|
Mind the membrane
Joachim Pietzsch
From inert to dynamic: our perception of the cell membrane has changed dramatically over the years, in ways that continue to surprise and amaze researchers.
Picture yourself in an olive oil river, with protein-filled trees and sugary skies. Imagine a storm: everything flows and rapidly floats and constantly changes its shape. Rafts emerge and dissolve almost immediately, trees receive signals that disappear as quickly as they arrive, tiny ions whizz around and dive in and out of the river, and the traffic around you is dizzily dense. Relax, this is no psychedelic fantasy; it's our view of the biological world's most useful border: the cell membrane.
All that forms the scaffold for this living frontier in each of the 100 trillion cells in our body is a film of fat between 5-10 nm thick. This wafer-thin layer, constructed from many different types of lipids, houses countless proteins - for example, a liver cell whose surface covers 110,000 �m� can, depending on the metabolic situation, carry as many as 300,000 insulin receptors on its membrane, not to mention all the other protein receptors and channels that populate this border.
The membrane creates an almost impermeable barrier to the passage of water-soluble molecules. Providing a border its not the membrane's only role, it also acts as a switchboard and a gatekeeper, as a receiver and an amplifier, as a regulator and a static stabilizer. Without this frontier, we would not exist: the membrane potential, for example, between the cytosol and the environment of a cell, is an absolute prerequisite for life. Life means valuing the differences between dynamic polarities. It resembles the continuous calculation of a differential equation based on matter and energy; and, in practice, membranes are linking both sides of this equation.
Membranes are everywhere. The outer cell membrane accounts for only a small proportion of the entire membrane fraction of a cell. The rest is compartmentalizing its cytosol. More than 50% is reserved for an organelle called the endoplasmatic reticulum, which is present in nearly all of the 220 different cell types of our body, and is involved in processes such as protein synthesis and insertion of proteins into the membrane.
The basic structure of the membrane is the lipid bilayer (FIG. 1),which has become a popular icon in biology textbooks. The existence of the bilayer was first postulated in 1925 by Gorter and Grendel who had studied lipid extracts of membranes from red blood cells. It took several decades and required major advances in imaging technology, however, until their hypothesis was generally accepted. The structure of the bilayer is derived from the amphiphatic nature of each phospholipid - the type of lipid predominantly used to make up the cell membrane. Phospholipids have a phosphate head that likes water and lipid tail that hates it. The natural repulsion between fats and water creates hydrophobic forces that pushes the hydrophobic tail from one phospholipid towards the tail from another. Tail-by-tail the lipids line up to form a bilayer whose inner space is hydrophobic. Only its surfaces can be exposed to water, and once the bilayer forms, hydrogen bonds, electrostatic attractions and van der Waals forces further stabilize the membrane. All these natural forces holding the bilayer together means that the membrane could spontaneous reform should it become damaged.
With the development of more advanced analytical methods, such as electron microscopy and NMR spectroscopy, scientists learnt more about the composition of the membrane. They discovered that the bilayer behaves more like a fluid than a solid, and they made inroads in examining and characterizing the many kinds of proteins that are embedded in or associated with the membrane - a task that is still challenging today because of the poor solubility of most of these proteins in water.
As a result of these greater insights, in 1972, Singer and Nicholson proposed a new view of the membrane: the Fluid-Mosaic-Model (FIG. 2). Here, the proteins are embedded in a fluid bilayer, partly integrated, partly peripherally attached, with sugar moieties anchored on many proteins of the outer surface. In this fluid mosaic, both lipids and proteins diffuse laterally in the plane of the membrane, at 2 �m and 1 �m per second, respectively. This is relatively quick, considering that a cell from the bacterium Escherischia coli is 2 �m long. So, there is constant motion on the membrane, yet only in two dimensions, along and across the membrane, because there is no diffusion from one lipid layer to the other.
