Conformational changes of NBDs in CFTR. (A) Widely separated NBDs in unphosphorylated CFTR. Top left: Superimposed cryo-EM structures for hCFTR (blue; Protein Data Bank [PDB] accession no. 5UAK) and zCFTR (yellow; PDB accession no. 5UAR). Bottom: Top view of NBDs. All conserved motifs are labeled as ABC, ABC signature sequence; Q, Q loop; H, H loop; A, Walker A; B, Walker B; and D, D loop. Right: magnified views of those labeled i–vi in the top view. (B) Phosphorylated E1372Q-zCFTR (PDB accession no. 5W81). The catalytic glutamate E1372 (E1371 in hCFTR) was mutated to glutamine to abolish ATP hydrolysis in site 2. Top left: cryo-EM structure of zCFTR (PDB accession no. 5W81; TMD1 in blue; TMD2 in cyan; NBD1 in gray; NBD2 in green). Bottom: Top view of NBD. Right: i and ii are magnified images showing the interactions between conserved motifs and ATP in dimeric NBDs.