For diffraction data collection, crystals were immersed in liquid nitrogen after cryoprotection in 20% glycerol. Data were collected at 110 °K on beamline 8.2.1 at the Advanced Light Source (Lawrence Berkeley National Laboratory, Berkeley). Integration, scaling and merging of the diffraction data were done with the Mosflm program 52 . The structures were solved by molecular replacement using the programs Phaser 53 and Phenix 54 . For CNGA1, the structure of HIV gp41 N-trimer pocket region 33 (3L36) was used as a search probe. For CNGA3, the structure of SARS virus S2 protein 34 (1ZVB) was used as a search probe. Molecular models were rebuilt using ARP/wARP (ref. 55) (CNGA1) or the Phenix software suite 54 (CNGA3). Structure refinement and validation were performed using the Phenix software suite 54 and the program Coot 56 using Fo -Fc ′omit′ maps. Based on the Ramachandran plot, the percent of amino acids in favoured/allowed/ disallowed conformations were 99.37/0.63/0.0 for CNGA1 and 100.00/0.0/0.0 for CNGA3. The coiled-coil parameters were analysed by the program TWISTER 57 .