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ScienceWeek
CELL BIOLOGY: ON THE PATCHY STRUCTURE OF CELL MEMBRANES
The following points are made by Donald M. Engelman (Nature 2005 438:578):
1) Given their biological importance, membranes have been surprisingly neglected by biochemists until recently. Perhaps this is understandable in view of the technical hurdles that working with them presents. Most methods require purification and observation in aqueous environments alien to the molecular design of a membrane, and so the field had to rely on oversimplified views that still dominate the texts and teaching in this area. But now we have a rising number of high-resolution structures, an abundance of functional data and an evolving conceptual basis for framing more pointed questions. This is leading to a great expansion of interest in the area.
2) The reductionist view of biology, to which many adhere, rests in part on the structure-function hypothesis, proposing that the structures we find are there for specific functional reasons selected by evolution. In the case of membranes, we might start with the origin of life, noting that compartmentalization is essential for an organism, and that with compartmentalization must come specific ways to surmount the barrier defining the boundary of the compartment -- the membrane. Thus, the lipid bilayer, which spontaneously forms permeability barriers surrounding aqueous interiors, must be modified by macromolecules for the uptake of nutrients and the disposal of waste. Further refinements led to the use of the barrier for its energy-storage properties and to the creation of ways to pass information between a cell and its environment.
3) An influential step in the study of membranes was taken with the development by Singer and Nicholson in 1972 of the "fluid mosaic model", which pulled together findings and ideas from the preceding decade. The model has become the standard conceptualization of membrane architecture and appears in virtually all biochemistry texts. As important and insightful as this model has been, the emergence of new findings during the passage of 33 years has weakened the generalizations it contains, and it is now appropriate to examine some of them more closely. The model includes the ideas that the proteins of a membrane are dispersed, are at low concentration and that they match the hydrophobic dimension of an unperturbed lipid bilayer with peripheral belts of exposed hydrophobic side chains. The lipid is seen as a sea in which mainly monomeric proteins float unencumbered, and the bilayer surface is exposed directly to the aqueous environment. Each of these ideas is misleading. Most current write of the preferential associations of molecules in the membrane plane, and such associations are expected: membranes are typically crowded and their bilayers vary considerably in thickness.
4) Is a membrane a random two-dimensional liquid? In the Singer-Nicholson model, molecules are distributed randomly in two dimensions. But we know from first principles and from experimental observation that non-randomness is the rule. Consider a mixture of (n) lipid and protein components in a membrane. The planar distribution can be random only if all pairwise interaction energies of the (n) different molecular species are within thermal energies (about 0.6 kcal/mol at room temperature) of each other. In a plasma membrane there are many species of lipids and proteins. The Escherichia coli genome, for example, codes for more than a thousand putative helical transmembrane proteins, giving more than half a million kinds of pairwise combinations. A narrow range of interaction energies is a highly improbable condition given the range of known intermolecular interactions from hydrogen bonds, packing, electrostatics, and the hydrophobic effect.
5) Indeed, simply rotating a pair of identical helices against each other or changing a single interfacial side chain can result in interaction variations of several times kT. Thus, it should have been expected that regions of biased composition would exist and that the environments of proteins should vary, because it is highly improbable that interaction energies will match each other across all protein and lipid species in a membrane. Time-invariant complexes, transient associations and biased distributions should be the norm. Evolution, ever seeking to exploit the natural tendencies of molecules, has seized the opportunity to craft functional associations, and it is clear that there are functional protein complexes, separated lipid compositional areas, and regions of functional specialization, although we do not yet know their extent.
Nature http://www.nature.com/nature
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CELL BIOLOGY: MEMBRANE RAFTS AND SIGNALING PROTEINS
The following points are made by Ben Nichols (Nature 2005 436:638):
1) Signal transduction -- the relay of signals from outside a cell to inside -- frequently involves bewildering patterns of interactions between several different types of protein at the cell surface. Work attempting to make sense of this complexity suggests that specific lateral organization of the interacting proteins in the membrane is key to their signalling functions. But how is such organization generated? New work [1] begins to provide some answers and emphasizes the utility of recently developed single-molecule imaging techniques in addressing the dynamic properties of signalling networks. Moreover, the authors' experiments directly address the controversy over the mechanisms that generate localized variations in the composition of the cell membrane.
2) Simple mixtures of lipids in artificial membrane bilayers can segregate into regions that differ in the way the acyl chains of the lipids are packed together. This can spontaneously generate heterogeneity in the membrane, as lipids that prefer different local environments tend to separate out from one another. More-ordered acyl-chain packing is associated with the presence of increasing amounts of cholesterol and sphingolipids -- both found in natural membranes -- and the resulting lipid domains tend not to be soluble in non-ionic detergents[2]. Detergent-insoluble fractions enriched for particular proteins and lipids can also be isolated from cells. Extrapolating from the artificial membrane data, it has been proposed that these detergent-resistant fractions might be derived from functional domains, or "lipid rafts", in cell membranes, where self-organization of the membrane lipids leads to the recruitment of specific proteins[3].
