Characterization of bivalent peptides that bind to X‐linked inhibitor of apoptosis protein

Abstract

X‐linked inhibitor of apoptosis protein (XIAP) is a regulator of programmed cell death and an important target in the development of cancer therapeutics. Biochemical studies reveal that the homodimeric protein Smac, the natural binding partner of XIAP, binds to XIAP via simultaneous contact to the BIR2 and BIR3 domains. Furthermore, the interaction is mediated primarily via the first four N‐terminal amino acids Ala‐Val‐Pro‐Ile. We and others have shown that dimeric peptides and peptidomimetics based upon the Ala‐Val‐Pro‐Ile motif also bind to XIAP through both the BIR2 and BIR3 domains. Despite the high structural and sequence similarities between the BIR2 and BIR3 domains, Ala‐Val‐Pro‐Ile binds to BIR2 with substantially lower affinity than to BIR3. Herein we explore the potential for asymmetric bivalent Smac mimetics wherein one binding motif has been optimized for the BIR2 domain. We show that the tetrapeptide Ala‐Glu‐Ala‐Val has a higher affinity for BIR2 than Ala‐Val‐Pro‐Ile, and we have incorporated this motif into dimeric peptides with both BIR2 and BIR3 targeting motifs. We also will present mutagenesis experiments aimed at restoring the tight binding of the BIR3 domain to BIR2. The implications of these results in the design of XIAP antagonists will be discussed. We acknowledge the Research Corporation (Cottrell College award to K.E.S.) and the Beckman Scholars Program (undergraduate research fellowship to K.F.S.) for support of this research.