Structural correlates of a functional streptokinase antigenic epitope: serine 138 is an essential residue for antibody binding

We determined the pattern of cross-reactivity of a panel of anti-streptokinase (SK) monoclonal antibodies (mAbs) with SK variants in order to map the antigenic and functional epitope of SK. Comparison of the pattern of cross-reactivity of the anti-SK mAb A4.3 with SK variants and sequence alignments of SK variants and native (n) SK suggested that mutation of Ser 138 to Lys results in loss of binding of mAb A4.3 to SK variants. However, this mutation does not affect formation of activator complex by these proteins. The epitope specificity of the mAb A4.3 was further confirmed by mutating Ser 138 to Lys in n SK. Monoclonal Ab A4.3 did not bind to mutant SK (Ser138Lys). Activator activity of mutant SK (Ser138Lys) was indistinguishable from that of n SK and recombinant n SK. Since addition of A4.3 mAb to an equimolar mixture of SK and human plasminogen inhibits activator complex formation, the sequences spanning position 138 are likely important for formation of streptokinase-plasminogen activator complex or processing of the plasminogen substrate.