Identification of mammalian mitochondrial proteins that interact with IAPs via N-terminal IAP binding motifs

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) Inhibitor of apoptosis proteins (IAPs) suppress apoptosis by binding to and inhibiting active caspases, cysteine proteases that otherwise cleave proteins, resulting in disintegration of the cell. The IAPs X-linked IAP (XIAP), cIAP1, cIAP2 and ML-IAP bear one to three baculoviral IAP repeats (BIRs) and a Cterminal RING domain that has ubiquitin E3 ligase activity.

The mechanism of peptide-binding specificity of IAP BIR domains

“…Our results are in agreement with a recent study that described several novel IBM-containing XIAP-BIR2 interacting proteins, many of which lacked Pro in P3 0 , 22 and also the XIAP-BIR domain-profiling studies mentioned above. 5,7 Although the BIR2 did not select for Pro in P3 0 , it is known that the presence of this residue does not preclude BIR2 binding, since this BIR domain has been shown to bind several proteins with Pro in this position.…”

“…These proteins remain unacetylated and comprise the most likely pool of candidate IAP target proteins, exemplified by SMAC and HtrA2. Figure 4 presents a collective set of previously identified IBM-containing proteins [11][12][13][14][15]20,22,23,36,37 which we have divided based on whether they were shown to interact with type II or type III BIRs. The sequences of these proteins naturally fall into two categories that closely resemble the profiles elucidated for type II and type III BIR domains in our studies.…”

“…It has been suggested that different types of BIR domains select different target proteins. 22 Several IAPs contain tandem type II and type III BIRs, and this seems to be important in enhancing avidity toward target proteins. For example, SMAC and caspase-9 have an IBM that is optimal for binding type III BIRs, indeed this is part of the mechanism of inhibition of caspase-9 by XIAP-BIR3, as well as derepression of inhibition of caspase-9 by SMAC.…”

Alternative splicing generates multiple transcripts of the inhibitor of apoptosis protein 1 in Aedes and Culex spp. mosquitoes

“…IAPs bind to and inhibit the activity of caspases, a family of cysteine proteases that orchestrate rapid cellular destruction by cleaving various target proteins (Shi, 2002;Shi, 2004). The anti-apoptotic activities of IAPs are antagonized by IAP binding motif (IBM) proteins, a family of proapoptotic proteins that share an IAP-binding tetrapeptide motif (Bergmann et al, 2003;Vaux and Silke, 2003;Verhagen et al, 2006). Members of this family include Reaper, Hid, Grim, Sickle, and Jafrac2 in Drosophila, michelob_x in Anopheles gambiae, the michelob_x homolog in Aedes aegypti, and Smac/DIABLO, Omi/HtrA2 and GSPT1/eRF3 in mammals.…”

Combination of IAP antagonist and IFNγ activates novel caspase-10- and RIPK1-dependent cell death pathways

“…1 Mammalian IAPs can be antagonised by endogenous proteins such as Smac/DIABLO and HtrA2/Omi, 2,3 and the finding that cell-permeable peptides containing the four N-terminal residues of Smac-sensitised tumour cells to apoptosis 4 hastened the development of synthetic Smac mimetics (SMs) that proved similarly efficacious. [5][6][7][8] cIAPs play a central role in regulating tumour necrosis factor receptor 1 (TNFR1) signalling.…”