A. The tetramer is shown in two different orientations, perpendicular to the plane of the ring, and parallel to the plane of the ring. Subunit A (red), subunit B (green), subunit X (purple -gray) and subunit Y (yellow). Bound CoA molecule is shown as a space-filling model. The two 2-fold non-crystallographic rotation axes in the plane of the ring are indicated by arrows and the 2-fold non-crystallographic rotation axis perpendicular to the plane of the ring is indicated by a filled oval. B. ecNAGK-like AAK-AAK domain interface between subunits A and B; α-helices, H5 and H6, and β-strands, B3 and B8, form this interface. C. N-terminal helix interface between subunits A and X, formed by interactions between the two N-terminal α-helices and two neighboring helices, H3 and H10. D. NAT-NAT domain interface between subunits A and X. Two α-helices, H14 and H15, and one β-strand, B18, form this interface.