The N-terminal region of SK has the sequence $^42LTSRPAHG^50$ homologous to that of fibronectin. Although the functions of this motif is not elucidated, it is supposed that the motif function as a binding site to form SK-plasminogen complex or as a plasminogen recognition site of the complex in the activating reaction. Mutations were introduced at these residues ($^44S$, $^45R$, $^46P$) by PCR method. Among these mutants, the mutant which had Cys instead of 44Ser showed the lowest activity. The others showed similar activity to that of wild-type SK. To define if this mutant has intact structure or not, CD spectroscopy analysis was performed and this analysis showed that the mutant, Ser44→Cys, had slightly changed structure. Also to prove binding of this mutant to plasminogen, size-exclusion column chromatographic analysis was performed. The results showed that this mutant had binding activity. Therefore the residue, $^44Ser$, is important in catalytic activity of SK-plasminogen complex and the sequence $^42LTSRPAHG^50$ plays a catalytic role in this complex.
On the other hand, although SK is not a protease, it has been suggested that SK has evolved from an ancestral serine protease by gene duplication and fusion. The chimeric protein, which has Streptomyces griseus trypsin-type protease as the N-terminal region and 165 residues of SK C-terminus as the C-terminal region, is expected to activate plasminogen for fibrinolysis by plasminogen binding through C-terminal region of SK. This chimera was expressed in Bacillus subtilis and checked activity of SK and trypsin. Although the trypsin activity remained, SK activity was not shown. This is that the expressed chimera was digested by its trypsin activity. And the candidates of intermolecular proteolytic digestion were $^228Arg$ or $^231Lys$. Therefore C-terminal region of SK alone was so unstable that N-terminal region of SK donate the stability of whole structure.