Regulation of cell death by the ubiquitin–proteasome system
Introduction
Ubiquitination, the conjugation of the protein ubiquitin to target proteins, has emerged as a prominent means of regulating cellular processes. Discovered more than three decades ago for its role in the degradation of unwanted proteins by the proteasome [1, 2], it is now known to have additional roles in signaling, transcription, DNA repair, endosomal trafficking and cell viability [3•, 4]. Over the past few years it has become increasingly clear that the ubiquitin–proteasome system (UPS) plays a central and complex role in regulating apoptosis by directly targeting key cell death proteins, including caspases, the key executioners of apoptosis.
Apoptosis is a tightly controlled form of active cell death that is necessary for development and organismal homeostasis [5, 6]. Death is achieved by the activation of a family of highly potent and specific proteases, termed caspases (for cysteine-aspartate protease) [7, 8, 9]. Given the potentially fatal consequence of their activity, these enzymes are tightly regulated; the cell maintains several ‘checkpoints’ before it enables them to act. The first level of regulation is intrinsic to caspases themselves. Caspases are initially transcribed as weakly active zymogens, which upon proper stimulation are cleaved to form the active enzyme. This step is brought forth by either internal signals that initiate the formation of the apoptosome [10], or by external cues through receptors that make up the Death Inducing Signaling Complex (DISC) [11]. The second level of caspase regulation is achieved by inhibitors, namely by a family of proteins called IAPs (Inhibitor of Apoptosis Proteins) [12, 13•, 14, 3•]. IAPs harbor between one to three copies of a baculovirus IAP repeat (BIR) domain that enables interaction with activated caspases. The BIR-domains of certain IAPs, in particular XIAP, have the ability to directly inhibit caspase activity in vitro [15, 16]. Some IAPs also contain a Really Interesting New Gene (RING) domain, which mediates binding to E2 ubiquitin-conjugating enzymes and enables these IAPs to act an E3 ubiquitin-ligases [17•]. E3-ligases serve as the substrate-binding module and thus convey substrate specificity. Studies in both mammalian systems and Drosophila revealed that the RING domain catalyzes many of the ubiquitination events associated with regulating apoptosis.
Section snippets
Role and regulation of IAPs in apoptosis
In cells that are destined to die, IAPs are inactivated by pro-apoptotic IAP-antagonists, which bind to BIR-domains with higher affinity than caspases [18, 14]. IAP-antagonists, such as Reaper, Hid, and Grim (RHG), were initially identified in Drosophila on the basis of their essential role for the initiation of apoptosis ([19] and reviewed in [14]). Reaper-family proteins contain a short N-terminal motif, termed IBM (IAP-Binding-Motif), which is required for IAP-binding and cell killing [14].
Ubiquitination in non-apoptotic caspase activation
An added level of complexity to caspase regulation is their non-apoptotic role in certain contexts. Caspases are involved in diverse pathways, including processing of pro-inflammatory cytokines, cellular signaling, differentiation and remodeling [49, 50••]. At this point, very little is known about how the potentially lethal activity of caspases is restricted in time and space to specific cellular compartments. However, several studies indicate that the ubiquitination machinery is also used in
Conclusion
Virtually all our cells constitutively express all the components necessary to carry out apoptosis and can rapidly self-destruct in response to a wide range of cell death stimuli. At the same time, healthy cells can avoid the activation of a caspase cascade and live for many years, or even the lifetime of an organism. Therefore, very stringent yet responsive mechanisms must govern caspase activation and cell death. The UPS, with its ability to rapidly eliminate and modify regulatory proteins in
References and recommended reading
Papers of particular interest published within the period of review have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank Sigi Benjamin and Samara Brown for critically reading this manuscript. H Steller is an investigator of the Howard Hughes Medical Institute. This work was supported by NIH grant RO1GM60124 to HS.
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