Crystal structure of streptokinase β-domain

Streptokinase, a 47 kDa secreted protein of hemolytic strains of streptococci, is a human plasminogen activator and contains three structural domains linked by flexible loops. We describe here the crystal structure of the isolated streptokinase middle (SK beta) domain determined at 2.4 Angstrom resolution, Among the functionally important structural features is a putative binding site for a kringle domain of plasminogen located at the tip of a fully exposed hairpin loop. The distribution of genetically conserved residues of SK beta is strongly correlated with their functions. The extensive interface of the SK beta dimer suggests that such dimers may also exist in solution for free SK beta. (C) 1999 Federation of European Biochemical Societies.