\The purpose of biological membranes is to create compartments suitable for specific biological functions. Aside from generating internal environments, membranes also carry out their own functions. For example, membranes carry out critical functions in signal transduction and vesicle (de)formation . The idea of lipid “rafts”, or distinct domains within the membrane, has been proposed as a way to spatially and temporally regulate membrane function . Theoretically, the formation of rafts are thermodynamically favorable. In order to maximize the exclusion of water and increase entropy, acyl chains of similar lengths interact with one another. Similarly, transmembrane (TM) proteins, depending on the length of their TM domain, also preferentially interact with acyl chains of a similar length. The matching of hydrophobic chains and TM domains also decreases enthalpy which is even more favorable. The thermodynamic separation of lipids will also result in phase separations throughout the membrane. Phase separation is one of the simplest examples of membrane asymmetry. Alternatively, membranes can be mechanically manipulated by other molecules such as sterols or proteins, changing the phase and/or shape of the membrane. The resulting 'microdomains' are a simple example of membrane asymmetry. Just as the name suggests, membrane asymmetry happens when a membrane no longer resembles uniformity in terms of lipid or protein distribution and relative leaflet curvature. Asymmetry can occur on both sides of a biological membrane or on just one. Biology has evolved a wide variety of lipids, sterols, and membrane-associated proteins in order to achieve a wide diversity of membrane symmetry (FIG1).
Figure 1. The different phase transitions of different lipids form the basis of lipid rafts.
This wiki will discuss some of the early evidence for lipid rafts, which was at one time a highly controversial topic. The contribution of lipids, sterols, and proteins to domain formation will also be presented with specific examples.
The lipids within a membrane are responsible for the overall shape. The charge of the head group, number of tails, length of tails, and size of the head group are all important factors in deciding membrane symmetry. For example, large, charged head groups are more likely to form curved structures. On the other hand, uncharged lipids with longer acyl chains and smaller head groups would be less likely to form curved structures (FIG 2). Thus, local enrichment of specific lipids can induce different curvatures and domains .
Figure 2. Intrinsic lipid properties cause zero, positive, or negative curvature, respectively. Lipid types have different phase transitions and preferences of interactions. An example of each type is listed below the representation.
The charge of the head group, size of the head group, length of the acyl chain, and saturation of the acyl chain are all factors in membrane asymmetry. Lipid molecules tend to group thermodynamically with those that have similar tail length and saturation thus leading to phase separation and domains within the membrane. As stated earlier, lipid molecules are not the sole reason for membrane asymmetry. Sterols and proteins also have a key role in changing the phase and shape of the membrane.
Cholesterol can be thought of as the moderator of order within the lipid bilayer. Through its simultaneously bulky head and long tail, cholesterol is able to disorder the gel phase and order the liquid phase of lipids (FIG 3) .
Figure 3. Sterols are able to modulate the order of lipid bilayers. Both the gel phase and the liquid disordered phase are mediated toward the liquid ordered phase.
Another cause of membrane asymmetry is protein manipulation. Proteins tie into the earlier concepts discussed because they interact preferentially with different lipid head groups and tails.
Membrane domains and protein
Perhaps the best example of protein’s influence on membrane curvature is the Bin-amphiphysin-Rvs (BAR) domain. Simulation of membranes is one of the most common ways to study membrane asymmetry on very small time scales. BAR domains are seen in many important biological context including vesicle and exosome budding. It is no surprise that proteins containing a BAR domain are among the most conserved in biology. BAR domains are banana-shaped dimers that both sense and induce membrane curvature (FIG 4). They interact with the negatively charged lipids electrostatically with positively charged residues on one side of their surface, leading to the amphipathic nature of these helical domains.
Figure 4. The effect of the BAR activity is additive. A simulation visualizes 6 BAR domains cooperate over time to bend a lipid bilayer.
The computational study in figure 3 was important because it suggested the additive model was correct. This simulation provided some of the first evidence that BAR domains bind linearly along a membrane and that their effects are cooperative .
Membrane spanning proteins also have an extremely important role in membrane manipulation. Local curvature of membranes can be heavily influenced by helical protein insertion . This is true for both anchored and membrane-spanning proteins. The former are typically amphipathic helices, which means they have both a polar and a non-polar face. Non-polar faces typically embed in the membrane and interact with the hydrophobic acyl chains while the polar surface remains near the phospholipid head groups. This kind of asymmetric insertion induces positive curvature in the membrane (FIG 5).
Figure 5. Example of amphipathic helix anchoring. The hydrophobic face buries into the acyl tails; the polar face remains solvent exposed. H.T. McMahon, J. L. Gallop Nature 438, 590-596 (2005).
One way to study the effects of proteins on membrane curvature is to utilize light microscopy to visualize liposomes or giant lamellar vesicles (GUVs) with and without the presence of protein. With these methods it has been determined that membrane-spanning helices can also manipulate membranes. One common mechanism is by protein oligomerization within the membrane. An example of this membrane manipulation is seen within a specific subdomain of the mitochondrial inner membrane called the cristae junction (CJ). CJs connect the inner boundary membrane to the cristae membranes where oxidative phosphorylation takes place. CJs are highly curved membranes that are responsible for the large surface area of the inner membrane. In fact, Mic10, a subunit of the mitochondrial contact site and cristae organizing system, coordinates the curvature of CJs by oligomerization through its transmembrane helices .
Figure 6. Mic10 is able to curve lipid bilayers via two glycine-rich motifs. Mic10 is able to cause liposomes to form tubule structures vs the control detergent (DDM) and protein (Tim23). Barbot et al, Cell Metabolism (2015). Copyright © 2015 Elsevier Inc. All rights reserved. This is an unofficial translation of an article that appeared in an Elsevier publication. Elsevier has not endorsed this translation.
Through a glycine-rich oligomerization domain in the transmembrane helices, Mic10 is able to self-assemble and curve membranes (FIG 6). Deletion of Mic10 has severe consequences in respiration due to loss of membrane surface area.
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