3) This lipid raft hypothesis has received much attention and is certainly appealing, but the correlation between detergent resistance and domain formation in vivo is a topic of some debate[4]. Artificial membranes may not be good models for cell membranes that are rich in protein and have two asymmetric layers of hundreds of different types of lipid.
4) The case of signalling through T-cell receptors is particularly germane to this debate. At the start of an immune response, T cells are activated when antigen molecules bind to the receptors on their surface. Stimulation of T cells usually occurs when stable contacts -- referred to as 'synapses' -- form between the T cell and so-called antigen-presenting cells[5]. There is a striking degree of spatial organization within the synapse; for example, molecules involved in the adhesion of the interacting cells, and activators and inhibitors of the signalling cascade, segregate and take on highly specific patterns[5]. Moreover, because several of the proteins recruited to the T-cell synapse are found in detergent-resistant membrane fractions, various functions have been ascribed to lipid rafts in organizing these proteins during T-cell-receptor signalling. This instance of a signalling machine in which the appropriate spatial distribution of its components is likely to be central to its function is a promising place to look for direct evidence of a physiological role for lipid rafts.
References (abridged):
1. Douglass, A. D. & Vale, R. D. Cell 121, 937-950 (2005)
2. London, E. & Brown, D. A. Biochim. Biophys. Acta 1508, 182-195 (2000)
3. Simons, K. & Ikonen, E. Nature 387, 569-572 (1997)
4. Munro, S. Cell 115, 377-388 (2003)
5. Davis, D. M. & Dustin, M. L. Trends Immunol. 25, 323-327 (2004)
Nature http://www.nature.com/nature
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VISUALIZATION OF LIPID RAFTS IN MODEL MEMBRANES
The following points are made by Deborah A. Brown (Proc. Natl. Acad. Sci. 2001 98:10517,10642):
1) The question of whether membrane lipids mix uniformly or are arranged in discrete microdomains has vexed membrane biochemists since the structure of the membrane bilayer was first elucidated. The last few years have seen the emergence of interest in one type of microdomain: lipid rafts, or sphingolipid-rich domains in the liquid-ordered phase. The raft hypothesis states that separation of discrete liquid-ordered and liquid-disordered phase domains occurs in membranes containing sufficient amounts of sphingolipid and sterol.
2) Although significant evidence for the existence of rafts has been presented, compelling proof has remained elusive. The liquid-ordered phase domains are proposed to have properties similar to domains described in binary mixtures of a single order-preferring phospholipid and cholesterol. They are fluid, and lipids in these domains are believed to have a relatively high diffusion rate. The acyl chains are tightly packed and highly ordered. Lipids and proteins that prefer such an ordered environment are proposed to partition favorably into the liquid-ordered phase domains and thus to be enriched in rafts.
3) Dietrich et al (2001) have pioneered a new approach that allows direct microscopic visualization in model membranes of domains with properties expected of rafts, and the results support the tenets of the raft hypothesis to a remarkable degree.
Proc. Nat. Acad. Sci. http://www.pnas.org
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ON IMMUNE CELLS AND MEMBRANE LIPID RAFTS
The following points are made by L.M. Pierini and F.R. Maxfield (Proc. Nat. Acad. Sci. 2001 1998:9471):
1) To effectively combat invading pathogens, immune cells must rapidly switch from approximately spherical resting cells to polarized migratory cells, which then move in a directed fashion to the site of infection. The dramatic metamorphosis of leukocytes into polarized cells and their subsequent migration are two of the most fascinating phenomena in cell biology. Polarization and migration require the spatial and temporal control of signal transduction molecules so that substrate attachment and membrane extension occur at the cell front, while detachment and membrane retraction happen at the rear.
2) How do cells coordinate signaling molecules to perform contrasting functions at opposite poles? It has long been appreciated that there is polarization in the protein machinery involved in cell migration. However, it is becoming evident that lipids are also distributed nonuniformly, and that the distribution of lipids is an important factor for directional integration. But although it is now understood that lipids are distributed nonrandomly in the plasma membrane and that this has important consequences for cell signaling and other functions, the precise nature of these lipid inhomogeneities ("microdomains") remains somewhat enigmatic --partly because the lipid microdomains are apparently in a size range (10 to 300 nanometers) that is below the resolution of optical microscopy.
3) In the current view of the plasma membrane, certain lipids and proteins assemble into dynamic, sub-micron-sized lateral organizations that function to facilitate signal transduction events. Regions of the plasma membrane that are enriched in sphingolipids and cholesterol are thought to exist in a liquid-ordered phase that confers detergent resistance to these structures and allows for their ready isolation by flotation on sucrose density gradients. One model is that signaling molecules are recruited to these small "rafts" from a largely liquid-disordered membrane.
Proc. Nat. Acad. Sci. http://www.pnas.org
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