To cross through both layers of the membrane is an energy-consuming task that requires specialized proteins, such as glucose transporters and ion channels that span the bilayer (FIG. 3). A family of proteins called G-protein-coupled receptors traverse the membrane seven times. They transduce signals from external mediators to G-proteins at the inner side of the membrane which then activate second messengers that trigger processes as diverse as sensory perception and cell communication. Other receptors span the membrane only once, like adhesion molecules that are involved in interactions with other cells and the extracellular matrix. To bury themselves inside the inner membrane all these proteins need to have long sequences of hydrophobic rather than water-loving amino acids. Presumably, none of these proteins are present in the membrane at a steady state. All of them are subject to overriding interests of the organism, and as a consequence, the membrane is permanently under reconstruction and there's traffic all the time.
The need for these proteins is of the utmost importance: at least one-third of all protein-coding genes are thought to code for membrane proteins. So it is no wonder that the importance of the membrane was for a long time attributed almost exclusively to its proteins. Growth and development, hormone action, energy production, glucose uptake, immune response and neuronal signalling, connection to the cytoskeleton and to neighbouring cells and many other membrane-dependent features of life - why would the boring lipid bilayer have anything to do with these tasks?
As it turns out, these dull, passive lipids are not half as boring as first thought. Since the 1990s, scientists are compiling increasing evidence that membrane lipids might be co-players in many physiological processes rather than merely a playground for proteins. Membrane domains have been discovered that have a different composition and physical state than the surrounding bilayer (FIG. 4). They are rich in a particular type of lipid called sphingolipids and cholesterol and appear as small invaginations (caveolae) or elevations (lipid rafts). They change their size and shape considerably within seconds, depending on the metabolic state of the cell. One cannot imagine them as being unstable enough, according to Michael Edidin, who describes membrane domains as resembling "a mixture of heavy and light cream that is on the verge of blending into a single fluid, but that is refreshed by new deliveries of one type of cream or the other"1.
 |
|
|
Figure 4 | Model for the organization of rafts and caveolae in the plasma membrane.
Lipid rafts (red/green) segregate from the other regions (blue) of the bilayer, which have a different lipid composition. The lipid bilayer in rafts is asymmetric, with sphingomyelin and glycosphingolipids (both red) enriched in the 'outer' layer known as the exoplasmic leaflet, and glycerolipids (green) in the 'inner' layer known as the cytoplasmic leaflet. a | Rafts contain proteins attached to the bilayer by their GPI anchors, by acyl tails (for example, the Src-family kinase Yes), or through their transmembrane domains, like the influenza virus proteins neuraminidase and haemagglutinin (HA). b | Caveolae are formed by self-associating caveolin molecules making a hairpin loop in the membrane. Interactions with raft lipids may be mediated by binding to cholesterol (brown) and by acylation of carboxy-terminal cysteines (not shown). Figure modified from Simons, K. & Ikonen, E. Functional rafts in cell membranes. Nature 387, 569-572 (1997) � Macmillan Publishers Ltd.
|
It is not yet proven that lipid rafts exist. A few scientists insist they are artefacts, but many more are convinced that lipid rafts and caveolae are proteins' little helpers and organizers. Lipid rafts probably help proteins to function together. They are thought to be involved in cholesterol transport. They might even help connect the two sides of the bilayer.
In any case, it seems to be clear that there are stronger interactions between lipids and proteins in the membrane than previously thought. For example, ion channel proteins in the membrane are thought to modulated by their lipid environments, as lipid rafts seem to determine the localization of these proteins.
These recent observations are again challenging scientists perspective of the membrane. As these new features cannot be integrated into the Fluid-Mosaic-Model, a whole new field of research has opened to interpret the dynamic dance between the lipids and proteins. This could help increase understanding of the role that membranes play in disease. Few diseases are directly caused by membrane deficiencies, but nearly all diseases somehow involve the membrane, for example, pertubations in ion channels lead to a host of diseases, such as cystic fibrosis and epilepsy. Within 100 years our perception of the membrane has changed from an inert border to a living, dynamic frontier, and now is an exciting time to consider the influences that this most useful of borders has on life.
|
