Background: SK, a heterogeneous PA protein from groups A, C, and G streptococci (GAS, GCS, GGS, respectively) contains three structural domains (SKα , SKβ , and SK ). Based on the variable region of SKβ , GAS-SKS (ska) are clustered as SK1 and SK2 (including SK2a/SK2b), which show low and high FG-dependent Plg activation properties, respectively. Despite being co-clustered as SK2a, GCS/GGS-SKS (skcg) variants display properties similar to SK1. Herein, by SKβ exchange between GGS (G88) and GAS-SK2a (STAB902) variants, the potential roles of SK domains in amidolytic/proteolytic activity and FG-bound-Plg activation are represented. Methods: Two parental SKG88 and SKSTAB902 genes were cloned into the NdeI/XhoI site of pET26b expression vector. The two chimeric SKβ-exchanged constructs (SKC1: α G88-β STAB-γ G88 and SKC2; α STAB-β G88-γ STAB) were constructed by BstEII/BsiWI digestion/crossligation in parental plasmids. SKS were expressed in E. coli and purified by Ni-NTA chromatography. PA potencies of SKS were measured by colorimetric assay. Results: SDS-PAGE and Western-blot analyses confirmed the proper expression of 47-kDa SKS. Analyses indicated that the catalytic efficiency (Kcat/Km) for amidolytic and proteolytic activity were less and moderately dependent on SKβ , respectively. The increase of FG-bound-Plg activation for SKSTAB902/SKC1 containing SK2aβ was around six times, whereas for SKG88/SKC2 containing skcgβ , it was four times. Conclusion: Although SKβ has noticeable contribution in FG-bound-Plg activation activity, it had minor contribution in fibrin-independent, amidolytic activity. These data might be of interest for engineering fibrinspecific versions of SK.