The nomenclature has been described in Fig. 1. Highlighted residues are important for various functions, including PPIase activity, substrate binding, and interaction with immunosuppressant drugs (CsA for the CyP domain and FK506 for the FKBP domain). A, homology of the FKBP domains. The prototype human FKBP (hFKBP12) and the P. falciparum FKBP35 (21, 22) are included for comparison. B, homology of the CyP domains. The prototype human CyP (hCyP18) is included for comparison. Residues important for CN binding are underlined. The W (for Trp), marked with overhead circle, is especially important for CsA binding but is replaced by H (for His) in Toxoplasma, which explains the relative CsA resistance of the latter (Fig. 4). Note how the residue numbers differ between the Toxoplasma and the bacterial FCBPs because of the reversal of the domain arrangement.