ABC transporter structure. Two structures of the bacterial lipid transporter MsbA, a representative ABC transporter. The nucleotide-free, inward-facing form is on the left and the ATP-bound, outward-facing form is shown in two views (rotated by 90 degrees) in the center and on the right. The two ATP-binding cassettes (blue) are related to the P-loop NTPases and, like them, contain P-loops (green). The α helix adjacent to the P-loop is shown in red. [Drawn from 3B5W and 3B60.pdb.]