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Lipid Polymorphisms and Membrane Shape Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press Lipid Polymorphisms and Membrane Shape Vadim A. Frolov1,2,3, Anna V. Shnyrova1,2, and Joshua Zimmerberg4 1Unidad de Biofisica (Centro Mixto CSIC-UPV/EHU), Leioa 48940, Spain 2Departamento de Biochimica y Biologı´a Molecular, Universidad del Pais Vasco, Leioa 48940, Spain 3IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain 4Program in Physical Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 Correspondence: [email protected] Morphological plasticity of biological membrane is critical for cellular life, as cells need to quickly rearrange their membranes. Yet, these rearrangements are constrained in two ways. First, membrane transformations may not lead to undesirable mixing of, or leakage from, the participating cellular compartments. Second, membrane systems should be metastable at large length scales, ensuring the correct function of the particular organelle and its turnover during cellular division. Lipids, through their ability to exist with many shapes (polymor- phism), provide an adequate construction material for cellular membranes. They can self- assemble into shells that are very flexible, albeit hardly stretchable, which allows for their far-reaching morphological and topological behaviors. In this article, we will discuss the importance of lipid polymorphisms in the shaping of membranes and its role in controlling cellular membrane morphology. ipids are natural molecules whose ability to volumetric shape of lipid molecules is different Lself-assemble in dynamic macrostructures in different phases. This plasticity in molecular in water has been recognized as one of the im- shape was discovered in the first X-ray diffrac- portant mechanisms of evolution (Luisi et al. tion structural determinations of lipid/water 1999; Hanczyc and Szostak 2004). This ability mixtures under different osmotic stresses, and is driven by the amphiphilic nature of lipid the investigators used the appropriate term molecules which tend to aggregate so that the from crystallography, polymorphism, to de- hydrophobic (tails) and the hydrophilic (heads) scribe the “crystallization into two or more parts of the lipid are well separated, and the area chemically identical but crystallographically of the dividing surface is held by the hydropho- distinct forms” (Random House Dictionary, bic effect (Tanford 1980). 2nd ed.) (Luzatti et al. 1968). Amongst many Because lipid assemblies “hate edges,” they of such structures, the bilayer plays the most tend to form various closed structures or phases important role in our life. Besides forming the extended over the space, in which the average core of cellular membranes, it is quickly gaining Editor: Kai Simons Additional Perspectives on The Biology of Lipids available at www.cshperspectives.org Copyright # 2011 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a004747 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004747 1 Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press V.A. Frolov et al. technological popularity. Lipid capsules or To balance dynamics and stability, cells closed bilayer vesicles are widely used in cos- employ different lipid species in the membrane metics, drug delivery, etc., primarily because (Dowhan 1997). Although the core phospho- of their natural biological compatibility (Hafez lipids, such as PC, assemble stable lipid bilayers and Cullis 2001). These vesicles can be con- at physiological temperatures, many of cellu- structed from lipids or similar amphiphiles that lar lipids intrinsically destabilize bilayers and, sense the environment, temperature (through when purified, do not form bilayer phases at thermal expansion), pH (through protonation physiological conditions (Cullis and de Kruijff of the heads), and so on, so that they can be 1979). Such “nonbilayer” lipids as PE and destabilized and release their content at the tar- DAG, however, are important membrane con- get place (Hafez and Cullis 2001). Lipid sacs can stituents: they mediate proteolipid interactions also have different shapes (such as spherical, within the lipid bilayer (Dowhan 1997; Lee polyhedral, and tubular vesicles) which may 2004; Ces and Mulet 2006) and increase mor- be important for the flow properties of vesicular phological plasticity of the lipid bilayer (Hafez suspensions (Florence et al. 2004). Hence, the and Cullis 2001). The behavior of individual main technological interest resides in making lipid species can be regulated: charged lipid spe- the lipid vesicle shape and topology switchable cies, such as PS, can switch its “bilayer” identity (i.e., the