The structure of the NBDs, as exemplified by the MalK dimer of the maltose transporter MalEFGK₂ (Protein Data Bank accession no. 3RLF). (A) View along an axis perpendicular to the membrane plane from the trans-side onto the NBDs (The TMDs and SBP have been removed for clarity). Domains and highly conserved sequence motifs are color-coded: green, α-helical domain; light blue, RecA-like domain; faded gray, regulatory C-terminal domain; red, A-loop; magenta, Walker A; orange, Walker B; blue, D-loop; green, H-loop; cyan, ABC motif; yellow, Q-loop. The ATP analogue AMP-PNP is shown in sticks. (B) A closer look onto the nucleotide-binding site. The key amino acids are indicated (see NBD for details).