Über die Autorin bzw. den Autor

Christopher Fielding is Neider Professor of Cardiovascular Physiology at the University of California at San Francisco (UCSF). He graduated from University College in London (UK) where he also received his PhD. After appointments at Oxford University and at the University of Chicago, he joined the faculty at UCSF in 1971, being appointed full professor in 1985.
Professor Fielding's main research interest is in the trafficking of cholesterol, its regulation and its role in signal transduction.

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This keenly awaited first overview of the field represents a complete guide to the structure and function of the most important mammalian cell membrane organelles. Filling a huge gap in the primary literature, this book is the first to cover the subject in detail. Following an introduction by Kai Simons, the discoverer of lipid rafts and the most prominent scientist in the field, chapters include: - Historical Background - Distinct Structures and Functions - Structural Basis - Signaling - Viral Entry and Virion Budding - Cholesterol Transport - Caveolins - Lipid Shells - Cell Polarity and Intracellular Trafficking - Cancer Cells The book is of prime importance to molecular and cell biologists, biochemists, membrane scientists, cancer researchers, and virologists.

Aus dem Klappentext

This keenly awaited first overview of the field represents a complete guide to the structure and function of the most important mammalian cell membrane organelles. Filling a huge gap in the primary literature, this book is the first to cover the subject in detail.
Following an introduction by Kai Simons, the discoverer of lipid rafts and the most prominent scientist in the field, chapters include:
 
- Historical Background
- Distinct Structures and Functions
- Structural Basis
- Signaling
- Viral Entry and Virion Budding
- Cholesterol Transport
- Caveolins
- Lipid Shells
- Cell Polarity and Intracellular Trafficking
- Cancer Cells
 
The book is of prime importance to molecular and cell biologists, biochemists, membrane scientists, cancer researchers, and virologists.

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Lipid Rafts and Caveolae

John Wiley & Sons

Chapter One

1.1 Introduction

Cell membranes are dynamic assemblies of a variety of lipids and proteins. They form a protective layer around the cell, but also mediate the communication with the outside world - that is, neighboring cells in a tissue, hormones and growth factors arriving with the blood supply, or pathogens trying to enter the system. The unique feature of cell membranes is that their lipid and protein constituents can self-assemble into 5 nm-thin, two-dimensional fluids composed of two apposing lipid monolayers that form a hydrophobic interior and two polar interfacial regions oriented towards the aqueous medium. This organizing principle - the lipid bilayer - is the oldest, still valid molecular model of biological structures. The first model that incorporated proteins was proposed by Danielli and Davson, and assumed that the bilayer was made up entirely of lipids and that proteins covered the two polar surfaces. Some 40 years later, the fluid mosaic model of the cell membrane proposed by Singer and Nicolson was a conceptual breakthrough. Amphipathic membrane proteins were recognized to reside within, and even span, the whole bilayer that was depicted as a dynamic structure, the components of which are laterally mobile. However, the view that the lipids in the bilayer mainly serve as a homogeneous solvent for proteins has been proven to be too simplistic. Lipids are not only distributed asymmetrically between the two leaflets of the bilayer, but also within the leaflet they are heterogeneously arranged. This chapter will recapitulate the history and recent advances in membrane biology including the lipid raft concept, and then summarize current views on the functions of rafts and caveolae in membrane traffic.

1.2 Basic Organization Principles of a Cell Membrane

The lipid bilayer is a two-dimensional fluid, where lipid molecules exchange slowly between leaflets but are mobile within the leaflet. This mobility consists of two parts:

the "translational freedom" of a molecule - that is, its lateral mobility; and

the "configurational freedom" that is, the ability to flex parts of the molecule and to rotate bonds in its carbon backbone.

