Regulation of Cell Death by IAPs and Their Antagonists

Abstract

Inhibitors of apoptosis (IAPs) family of genes encode baculovirus IAP-repeat domain-containing proteins with antiapoptotic function. These proteins also contain RING or UBC domains and act by binding to major proapoptotic factors and ubiquitylating them. High levels of IAPs inhibit caspase-mediated apoptosis. For these cells to undergo apoptosis, IAP function must be neutralized by IAP-antagonists. Mammalian IAP knockouts do not exhibit obvious developmental phenotypes, but the cells are more sensitized to apoptosis in response to injury. Loss of the mammalian IAP-antagonist ARTS results in reduced stem cell apoptosis. In addition to the antiapoptotic properties, IAPs regulate the innate immune response, and the loss of IAP function in humans is associated with immunodeficiency. The roles of IAPs in Drosophila apoptosis regulation are more apparent, where the loss of IAP1, or the expression of IAP-antagonists in Drosophila cells, is sufficient to trigger apoptosis. In this organism, apoptosis as a fate is conferred by the transcriptional induction of the IAP-antagonists. Many signaling pathways often converge on shared enhancer regions of IAP-antagonists. Cell death sensitivity is further regulated by posttranscriptional mechanisms, including those regulated by kinases, miRs, and ubiquitin ligases. These mechanisms are employed to eliminate damaged or virus-infected cells, limit neuroblast (neural stem cell) numbers, generate neuronal diversity, and sculpt tissue morphogenesis.

Keywords: ARTS; Apoptosis; BRUCE; DIAP1; HID; IAP-antagonist; REAPER; SMAC; XIAP; c-IAP1.

Figure 1. Domain maps of IAPs and their antagonists from various model systems

All known IAPs contain at least one Baculovirus IAP-Repeat (BIR) domain. In addition, most have RING domains. BRUCE is the largest IAP (as indicated by the break in sequence in figure) and an exception in that it does not contain a RING domain but instead has an Ubiqutin Conjugation domain (UBC). Most IAP-antagonists contain a short 5–10 amino acid IAP-Binding Motif (IBM) at their N-terminii, usually immediately after the Methionine, which is cleaved to expose the IBM. The mammalian IAP antagonists, Smac, ARTS and Omi/HtrA2, localize to the mitochondria for their function and hence contain a Mitochondria Localization Sequence (MLS) amongst other domains. ARTS belongs is a non-canonical IAP-antagonist that does not have an N-terminal IBM, and instead uses the C-terminal sequences to bind IAPs. Domain maps to scale, source: www.uniprot.org.

Figure 2. A schematic showing the Drosophila H99 locus and its regulation

The 3L chromosome arm contains all four of the IAP-antagonists, hid, grim, reaper (rpr) and sickle (skl), clustered in the H99 locus. A ~12kbp region upstream of rpr is the hub of most transcriptional regulation and controls transcription of rpr and hid, although the hid locus is more than 250kbp away. It contains a ~11kbp Irradiation-Responsive Enhancer Region (IRER) containing a 20bp p53 binding site critical for radiation induced apoptosis. The IRER region is heavily epigenetically regulated during development by histone deacetylases (HDAC) and methylases. The rpr upstream promoter region contains sites for binding several transcription factors, notably Jun/Fos and Schnurri (Shn), which mediate context dependent JNK-driven apoptosis.

Figure 3. A schematic diagram of IAP-antagonists and their targets

Upper panel shows the relationships between Drosophila genes, whereas the lower panel shows mammalian IAPs and their antagonists.