Singer-Nicolson membrane model

The first membrane model to be widely accepted was that proposed by Danielli and Davson in 1935 [528]. On the basis of the observation that proteins could be adsorbed to oil droplets obtained from mackerel eggs and other research, the two scientists at University College in London proposed the sandwich of lipids model (Fig. 7.2), where a bilayer is covered on both sides by a layer of protein. The model underwent revisions over the years, as more was learned from electron microscopic and X-ray diffraction studies. It was eventually replaced in the 1970s by the current model of the membrane, known as the fluid mosaic model, proposed by Singer and Nicolson [529,530]. In the new model (Fig. 7.3), the lipid bilayer was retained, but the proteins were proposed to be globular and to freely float within the lipid bilayer, some spanning the entire bilayer. 

L., Matyus, L., Waldmann, T.A., Damjanovich, S. (2003) Dynamic, yet structured the cell membrane three decades after the Singer-Nicolson model. Proc. Natl. Acad. Sci. USA 100, 8053-8058. 

Fig. 2.3. Diagrammatic representation of the molecular organisation of the tegument plasma membrane (based on the fluid mosaic model of membrane structure of Singer Nicolson (1972)). The carbohydrate moieties of the membrane glycoproteins and glycolipids are exposed on the external face as the glycocalyx. (After Smyth Halton, 1983.)...

G. Vereb, J. Szollosi, J. Matko, P. Nagy, T. Farkas, L. Vigh, L. Matyus, T.A. Waldmann, S. Damjanovich, Dynamic, Yet Structured The Cell Membrane Three Decades After the Singer-Nicolson Model , Proc. Nad. Acad. Sci. USA, 100,8053 (2003)... 

Figure 1 Singer-Nicolson model of fluid membrane. (From Reference 2.)...

The Singer-Nicolson model of the membrane played a very important role in understanding membrane structure and function. However, many properties of biomembranes are not consistent with this model. In recent years, a growing consensus points at more complex membrane structure, which can be characterized as dynamically structured fluid mosaic. Compared with the original fluid mosaic model, the emphasis has shifted from fluidity to mosaicity. Experimental observations have led to the membrane microdomain concept that describes compartmen-talization/organization of membrane components into stable or transient domains. 

Because the assumption of simple Brownian diffusion breaks down, the diffusion in biomembranes cannot be described by a single diffusion coefficient. For instance, FRAP experiments in the plasma membrane showed that the observed translational diffusion rates depend on the size of the initial photobleached spot, which is also inconsistent with a simple Singer-Nicolson model. 

To reconcile this apparent contradiction the membrane skeleton fence and anchored transmembrane picket model was proposed (54). According to this model, transmembrane proteins anchored to and lined up along the membrane skeleton (fence) effectively act as a row of posts for the fence against the free diffusion of lipids (Fig. 11). This model is consistent with the observation that the hop rate of transmembrane proteins increases after the partial removal of the cytoplasmic domain of transmembrane proteins, but it is not affected by the removal of the major fraction of the extracellular domains of transmembrane proteins or extracellular matrix. Within the compartment borders, membrane molecules undergo simple Brownian diffusion. In a sense, the Singer-Nicolson model is adequate for dimensions of about 10 x lOnm, the special scale of the original cartoon depicted by the authors in 1972. However, beyond such distances simple extensions of the fluid mosaic model fail and a substantial paradigm shift is required from a two-dimensional continuum fluid to the compartmentalized fluid. 

This fluid-mosaic membrane model allows proteins their freedom to diffuse in the plane of the lipid membrane and hence to become distributed over the cell surface in a pattern that is sometimes random and sometimes homogeneous (Singer and Nicolson, 1972). 

Looking at the Singer-Nicolson model (1) biomembranes mainly consist of lipids and proteins, the latter being either partially (peripheral proteins) or completely (integral proteins) embedded into the lipid matrix. These proteins contribute a major part to the stability of natural membranes (2). The use of these proteins, however, to stabilize an artificial membrane system via incorporation is rather limited and not yet realizable. The stabiliztation of the lipid matrix itself, therefore, appears to be a more convenient method also providing a greater potential use. 

Goni FM. The basic structure and dynamics of ceU membranes an update of the Singer-Nicolson model. Biochim Biophys Acta. 2014 1838(6) 1467-1476. 

The major components of biological membranes are lipids and proteins. The lipids are arranged in a bimoleciilar leaflet. The proteins are either incorporated into the bilayer or are bound to the surface by electrostatic interactions, the two protein classes being called intrinsic (or integral) and extrinsic (or peripheral) membrane proteins. The cuiTent membrane model is still based on the proposal by Singer and Nicolson from 1972 [I]. Figure 1 shows a cartoon of a biological membrane with its intrinsic proteins. 

This fluid-mosaic membrane model allows proteins their freedom to diffuse in the plane of the lipid membrane and hence to become distributed over the cell surface in a pattern that is sometimes random and sometimes homogeneous (Singer and Nicolson, 1972). There are (a) integral proteins running perpendicularly to the plane of the lipid layer, some passing right through the bilayer, others only half way, and (b) extrinsic proteins that have become adsorbed on one or other of the surfaces of the bilayer and are easily lost. 

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

Singer, S. J., and Nicolson, G. L., 1972. The fluid mosaic model of the structure of cell membranes. Science 175 720-731. 

The overall picture of a membrane then becomes one in which the surface presents a background of lipids in- and onto which are placed proteins with specific positions and functions. This is called the fluid mosaic model as proposed by Singer and Nicolson. 

Figure 8-5 The fluid mosaic model of Singer and Nicolson.61 Some integral membrane proteins, which are shown as irregular solids, are dissolved in the bilayer. Transmembrane proteins protrude from both sides. One of these is pictured as a seven-helix protein, a common type of receptor for hormones and for light absorption by visual pigments. Other proteins adhere to either the outer or the inner surface. Many membrane proteins carry complex oligosaccharide groups which protrude from the outer surface (Chapter 4). A few of these are indicated here as chains of sugar rings.

Membranes of plant and animal cells are typically composed of 40-50 % lipids and 50-60% proteins. There are wide variations in the types of lipids and proteins as well as in their ratios. Arrangements of lipids and proteins in membranes are best considered in terms of the fluid-mosaic model, proposed by Singer and Nicolson % According to this model, the matrix of the membrane (a lipid bilayer composed of phospholipids and glycolipids) incorporates proteins, either on the surface or in the interior, and acts as permeability barrier (Fig. 2). Furthermore, other cellular functions such as recognition, fusion, endocytosis, intercellular interaction, transport, and osmosis are all membrane mediated processes. 

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