Domain structure and domain-domain interactions of recombinant tissue plasminogen activator

The melting of recombinant tissue plasminogen activator (rtPA) has been investigated by differential scanning calorimetry and fluorescence spectroscopy. At neutral pH, rtPA melts with only partial reversibility in a single sharp peak that can be deconvoluted into four transitions. By contrast, at acidic pH the melting process is spread over a broad range of temperature and is highly reversible. Under these conditions five transitions are resolved by deconvolution analysis. Additional measurements in 6 M guanidinium chloride reveal a sixth transition representing an extremely stable domain. Comparison of the melting curves of several fragments with those of the parent protein allowed all of the transitions to be assigned. The results indicate that rtPA is comprised of six independently folded domains. Two of these domains correspond to the two kringle modules whose thermodynamic properties are similar to those of the kringles in plasminogen. Two additional domains are formed by the epidermal growth factor (EGF)-like and finger modules, the latter of which is extremely stable, requiring the presence of a chemical denaturant for its melting to be observed. The serine protease module contains two more domains which at neutral pH melt cooperatively in a single transition but at low pH melt independently, accounting for the greater number of transitions observed there. Measurements with a 50-kDa fragment lacking the C-terminal half of the serine protease module and with a variant lacking the finger and EGF domains indicate that the serine protease domains interact strongly with and are stabilized by the finger and/or EGF domains in the intact protein. This interaction between domains located at opposite ends of the rtPA molecule produces a more compact structure. A better understanding of such interactions may enhance efforts to engineer plasminogen activators with improved thrombolytic properties.