Leucine 42 in the fibronectin motif of streptokinase plays a critical role in fibrin-independent plasminogen activation

Lin Liu, Irina Yurievna Sazonova, Ryan B. Turner, Shakeel A. Chowdhry, Judy Tsai, Aiilyan K. Houng, Guy L. Reed

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Abstract

The NH2 terminus (residues 1-59) of streptokinase (SK) is a molecular switch that permits fibrin-independent plasminogen activation. Targeted mutations were made in recombinant (r) SK1-59 to identify structural interactions required for this process. Mutagenesis established the functional roles of Phe-37and Glu-39, which were projected to interact with microplasmin in the activator complex. Mutation of Leu-42 (rSK1-59(L42A)), a conserved residue in the SK fibronectin motif that lacks interactions with microplasmin, strongly reduced plasminogen activation (k(cat) decreased 50-fold) but not arnidolysis (k(cat) decreased 1.5-fold). Otherwise rSK1-59(L42A) and native rSK1-59 were indistinguishable in several parameters. Both displayed saturable and specific binding to Glu-plasminogen or the remaining SK fragment (rSKA59). Similarly rSK1-59 and rSK1-59(L42A) bound simultaneously to two different plasminogen molecules, indicating that both plasminogen binding sites were intact. However, when bound to SKΔ59, rSK1-59(L42A) was less effective than rSK1-59 in restructuring the native conformation of the SK A domain, as detected by conformation-dependent monoclonal antibodies. In the light of previous studies, these data provide evidence that SK1-59 contributes to fibrin-independent plasminogen activation through 1) intermolecular inter-actions with the plasmin in the activator complex, 2) binding interactions with the plasminogen substrate, and 3) intramolecular interactions that structure the A domain of SK for Pg substrate processing.

Original languageEnglish (US)
Pages (from-to)37686-37691
Number of pages6
JournalJournal of Biological Chemistry
Volume275
Issue number48
DOIs
StatePublished - Dec 1 2000

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ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

Cite this

Liu, L., Sazonova, I. Y., Turner, R. B., Chowdhry, S. A., Tsai, J., Houng, A. K., & Reed, G. L. (2000). Leucine 42 in the fibronectin motif of streptokinase plays a critical role in fibrin-independent plasminogen activation. Journal of Biological Chemistry, 275(48), 37686-37691. https://doi.org/10.1074/jbc.M003963200