Synthetic bilayers change from a liquid state with high translational and configurational freedom into a rigid gel state at a characteristic freezing point. Cell membranes at physiological temperatures are almost always in the liquid state, but can contain regions with high configurational order, as will be described later. Importantly, the lipid bilayer of cell membranes is asymmetric, with a different lipid composition in the two leaflets. The main lipid components of cellular membranes are glycerophospholipids, with the most abundant species being phosphatidylcholine (PC) in the exoplasmic leaflet and phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the inner leaflet, as well as sphingolipids with glycosphingolipids and sphingomyelin (SM) mostly localized to the exoplasmic leaflet. Sterols make up the third lipid class, and are present in both leaflets. Mammalian cell membranes contain only one sterol, namely cholesterol, but probably more than thousand different glyco- and sphingolipid species, emerging from the combinatorial propensity to assemble lipids from different backbones linked in different ways with two varying hydrocarbon chains and a vast number of headgroups. A large number of flippases and translocators tightly control the asymmetric distribution of all these lipids across the bilayer.

Lipids are differentially distributed between cellular organelles. The endoplasmic reticulum and the Golgi-complex contain mainly glycerophospholipids and only small amounts of sphingolipids, whereas the plasma membrane is relatively enriched in SM and glycosphingolipids. Also within the membrane plane of one organelle, lipids are believed to be heterogeneously arranged. Caveolae - small invaginations of the plasma membrane - are enriched in glycosphingolipids, and phosphatidylinositol-3'-phosphate (PI(3)P) is concentrated in subdomains of early endosome membranes. Recently, vacuole-fusion in yeast has been shown to be controlled by microdomains of ergosterol, diacylglycerol and phosphoinositide-3-and-4-phosphate. Furthermore, membranes are differentially susceptible to extraction by detergents such as Triton X-100 or CHAPS at 4 C, with some proteins and lipids being completely solubilized and others forming so-called "detergent-resistant membranes" (DRM; for a review, see). These findings suggested that cell membranes contained microdomains in which lipids were more tightly packed and thus not accessible to the detergent, although it is widely accepted that DRMs do not have an exact in-vivo correlate but are defined by being formed during the detergent treatment. These microdomains were later termed "rafts" and were described as sphingolipid-cholesterol assemblies containing a subset of membrane proteins. Currently, the raft hypothesis is heavily debated, with the main discussion points being the methodologies to study rafts and the size of the domains (see below). The core of the raft concept is that cell membranes phase-separate into different domains and that this is a lipid-driven process. In light of the ongoing discussion in the field, the following sections will provide an overview about what is known about phase separation, first discussing the studies conducted in model membrane systems and later in cell membranes.

1.3 Evidence for Phase Separation in Model Membrane Systems: Liquid-Ordered and Liquid-Disordered Phases

Various model membrane systems have been used by physicists and chemists to study phase separation in lipid mixtures. They are either monolayers or bilayers. Monolayers are either assembled at an air-water interface with the packing density of the lipids being adjusted by applying lateral pressure, or on a supporting lipid monolayer that is fixed to a solid support. Bilayers are used in the supported version as described above, or in the form of vesicles. The most commonly used vesicles are large or giant unilamellar vesicles (LUV or GUV, respectively) composed of only a single bilayer, but also multilamellar vesicles (MLV) are used. The basic principles were first established in simple binary lipid mixtures, but recently ternary mixtures which more closely mimic the composition of the cell plasma membrane have been used. The mixtures usually contain one lipid with a high melting temperature ([T.sub.m]), one with a low [T.sub.m], and cholesterol. GUVs are probably the system closest to a cell membrane, because artifacts from a support are excluded. Still, cell membranes are asymmetric with different lipid compositions of the outer versus the inner leaflet, while the GUVs used so far were all symmetric. Since maintaining an asymmetric lipid distribution is energy-consuming, perhaps by reconstituting lipid translocators into liposomes this drawback can be overcome in the future. Although model membrane systems produce very simplified pictures of cell membranes, there are many examples of a close correlation with experimental data obtained in living cells.

Ipsen et al. were the first to describe the formation of a liquid-ordered phase by cholesterol and saturated phospholipids. This phase can coexist with other lipid phases, and its characteristics...