Streptococcus equisimilis streptokinase (SK) is a single-chain protein of 414 residues that is used extensively in the clinical treatment of acute myocardial infarction due to its ability to activate human plasminogen (Plg). The mechanism by which this occurs is poorly understood due to the lack of structural details concerning both molecules and their complex. We reported recently (Parrado J et al., 1996, Protein Sci 5:693-704) that SK is composed of three structural domains (A, B, and C) with a C-terminal tail that is relatively unstructured. Here, we report thermal unfolding experiments, monitored by CD and NMR, using samples of intact SK, five isolated SK fragments, and two two-chain noncovalent complexes between complementary fragments of the protein. These experiments have allowed the unfolding processes of specific domains of the protein to be monitored and their relative stabilities and interdomain interactions to be characterized. Results demonstrate that SK can exist in a number of partially unfolded states, in which individual domains of the protein behave as single cooperative units. Domain B unfolds cooperatively in the first thermal transition at approximately 46 degrees C and its stability is largely independent of the presence of the other domains. The high-temperature transition in intact SK (at approximately 63 degrees C) corresponds to the unfolding of both domains A and C. Thermal stability of domain C is significantly increased by its isolation from the rest of the chain. By contrast, cleavage of the Phe 63-Ala 64 peptide bond within domain A causes thermal destabilization of this domain. The two resulting domain portions (A1 and A2) adopt unstructured conformations when separated. A1 binds with high affinity to all fragments that contain the A2 portion, with a concomitant restoration of the native-like fold of domain A. This result demonstrates that the mechanism whereby A1 stimulates the plasminogen activator activities of complementary SK fragments is the reconstitution of the native-like structure of domain A.