The trimerization motif is structurally conserved. (A) Frequency of occurrence of the sequence pattern R-[ILVM]-X-X-[ILV]-E in a structurally verified set of short (≤50 amino acid residues) parallel (black bars) and antiparallel (gray bars) coiled coils. (B) Superimposition of trimerization motifs found in ccCor1 (red), simian immunodeficiency virus gp41 (blue; PDB ID code 1qbz), visna TM (yellow, PDB ID code 1jek), ccβ-p (magenta; PDB ID code 1s9z), p4 (cyan; PDB ID code 1kyc), and influenza hemagglutinin (salmon; PDB ID code 1eo8) is shown in stereo and in line representation.