The packing of the coiled-coil domains followed the characteristic 'knobs into holes' arrangement with the 'a' and 'd' positions forming alternating layers of the hydrophobic core of the coiled coil ( Fig. 7) 32, 35 . The two noncontiguous heptad repeat regions assembled into a single continuous coiled coil. Interestingly, the CNGA1 and CNGA3 structures differed in the length of the two heptad repeat regions. For CNGA1, the first region contained 3 heptad repeats and the second contained 3.5 repeats (Fig. 7b) . For CNGA3 the first region contained 4 heptad repeats and the second contained 2.5 repeats (Fig. 7c) . The difference appears to be the substitution of an isoleucine for a leucine after the third heptad repeat in CNGA1 (Fig. 1b) .