FIG. 1.

(A) Residues in and around the substrate binding pocket of FIV PR, shown on one chain of the homodimer. These residues were the focus of the substitutions in this study. Earlier studies had identified the I35³⁰D and I57⁴⁸G substitutions as intolerant to change in the background of FIV PR, whereas other substitutions allow maintenance of activity and contribute to the substrate and inhibitor specificity significantly. Residues outside the substrate-binding pocket but in close proximity to I35³⁰D or I57⁴⁸G are shown in gray and black, respectively. (B) Amino acid sequence alignment of FIV and HIV-1 PRs based on crystal structures (41). *, catalytic aspartic acids (D30 of FIV PR and D25 of HIV-1PR). Numbers indicate the positions of structurally equivalent residues.