Regulation of plasmin-dependent fibrin clot lysis by annexin II heterotetramer

In a previous report we showed that plasmin-dependent lysis of a fibrin pol ymer, produced from purified components, was totally blocked if annexin II heterotetramer (AIIt) was present during fibrin polymer formation. Here, we show that AIIt inhibits fibrin clot lysis by stimulation of plasmin autode gradation, which results in a loss of plasmin activity. Furthermore, the C- terminal lysine residues of its pll subunit play an essential role in the i nhibition of fibrin clot lysis by AIIt, We also found that AIIt binds to fi brin with a K-d of 436 nM and a stoichiometry of about 0.28 mol of AIIt/mol of fibrin monomer. The binding of AIIt to fibrin was not dependent on the C-terminal lysines of the p11 subunit, Furthermore, in the presence of plas minogen, the binding of AIIt to fibrin was increased to about 1.3 mol of AI It/mol of fibrin monomer, suggesting that AIIt and plasminogen do not compe te for identical sites on fibrin. Immunohistochemical identification of p36 and p11 subunits of AIIt in a pathological clot provides important evidenc e for its role as a physiological fibrinolytic regulator. These results sug gest that AIIt may play a key role in the regulation of plasmin activity on the fibrin clot surface.