Mutagenesis of the Y176 and L177 residues in helix 9 of the FIV CA protein. (A) Ribbon representation of the crystal structure of the FIV CA (PDB 5NA2). The α-helices H1–H7 and H8–H11, corresponding to the N-terminal domain (CA-NTD) and C-terminal domain (CA-CTD), respectively, are shown. The N-terminal β-hairpin and the cyclophilin A-binding loop (CypA-BL) are also indicated. (B) Alignment of the amino acid sequences of helix 9 of FIV [33], human immunodeficiency virus type 1 (HIV-1) [51], and equine infectious anemia virus (EIAV) [31] CA proteins. Identical amino acids at the same positions in two or three sequences are highlighted in red, whereas similar residues are indicated in blue. The assembly-relevant motif W184/M185 of HIV-1 CA as well as the amino acids present at equivalent positions in the FIV and EIAV CA helix 9 are underlined. (C) Amino acid substitutions that were designed to modify the Y176/L177 motif in the FIV CA. Below the sequence of the wild-type (WT) FIV CA helix 9, the amino acids replacing Y176, L177, or both residues of the motif are listed.