Scythe Regulates Apoptosis-inducing Factor Stability during Endoplasmic Reticulum Stress-induced Apoptosis

Abstract

Scythe (BAT3; HLA-B associated transcript 3, Bag 6) is a protein that has been implicated in apoptosis because it can modulate the Drosophila melanogaster apoptotic regulator, Reaper. Mice lacking Scythe show pronounced defects in organogenesis and in the regulation of apoptosis and proliferation during mammalian development. However, the biochemical pathways important for Scythe function are unknown. We report here multiple levels of interaction between Scythe and the apoptogenic mitochondrial intermembrane protein AIF (apoptosis-inducing factor). Scythe physically interacts with AIF and regulates its stability. AIF stability is markedly reduced in Scythe ؊/؊ cells, which are more resistant to endoplasmic reticulum stress induced by thapsigargin. Reintroduction of Scythe or overexpression of AIF in Scythe ؊/؊ cells restores their sensitivity to apoptosis. Together, these data implicate Scythe as a regulator of AIF.Apoptosis is the physiological process responsible for the demise of superfluous, aged, damaged, and ectopic cells and is essential during embryonic development and for maintenance of adult tissue homeostasis (1). Among the numerous proteins implicated in apoptosis is Scythe (BAT3; HLA-B associated transcript; Bag 6). Scythe has been shown to interact with Reaper, a central regulator of developmental apoptosis in Drosophila melanogaster (2, 3). To date, no Reaper homologues have been discovered in vertebrate species, but Reaper and associated Drosophila regulators Hid and Grim can induce apoptosis in mammalian systems (4, 5). The interaction between Scythe and Reaper is required for Reaper-induced apoptosis in Xenopus egg extracts and results in the release of an as yet unidentified Scythe-bound apoptotic-inducing factor, leading to rapid mitochondrial cytochrome c release, caspase activation, and nuclear fragmentation. Immunodepletion of Scythe prevented reaper-induced apoptosis (2, 3). Our previous study also indicated a role for Scythe during apoptosis in a mammalian system. We found that inactivation of Scythe in the mouse resulted in lethality associated with pronounced developmental defects in the lung, kidney, and brain (6). Co-incident with organogenesis defects was widespread aberrant apoptosis and proliferation, and a resistance of Scythe-null cells to apoptosis (6).A critical step in apoptosis is often the permeabilization of mitochondrial membranes, leading to the release of proteins that are normally localized behind the outer mitochondrial membrane (7). One of those proteins is the apoptosis-inducing factor (AIF) 2 (8). In healthy cells, AIF is located in the mitochondrial intermembrane space where its physiological function is not entirely clear, but may involve protection against oxidative stress (8 -11). In apoptotic cells, AIF relocates from the mitochondria to the nucleus where it exerts its pro-apoptotic activity via chromatin condensation and large scale DNA fragmentation (12-14). Although AIF is strongly linked to apoptosis, many aspects of its activity remain to be clarified.Her...

Bat3 promotes the membrane integration of tail-anchored proteins

“…S3A). This is consistent with studies in cultured mammalian cells where populations of Bat3 in both the nucleus and the cytosol can be observed (Desmots et al, 2008). Bat3 expression had no effect on the location of GFP-Sed5 in wild-type cells, where numerous punctae were seen, consistent with efficient membrane integration and Golgi localization (Fig.…”

Bag6 complex contains a minimal tail-anchor–targeting module and a mock BAG domain

“…It is expected that the mammalian homologs, along with Bag6, play a similar role (23)(24)(25)(26)(27). Bag6 also interacts with other proteins such as apoptosisinducing factor, glycoprotein 78 (gp78), regulatory particle 5, and brother of regulator of imprinted sites (BORIS) (16,(27)(28)(29)(30)(31)(32) and can homo-oligomerize, increasing the level of complexity (30). These findings build a picture of Bag6 as a central hub for a diverse physiological network of proteins.…”

BAT3 modulates p300-dependent acetylation of p53 and autophagy-related protein 7 (ATG7) during autophagy

“…BAT3 has nuclear and cytosolic functions. In the cytosol, BAT3 has roles in quality control (8) and apoptosis (9)(10)(11)(12). In the nucleus, thanks to its NLS domain (13), BAT3 modulates the methylase activity of SET1A and DOT1L, with roles in transcription and DNA damage response, respectively (14,15).…”