Cell membranes display a tremendous complexity of lipids and proteins designed to perform the functions cells require. To coordinate these functions, the membrane is able to laterally segregate its constituents. This capability is based on dynamic liquid-liquid immiscibility and underlies the raft concept of membrane subcompartmentalization. Lipid rafts are fluctuating nanoscale assemblies of sphingolipid, cholesterol, and proteins that can be stabilized to coalesce, forming platforms that function in membrane signaling and trafficking. Here we review the evidence for how this principle combines the potential for sphingolipid-cholesterol self-assembly with protein specificity to selectively focus membrane bioactivity.

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Lipid Rafts As a Membrane-Organizing Principle

Death receptors are members of the tumor necrosis factor receptor superfamily characterized by a cytoplasmic region known as the “death domain” that enables the receptors to initiate cytotoxic signals when engaged by cognate ligands. Binding to the ligand results in receptor aggregation and recruitment of adaptor proteins, which, in turn, initiates a proteolytic cascade by recruiting and activating initiator caspases 8 and 10. Death receptors were once thought to primarily induce cytotoxic signaling cascades. However, recent data indicate that they initiate multiple signaling pathways, unveiling a number of nonapoptosis-related functions, including regulation of cell proliferation and differentiation, chemokine production, inflammatory responses, and tumor-promoting activities. These noncytotoxic cascades are not simply a manifestation of inhibiting proapoptotic pathways but are intrinsically regulated by adaptor protein and receptor internalization processes. Insights into these various death receptor signaling pathways provide new therapeutic strategies targeting these receptors in pathophysiological processes.—Guicciardi, M. E., Gores, G. J. Life and death by death receptors.

https://faseb.onlinelibrary.wiley.com/doi/pdf/10.1096/fj.08-111005

Life and death by death receptors

Protein palmitoylation is an essential post-translational lipid modification of proteins, and reversibly orchestrates a variety of cellular processes. Identification of palmitoylated proteins with their sites is the foundation for understanding molecular mechanisms and regulatory roles of palmitoylation. Contrasting to the labor-intensive and time-consuming experimental approaches, in silico prediction of palmitoylation sites has attracted much attention as a popular strategy. In this work, we updated our previous CSS-Palm into version 2.0. An updated clustering and scoring strategy (CSS) algorithm was employed with great improvement. The leave-one-out validation and 4-, 6-, 8- and 10-fold cross-validations were adopted to evaluate the prediction performance of CSS-Palm 2.0. Also, an additional new data set not included in training was used to test the robustness of CSS-Palm 2.0. By comparison, the performance of CSS-Palm was much better than previous tools. As an application, we performed a small-scale annotation of palmitoylated proteins in budding yeast. The online service and local packages of CSS-Palm 2.0 were freely available at: http://bioinfor

/pdf/css-palm-2-0-an-updated-software-for-palmitoylation-sites-3l2y8n762a.pdf

CSS-Palm 2.0: an updated software for palmitoylation sites prediction.

Protein palmitoylation, a classical and common lipid modification, regulates diverse aspects of neuronal protein trafficking and function. The reversible nature of palmitoylation provides a potential general mechanism for protein shuttling between intracellular compartments. The recent discovery of palmitoylating enzymes--a large DHHC (Asp-His-His-Cys) protein family--and the development of new proteomic and imaging methods have accelerated palmitoylation analysis. It is becoming clear that individual DHHC enzymes generate and maintain the specialized compartmentalization of substrates in polarized neurons. Here, we discuss the regulatory mechanisms for dynamic protein palmitoylation and the emerging roles of protein palmitoylation in various aspects of pathophysiology, including neuronal development and synaptic plasticity.

Protein palmitoylation in neuronal development and synaptic plasticity

In fluorescence correlation spectroscopy (FCS), information about molecular dynamics is extracted by recording the fluctuating signal of a pico- to micromolar concentration of fluorescent molecules in solution. As primary parameters, FCS provides time constants of the fluorescence emission, as well as numbers and dwell times of the observed particles diffusing through the open volume. A biochemical reaction or macromolecular rearrangement causing changes in these parameters, when linked to fluorescence readout, can be quantified by FCS. Since the measurements are now routinely performed in a laser-illuminated confocal spot, making measurements in living cells is straightforward. Different cellular compartments, such as the nucleus, the cytoplasm, or the plasma membrane, can be easily distinguished and addressed. In addition to biochemical reactions, the local environment of macromolecules, for example, ion concentrations, pH, or viscosity, can be probed. Thus, FCS is a versatile and attractive technique for researchers striving for a quantitative understanding of interactions and dynamics of biological and in particular cellular systems.

Fluorescence Correlation Spectroscopy in Living Cells

Localization of the death receptor Fas to specialized membrane microdomains is crucial to Fas-mediated cell death signaling. Here, we report that the post-translational modification of Fas by palmitoylation at the membrane proximal cysteine residue in the cytoplasmic region is the targeting signal for Fas localization to lipid rafts, as demonstrated in both cell-free and living cell systems. Palmitoylation is required for the redistribution of Fas to actin cytoskeleton-linked rafts upon Fas stimulation and for the raft-dependent, ezrin-mediated cytoskeleton association, which is necessary for the efficient Fas receptor internalization, death-inducing signaling complex assembly and subsequent caspase cascade leading to cell death.

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Britta Dost

Papers

Palmitoylation is required for efficient Fas cell death signaling