Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induced mitochondrial pathway to apoptosis and caspase activation is potentiated by phospholipid scramblase-3.

Tumor Necrosis Factor (TNF)-Related Apoptosis-Inducing Ligand (TRAIL) initiate pathways of cell death in which caspase activation is mediated either directly (without mitochondrial amplification), or indirectly via the release of apoptogenic factors from mitochondria. Phospholipid scramblases (PLS) are enzymes that play a key role in cellular function by inducing bidirectional movement of membrane lipids. Changes in mitochondrial membrane lipids, cardiolipin, are critical for mediating apoptotic response in many cell-types. PLS3 is a phospholipid scramblase that is localized to mitochondria and is thought to be involved in the regulation of apoptotic signals. Here we report that exogenous-expression of PLS3 enhances apoptotic death induced by TRAIL. This is acheived by potentiating the mitochondrial arm of the death pathway. Thereby, PLS3 expression facilitates changes in mitochondrial membrane lipids that promote the release of apoptogenic factors and consequent full activation and processing of the caspase-9 and effector caspase-3. Moreover, we show that knock-down of endogenous PLS3 suppresses TRAIL-induced changes in cardiolipin. Finally, we demonstrate that TRAIL-induced activation of PKC-delta mediates regulation of the PLS3-induced changes in cardiolipin.

The mitochondrial death pathway.

Mitochondria have long been known to be critical for cell survival due to their role in energy metabolism. However, not until the mid-1990s did it become evident that mitochondria are also active participants in programmed cell death (PCD). This chapter focuses mainly on the role the mitochondria in mammalian cell death and cancer progression and therapy.

Site-specific and synergistic stimulation of methylation on the bacterial chemotaxis receptor Tsr by serine and CheW.

BACKGROUND: Specific glutamates in the methyl-accepting chemotaxis proteins (MCPs) of Escherichia coli are modified during sensory adaptation. Attractants that bind to MCPs are known to increase the rate of receptor modification, as with serine and the serine receptor (Tsr), which contributes to an increase in the steady-state (adapted) methylation level. However, MCPs form ternary complexes with two cytoplasmic signaling proteins, the kinase (CheA) and an adaptor protein (CheW), but their influences on receptor methylation are unknown. Here, the influence of CheW on the rate of Tsr methylation has been studied to identify contributions to the process of adaptation. RESULTS: Methyl group incorporation was measured in a series of membrane samples in which the Tsr molecules were engineered to have one available methyl-accepting glutamate residue (297, 304, 311 or 493). The relative rates at these sites (0.14, 0.05, 0.05 and 1, respectively) differed from those found previously for the aspartate receptor (Tar), which was in part due to sequence differences between Tar and Tsr near site four. The addition of CheW generated unexpectedly large and site-specific rate increases, equal to or larger than the increases produced by serine. The increases produced by serine and CheW (added separately) were the largest at site one, approximately 3 and 6-fold, respectively, and the least at site four, no change and approximately 2-fold, respectively. The rate increases were even larger when serine and CheW were added together, larger than the sums of the increases produced by serine and CheW added separately (except site four). This resulted in substantially larger serine-stimulated increases when CheW was present. Also, CheW enhanced methylation rates when either two or all four sites were available. CONCLUSION: The increase in the rate of receptor methylation upon CheW binding contributes significantly to the ligand specificity and kinetics of sensory adaptation. The synergistic effect of serine and CheW binding to Tsr is attributed to distinct influences on receptor structure; changes in the conformation of the Tsr dimer induced by serine binding improve methylation efficiency, and CheW binding changes the arrangement among Tsr dimers, which increases access to methylation sites.

Electron microscopic analysis of membrane assemblies formed by the bacterial chemotaxis receptor Tsr.

The serine receptor (Tsr) from Escherichia coli is representative of a large family of transmembrane receptor proteins that mediate bacterial chemotaxis by influencing cell motility through signal transduction pathways. Tsr and other chemotaxis receptors form patches in the inner membrane that are often localized at the poles of the bacteria. In an effort to understand the structural constraints that dictate the packing of receptors in the plane of the membrane, we have used electron microscopy to examine ordered assemblies of Tsr in membrane extracts isolated from cells engineered to overproduce the receptor. Three types of assemblies were observed: ring-like "micelles" with a radial arrangement of receptor subunits, two-dimensional crystalline arrays with approximate hexagonal symmetry, and "zippers," which are receptor bilayers that result from the antiparallel interdigitation of cytoplasmic domains. The registration among Tsr molecules in the micelle and zipper assemblies was sufficient for identification of the receptor domains and for determination of their contributions to the total receptor length. The overall result of this analysis is compatible with an atomic model of the receptor dimer that was constructed primarily from the X-ray crystal structures of the periplasmic and cytoplasmic domains. Significantly, the micelle and zipper structures were also observed in fixed, cryosectioned cells expressing the Tsr receptor at high abundance, suggesting that the modes of Tsr assembly found in vitro are relevant to the situation in the cell.