakage is involved in the conformational changes of the protein during transport. In this review, we attempt to set forth a set of mechanistic principles that characterize all MFS antiporters.

Cystic fibrosis is caused by defects in the cystic fibrosis transmembrane conductance regulator (CFTR), commonly the deletion of residue Phe-508 (⌬F508) in the first nucleotide-binding domain (NBD1), which results in a severe reduction in the population of functional channels at the epithelial cell surface. Previous studies employing incomplete NBD1 domains have attributed this to aberrant folding of ⌬F508 NBD1. We report structural and biophysical studies on complete human NBD1 domains, which fail to demonstrate significant changes of in vitro stability or folding kinetics in the presence or absence of the ⌬F508 mutation. Crystal structures show minimal changes in protein conformation but substantial changes in local surface topography at the site of the mutation, which is located in the region of NBD1 believed to interact with the first membrane spanning domain of CFTR. These results raise the possibility that the primary effect of ⌬F508 is a disruption of proper interdomain interactions at this site in CFTR rather than interference with the folding of NBD1. Interestingly, increases in the stability of NBD1 constructs are observed upon introduction of second-site mutations that suppress the trafficking defect caused by the ⌬F508 mutation, suggesting that these suppressors might function indirectly by improving the folding efficiency of NBD1 in the context of the full-length protein. The human NBD1 structures also solidify the understanding of CFTR regulation by showing that its two protein segments that can be phosphorylated both adopt multiple conformations that modulate access to the ATPase active site and functional interdomain interfaces.Cystic fibrosis causes lung, liver, pancreas, and reproductive tract disorders, typically leading to death prior to middle age from deterioration in pulmonary function (1). CFTR 1 protein is composed of two membrane spanning domains (MSD1 and MSD2), two nucleotide-binding domains (NBD1 and NBD2), and a regulatory region (R). Although it functions as an ATPgated anion channel, CFTR is a member of the ATP-binding cassette (ABC) transporter superfamily (2) based on high sequence similarity between the NBDs and canonical ABC domains. Understanding the exact molecular pathology caused by the ⌬F508 mutation in CFTR is of great importance in the development of drugs to treat cystic fibrosis because of the prevalence of this mutation in the human population. ⌬F508 CFTR fails to mature appropriately in the endoplasmic reticulum and is poorly populated in the epithelial membrane (3-6). It has been proposed that the primary effect of the ⌬F508 mutation is to cause misfolding of NBD1, which leads to aberrant transport and ultimately targeted proteolytic degradation of CFTR (7,8). Channels harboring the deletion show enhanced sensitivity to proteolytic degradation (9) but have at least partial wild-type chloride conductance properties (4, 10). Canonical ABC domain structures are composed of three subdomains, a central F1-type ATP-binding core subdomain, an antiparallel ␤-sheet (ABC␤) s...