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.The serine receptor (Tsr), one of four methyl-accepting chemotaxis proteins (MCPs) that span the inner membrane of Escherichia coli, initiates responses and governs adaptation to changes in the serine concentration. MCPs belong to a large class of transducers (21, 46), which sense a variety of environmental cues and are the inputs to sensory pathways that bias cell movement toward favorable environments (12). The chemotaxis pathways belong to the two-component superfamily of signal transduction pathways (17, 42), which are chiefly found in prokaryotes. A two-component pathway consists of a sensor, which is frequently an integral membrane protein possessing kinase activity, and one or more cytoplasmic phosphate-accepting response regulator proteins. The transmembrane sensor-kinases of the chemotaxis pathways are often noncovalent complexes between MCPs (which have no enzyme activity) and two soluble cytoplasmic proteins, namely, an adaptor protein (CheW) and a kinase (CheA) (15, 39).Elucidation of the structure and distribution of receptors in the membrane of the cell is integral to understanding the molecular basis of signaling by the transmembrane sensor (MCP-CheW-CheA) complexes. X-ray structure determination of the soluble domains has clearly defined the dimeric organization of the 60-kDa receptor subunits (19,31,45), and functional studies have helped to elucidate the role of dimer organization in the mechanism of transmembrane signaling (references 32 and 12 and refer...

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.

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.

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.