Cystic fibrosis, one of the major human inherited diseases, is caused by defects in the CFTR (cystic fibrosis transmembrane conductance regulator), a cell-membrane protein. CFTR acts as a chloride channel which can be opened by ATP. Low-resolution structural studies of purified recombinant human CFTR are described in the present paper. Localization of the C-terminal decahistidine tag in CFTR was achieved by Ni 2+ -nitriloacetate nanogold labelling, followed by electron microscopy and single-particle analysis. The presence of the gold label appears to improve the single-particle-alignment procedure. Projection structures of CFTR from twodimensional crystals analysed by electron crystallography displayed two alternative conformational states in the presence of nucleotide and nanogold, but only one form of the protein was observed in the quiescent (nucleotide-free) state.The ABC (ATP-binding cassette) proteins are a superfamily of active transporter membrane proteins and are composed of approx. 50 functionally diverse classes of prokaryotic and eukaryotic transmembrane proteins [1]. Despite the differences in the type of substances that are transported by each protein, most members of the ABC proteins have a similar pattern of two NBDs (nucleotide-binding domains) and two TMDs (transmembrane domains) [2]. The CFTR (cystic fibrosis transmembrane conductance regulator), also termed ABCC7, is a unique member of the ABC superfamily of transporter proteins, which works as an ion channel and has an extra domain called the regulatory domain (R) located between NBD1 and TMD2 [3]. Channel activity is affected by phosphorylation (by protein kinases A and C) and by ATP. CFTR regulates secretion and reabsorption of ions at epithelial surfaces [4]. Defects in the channel function and/or folding and processing lead to cystic fibrosis [5].Structural data for many ABC transporter proteins suspected to be responsible for human diseases are mostly lacking. Structural studies of ABC proteins were successful in determining the high-resolution structures of MsbA (bacterial lipid transporter) [6,7] and BtuCD (bacterial vitamin B 12 transporter) [8] and the low-resolution structures

Cystic fibrosis, one of the major human inherited diseases, is caused by defects in the CFTR (cystic fibrosis transmembrane conductance regulator), a cell-membrane protein. CFTR acts as a chloride channel which can be opened by ATP. Low-resolution structural studies of purified recombinant human CFTR are described in the present paper. Localization of the C-terminal decahistidine tag in CFTR was achieved by Ni2+-nitriloacetate nanogold labelling, followed by electron microscopy and single-particle analysis. The presence of the gold label appears to improve the single-particle-alignment procedure. Projection structures of CFTR from two-dimensional crystals analysed by electron crystallography displayed two alternative conformational states in the presence of nucleotide and nanogold, but only one form of the protein was observed in the quiescent (nucleotide-free) state.