1. ¹The Walter and Eliza Hall Institute of Medical Research, 1G Royal Parade, Parkville, Victoria 3050, Australia
2. ²The Murdoch Children's Research Institute, Parkville, Victoria, 3052, Australia

Correspondence: AM Verhagen, The Walter and Eliza Hall Institute of Medical Research, IG Royal Parade, Parkville, Victoria 3050, Australia. Tel: 61 3 9345 2555; Fax: 61 3 9347 0852; E-mail: verhagen@wehi.edu.au

³Current address: Department of Biochemistry, La Trobe University, Bundoora Victoria, 3086, Australia

Received 30 March 2006; Revised 10 May 2006; Accepted 22 May 2006; Published online 23 June 2006.

Abstract

Direct IAP binding protein with low pI/second mitochondrial activator of caspases, HtrA2/Omi and GstPT/eRF3 are mammalian proteins that bind via N-terminal inhibitor of apoptosis protein (IAP) binding motifs (IBMs) to the baculoviral IAP repeat (BIR) domains of IAPs. These interactions can prevent IAPs from inhibiting caspases, or displace active caspases, thereby promoting cell death. We have identified several additional potential IAP antagonists, including glutamate dehydrogenase (GdH), Nipsnap 3 and 4, CLPX, leucine-rich pentatricopeptide repeat motif-containing protein and 3-hydroxyisobutyrate dehydrogenase. All are mitochondrial proteins from which N-terminal import sequences are removed generating N-terminal IBMs. Whereas most of these proteins have alanine at the N-terminal position, as observed for previously described antagonists, GdH has an N-terminal serine residue that is essential for X-linked IAP (XIAP) interaction. These newly described IAP binding proteins interact with XIAP mainly via BIR2, with binding eliminated or significantly reduced by a single point mutation (D214S) within this domain. Through this interaction, many are able to antagonise XIAP inhibition of caspase 3 in vitro.