The structures of both CNGA1#621-690 and CNGA3#626-672 consisted of long (65 Å) parallel three-helix coiled-coil domains (Fig. 6) . The sequence beyond the CLZ domain in CNGA1 (amino acids 665-675 for chain A; 665-676 for chain B; and 665-670 for chain C) flared outward and was partially involved in crystal packing interactions (Supplementary Fig. S4 ). In addition, the CNGA1#621-690 structure contained two bound heavy atoms per trimer with an electron density greater than 10 sigma ( Fig. 6 ; Supplementary Fig. S5 ). These heavy atoms were coordinated by acidic residues (E629 and D633 for the proximal site, and E671 and E623 for the distal ion) at the interface between different trimers in the crystal and, therefore, are presumed to be non-physiological ( Supplementary Fig. S5 ). They are likely to be Zn 2 + ions because of their coordination and the requirement for Zn 2 + in the crystallization solution to produce crystals.