Structural basis for binding of Smac/DIABLO to the XIAP BIR3 domain

The inhibitor-of-apoptosis proteins (IAPs)(1) regulate programmed cell deat h by inhibiting members of the caspase family of enzymes(2-5). Recently, a mammalian protein called Smac(6) (also named DIABLO(7)) was identified that binds to the IAPs and promotes caspase activation. Although undefined in t he X-ray structure, the amino-terminal residues of Smac are critical for it s function(8,9). To understand the structural basis for molecular recogniti on between Smac and the IAPs, we determined the solution structure of the B IR3 domain of X-linked IAP (XIAP) complexed with a functionally active nine -residue peptide derived from the N terminus of Smac. The peptide binds acr oss the third beta -strand of the BIR3 domain in an extended conformation w ith only the first four residues contacting the protein. The complex is sta bilized by four intermolecular hydrogen bonds, an electrostatic interaction involving the N terminus of the peptide, and several hydrophobic interacti ons. This structural information, along with the binding data from BIR3 and Smac peptide mutants reported here, should aid in the design of small mole cules that may be used for the treatment of cancers that overexpress IAPs(1 0-12).