NMR secondary structure of the plasminogen activator protein staphylokinase

Staphylokinase (Sak) is a 15.5 kDa protein secreted by several strains of Staphylococcus aureus. Due to its ability to convert plasminogen, the inactive proenzyme of the fibrinolytic system, into plasmin, Sak is presently undergoing clinical trials for blood clot lysis in the treatment of thrombovascular disorders. With a view to developing a better understanding of the mode of action of Sak, we have initiated a structural investigation of Sak via multidimensional heteronuclear NMR spectroscopy employing uniformly 15N- and 15N, 13C-labelled Sak. Sequence-specific resonance assignments have been made employing 15N-edited TOCSY and NOE experiments and from HNCACB, CBCA(CO)NH, HBHA-(CBCACO) NH and CC(CO)NH sets of experiments. From an analysis of the chemical shifts, 3JHNH alpha scalar coupling constants, NOEs and HN exchange data, the secondary structural elements of Sak have been characterized.