An anti-apoptotic protein human survivin is a direct inhibitor of caspase-3 and-7

Survivin, an apoptosis inhibitor/cell-cycle regulator, is critically requir ed for suppression of apoptosis and ensuring normal cell division in the G2 /M phase of the cell cycle. It is highly expressed in a cell cycle-regulate d manner and localizes together with caspase-3 on microtubules within centr osomes. Whether survivin is a physiologically relevant caspase inhibitor ha s been unclear due to the difficulties with obtaining correctly folded surv ivin and finding the right conditions for inhibition assay. In this study, recombinant, active human survivin was expressed in Escherichia coli and pu rified to homogeneity. The protein, existing as a homodimer in solution, bi nds caspase-3 and -7 tightly with dissociation constants of 20.9 and 11.5 n M, respectively, when evaluated by surface plasmon resonance spectroscopy. Consistently, survivin potently inhibits the cleavage of a physiological su bstrate poly(ADP-ribose) polymerase and an artificial tetrapeptide by caspa se-3 and -7 in vitro with apparent inhibition constants of 36.0 and 16.5 nM , respectively. The data suggest that sequestering caspase-3 and -7 in inhi bited states on microtubules is at least one mechanism of survivin in the s uppression of default apoptosis in the G2/M phase. The localization of surv ivin on microtubules, which is essential for its function, should increase the protective activity at the action site.