regulation of the shape of the lipid at elevated pH or Ca2þ (Hafez and Cullis 2001; bilayer). Fuller et al. 2003; Zimmerberg et al. 2006). Nev- Living cells have to solve the same task on a ertheless, the overall lipid mixture of cells is daily basis. Cellular membranes are extremely always balanced to form metastable lipid bi- dynamic, as cells (and biological matter in gen- layers. Microorganisms living at different tem- eral) are in constant flux. This motion has both peratures alter the ratio between bilayer and constitutive aspects that tend toward homeo- nonbilayer lipids dependently on the growth stasis, and regulated aspects that are governed temperature (de Kruijff 1997). Besides dynamic by signaling networks. Consequently, cellular regulation of the bulk lipid composition, cells membranes preserve their morphology at large specifically create nonuniform distribution of scale, forming organelles with distinct shape lipids within membrane organelles. First, in and function, whereas at smaller scales the most of the membrane systems, except endo- membranes are constantly remodeled to main- plasmic reticulum (van Meer et al. 2008), there tain dynamic material exchange between dif- is substantial compositional asymmetry be- ferent membrane formations (Paiement and tween the two monolayers: for example, in Bergeron 2001; McMahon and Gallop 2005). plasma membrane such active lipid species as This remodeling, however, may not interfere PS and PE are accumulated in the monolayer with the barrier function and the overall struc- facing the cytoplasm (Hafez and Cullis 2001; tural stability of the membranes. This combi- Janmey and Kinnunen 2006; van Meer et al. nation of flexibility and the resistance against 2008). Second, lipids and proteins can segregate rupture is brought by the lipid bilayer. Despite laterally by forming domains of distinct compo- the extreme protein crowding, the lipid bilayer sitions and functions (Lingwood et al. 2009). remains the structural core of cellular mem- These three factors, large amounts of nonbilayer branes. The protein area occupancy in mem- lipids, trans-bilayer asymmetry, and lateral in- branes generally does not exceed 20% (Dupuy teractions and segregation of protein and lipid and Engelman 2008), so that the mean distance species in the cellular membranes, provide a between membrane-inserting parts of the pro- basis for the active involvement of lipids in cel- teins is 10 nm, more than 10 lipid molecules lular morphogenesis, as we describe below. (Phillips et al. 2009). Thus, lipids “glue” the The morphological activity of the lipid membrane components together and in the bilayer is regulated by special enzymes main- same time actively participate in membrane taining or changing the lipid distribution in remodeling (Chernomordik and Kozlov 2003). the membrane. This way, the lipid content of 2 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004747 Downloaded from http://cshperspectives.cshlp.org/ on October 2, 2021 - Published by Cold Spring Harbor Laboratory Press Lipid Polymorphisms and Membrane Shape the membrane can be matched to the cellular profile, which determines the redistribution of “morphological” needs and to the physiological stresses across the lipid bilayer (Marsh 2007). function of a particular membrane compart- By looking at the cross section of the lipid ment, making lipids the mediators of the struc- bilayer, several distinct regions dominated by ture/function relationships at the organelle the proper set of forces become noticeable. level (Shnyrova et al. 2009). Some fundamental The most important region for the structural principles linking the molecular identity of lipid stability of the monolayer is the one near the species and their function are embedded into interface dividing the hydrophilic and hydro- the molecular architecture of the lipids. There- phobic parts of the lipids. Even a small increase fore, lipids can be designed (synthesized, me- in the area of this dividing surface leads to tabolized, or intracellularly remade) to have a hydrophobic tail exposure, a process that is predictable morphological behavior. very costly energetically. Thus, the lipid mono- layers are hardly expandable as lipids are held MOLECULAR ORGANIZATION OF LIPID together by the hydrophobic effect resulting POLYMORPHISM from the high negative pressure peaked at the dividing surface (Fig. 1). Changes in the divid- Forces Driving Self-Assembly ing surface affect the membrane morphology of Lipid Bilayers the most so that this region is a general target Although the intrinsic wish of lipids to avoid for proteins that control membranes via hydro- direct exposure
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