Isolation and characterization of Staphylocoagulase chymotryptic fragment. Localization of the procoagulant- and prothrombin-binding domain of this protein

Several strains of Staphylococcus aereus secrete a protein, staphylocoagulase, that binds stoichiometrically to human prothrombin, resulting in a coagulant complex designated staphylothrombin. In the present study, staphylocoagulase was digested with α-chymotrypsin and the resulting fragments were isolated by gel filtration. One fragment (M(r) 43,000) exhibited a high affinity for human prothrombin (K(d) = 1.7 x 10^-9 M), which is comparable to the affinity observed using intact staphylocoagulase (K(d) = 4.6 x 10^-10 M). A complex of the M(r) 43,000 fragment and prothrombin possessed both clotting and amidase activity essentially identical to that observed in a complex of intact staphylocoagulase and prothrombin. A second fragment (M(r) 30,000) exhibited weaker affinity for prothrombin (K(d) = 1.2 x 10^-7 M). While clotting activity was not observed with a complex of this fragment and prothrombin, it nonetheless possessed a weak amidase activity. A third fragment (M(r) 20,000) was found to bind to prothrombin, but the resultant complex did not exhibit clotting or amidase activity. Amino-terminal sequence analyses of these staphylocoagulase fragments revealed that the M(r) 43,000 fragment constitutes the amino-terminal portion of staphylocoagulase and also contains the M(r) 30,000 and 20,000 fragments. Moreover, the amino-terminal sequence of the M(r) 20,000 fragment was identical to that observed for the M(r) 30,000 fragment. From these results, we conclude that the functional region of staphylocoagulase for binding and activation of human prothrombin is localized in the amino-terminal region of the intact bacterial protein.