Apoptosis - the regulated destruction of a cell - is a complicated process. The decision to die cannot be taken lightly, and the activity of many genes influence a cell's likelihood of activating its self-destruction programme. Once the decision is taken, proper execution of the apoptotic programme requires the coordinated activation and execution of multiple subprogrammes. Here I review the basic components of the death machinery, describe how they interact to regulate apoptosis in a coordinated manner, and discuss the main pathways that are used to activate cell death.

The biochemistry of apoptosis

Apoptosis is a cell suicide mechanism that enables metazoans to control cell number in tissues and to eliminate individual cells that threaten the animal's survival. Certain cells have unique sensors, termed death receptors, on their surface. Death receptors detect the presence of extracellular death signals and, in response, they rapidly ignite the cell's intrinsic apoptosis machinery.

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Death receptors: signaling and modulation

Bcl-2 and related cytoplasmic proteins are key regulators of apoptosis, the cell suicide program critical for development, tissue homeostasis, and protection against pathogens. Those most similar to Bcl-2 promote cell survival by inhibiting adapters needed for activation of the proteases (caspases) that dismantle the cell. More distant relatives instead promote apoptosis, apparently through mechanisms that include displacing the adapters from the pro-survival proteins. Thus, for many but not all apoptotic signals, the balance between these competing activities determines cell fate. Bcl-2 family members are essential for maintenance of major organ systems, and mutations affecting them are implicated in cancer.

https://science.sciencemag.org/content/sci/281/5381/1322.full.pdf

The Bcl-2 Protein Family: Arbiters of Cell Survival

Apoptosis by death factor.

Cell Death: Critical Control Points

Heterodimerization between members of the Bcl-2 family of proteins is a key event in the regulation of programmed cell death. The molecular basis for heterodimer formation was investigated by determination of the solution structure of a complex between the survival protein Bcl-x L and the death-promoting region of the Bcl-2-related protein Bak. The structure and binding affinities of mutant Bak peptides indicate that the Bak peptide adopts an amphipathic α helix that interacts with Bcl-x L through hydrophobic and electrostatic interactions. Mutations in full-length Bak that disrupt either type of interaction inhibit the ability of Bak to heterodimerize with Bcl-x L .

https://science.sciencemag.org/content/sci/275/5302/983.full.pdf

Structure of Bcl-xL-Bak peptide complex: recognition between regulators of apoptosis.

PROGRAMMEDcell death (apoptosis) mediated by the cytokine receptor Fas is critical for the removal of autoreactive T cells1, the mechanism of immune privilege2,3, and for maintenance of immune-system homeostasis4. Signalling of programmed cell death involves the self-association of a conserved cytoplasmic region of Fas called the death domain5–7 and interaction with another death-domain-containing protein, FADD8 (also known as MORT1)9. Although death domains are found in several proteins10, their three-dimensional structure and the manner in which they interact is unknown. Here we describe the solution structure of the Fas death domain, as determined by NMR spectroscopy. The structure consists of six antiparallel, amphi-pathic α-helices arranged in a novel fold. From the structure and from site-directed mutagenesis, we have identified the region of the death domain involved in self-association and binding to the downstream signalling partner FADD.

NMR structure and mutagenesis of the Fas (APO-1/CD95) death domain.

When activated, membrane-bound receptors for Fas and tumour-necrosis factor initiate programmed cell death by recruiting the death domain of the adaptor protein FADD to the membrane. FADD then activates caspase 8 (also known as FLICE or MACH) through an interaction between the death-effector domains of FADD and caspase 8. This ultimately leads to the apoptotic response. Death-effector domains and homologous protein modules known as caspase-recruitment domains have been found in several proteins and are important regulators of caspase (FLICE) activity and of apoptosis. Here we describe the solution structure of a soluble, biologically active mutant of the FADD death-effector domain. The structure consists of six antiparallel, amphipathic alpha-helices and resembles the overall fold of the death domains of Fas and p75. Despite this structural similarity, mutations that inhibit protein-protein interactions involving the Fas death domain have no effect when introduced into the FADD death-effector domain. Instead, a hydrophobic region of the FADD death-effector domain that is not present in the death domains is vital for binding to FLICE and for apoptotic activity.

NMR structure and mutagenesis of the FADD (Mort1) death-effector domain.

NMR Structure and Mutagenesis of the N-Terminal Dbl Homology Domain of the Nucleotide Exchange Factor Trio

A mixture of dilauroyl phosphatidylcholine (DLPC) and 3-(cholamidopropyl)dimethylammonio-2-hydroxyl-1-propane sulfonate (CHAPSO) in water forms disc shaped bicelles that become ordered at high magnetic fields over a wide range of temperatures. As illustrated for the FK506 binding protein (FKBP), large residual dipolar couplings can be measured for proteins dissolved in low concentrations (5% w/v) of a DLPC/CHAPSO medium at a molar ratio of 4.2:1. This system is especially useful for measuring residual dipolar couplings for molecules that are only stable at low temperatures.

Matthias Eberstadt

Papers

Structure of Bcl-xL-Bak peptide complex: recognition between regulators of apoptosis.

NMR structure and mutagenesis of the Fas (APO-1/CD95) death domain.

NMR structure and mutagenesis of the FADD (Mort1) death-effector domain.

NMR Structure and Mutagenesis of the N-Terminal Dbl Homology Domain of the Nucleotide Exchange Factor Trio

A liquid crystalline medium for measuring residual dipolar couplings over a wide range of temperatures