Figure 2.
The protein S binding site on APC. APC is depicted with its membrane-binding GLA domain on the bottom followed upward by EGF1, EGF2, and protease domains. (A) The gray oval outlines the protein S binding surface containing 5 labeled amino acid residues (green) whose mutations reduce protein S enhancement of APC’s anticoagulant activity. This surface is on the side opposite from the catalytic triad. (B) When the model in panel A is rotated 180 degrees, the active site triad is visualized on top in red; this surface in panel B lacks residues L38, K43, I73, F95, and W115. Surfaces show Arg87 (purple) whose mutation does not affect protein S cofactor activity. (Images were created using the Python Molecular Viewer v.1.5.6, and the molecular surface was computed using the Maximal Speed Molecular Surface program.17,18 )