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Functional Consequences of Lipid Packing Stress

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Functional Consequences of Lipid Packing Stress Current Opinion in Colloid & Interface Science 5Ž. 2000 237᎐243 Functional consequences of lipid packing stress Sergey M. Bezrukov a,b,U aLaboratory of Physical and Structural Biology, NICHD, NIH, Bethesda, MD 20892-0924, USA bSt. Petersburg Nuclear Physics Institute, Gatchina, Russia 188350 Abstract When two monolayers of a non-lamellar lipid are brought together to form a planar bilayer membrane, the resulting structure is under elastic stress. This stress changes the membrane's physical properties and manifests itself in at least two biologically relevant functional aspects. First, by modifying the energetics of hydrophobic inclusions, it in¯uences protein᎐lipid interactions. The immediate consequences are seen in several effects that include changes in conformational equilibrium between different functional forms of integral proteins and peptides, membrane-induced interactions between proteins, and partitioning of proteins between different membranes and between the bulk and the membrane. Secondly, by changing the energetics of spontaneous formation of non-lamellar local structures, lipid packing stress in¯uences membrane stability and fusion. ᮊ Published by 2000 Elsevier Science Ltd. Keywords: Integral proteins; Intra-membrane pressure; Non-lamellar lipids 1. Introduction derived second messengers serve as ligands for highly speci®c biochemical reactions. Although a role for Membrane lipids are no longer regarded as a kind phosphoinositides in signal transduction was ®rst sug- wx of ®ller or passive solvent for the membrane protein gested about half a century ago, recent reviews 1,2 machinery. It is now well established that lipids play have demonstrated new exciting developments in this an important role at several levels of cell regulation. growing ®eld. Another example of speci®c regulation This functional involvement naturally explains why is the highly selective interaction of cytochrome c wx cells exquisitely control the lipid composition of their oxidase with cardiolipin 3 . Cardiolipin is unique to membranes. Still, the mechanisms of membrane᎐pro- the mitochondrial membrane of mammalian cells and tein interaction and the constraints upon the lipid is found to be a very ef®cient activator of this enzyme. composition of organelles and cell membranes are High speci®city is also reported in lipid-assisted pro- poorly understood. tein folding where lipids may play a role of molecular The ways by which lipids ful®ll their regulatory role chaperoneswx 4 . are complex and diverse, but they can be conditionally Non-speci®c regulation does not involve any divided into speci®c and non-speci®c. Probably the biochemical reactions or high selectivity with respect best known example of a speci®c mechanism is the to ®ne chemical detailswx 5 . Instead, it is realized inositol phospholipid signaling pathway. Here, lipid- through the changes in membrane physical parame- ters, such as membrane hydrocarbon thickness, sur- face charge density, polar layer potential, lipid head- U Tel.: q1-301-4024701; fax: q1-301-402-9462. group hydration, etc. This paper discusses only one E-mail address: [email protected]Ž. S.M. Bezrukov . aspect: non-speci®c regulation from the elastic stress 1359-0294r00r$ - see front matter ᮊ 2000 Published by Elsevier Science Ltd. PII: S 1 3 5 9 - 0 2 9 4Ž. 0 0 00061-3 238 S.M. Bezruko¨rCurrent Opinion in Colloid & Interface Science 5() 2000 237᎐243 of packing of non-lamellar lipid molecules into planar zero. The higher the lipid spontaneous curvature, the bilayer structures. higher the repulsion between hydrocarbon chains. Almost 40 years ago it was observedwx 6 that many Several physical properties of a membrane are phospholipids found in plasma membrane bilayers, modi®ed by lipid packing stress. The direction of the when puri®ed, do not form lamellar phases. Instead of change, however, depends on the particular way the forming a `stacked bilayer phase', they favor packing stress is introduced. Even if all manipulations lead to into inverted hexagonal bulk phases. This observation an increasing negative curvature strainŽ lipid led researchers to suggest that that these `non-bilayer' monolayers that tend to form inverted hexagonal or lipids have a special functional role in biological regu- cubic phase but held in a planar con®guration. , the lationwx 7᎐9 ; however, the range of functional conse- outcome depends on whether the repulsion between quences and underlying physical mechanisms are still headgroups is reduced or the repulsion between hy- energetically discussedwx 10,11 . drocarbon chains is increased. NMR experimentswx 16 When a planar membrane is formed by two show, for example, that going from phosphatidyl- monolayers of non-lamellar lipids, these monolayers cholineŽ. PC to smaller phosphatidylethanolamine undergo elastic deformation. Their spontaneous state Ž.PE reduces repulsive forces between headgroups with a ®nite equilibrium curvature is disturbed by and reduces the area per lipid molecule by a few ¯attening, which is necessary to form a planar struc- square A.Ê It also increases chain order and hy- ture. The resulting elastic stress can be seen as a drophobic membrane thickness. On the other hand, lateral pressure that varies with depth in the mem- an increase in the negative curvature strain obtained branewx 12᎐15 . Diagrams in Fig. 1 illustrate the idea by an increase in hydrocarbon chain length or in and also provide an example of possible pressure degree of unsaturation increases the area per distributions. The pressure pro®les are comprised of molecule and lowers the chain order. From osmotic repulsion between headgroups and between the hy- stressrX-ray diffraction experiments, it is also known drocarbon chains of adjacent lipid molecules, which is that going from PC to PE changes the hydration compensated by attractive interfacial tension. In the properties of lamellar phases. In the case of PE case of exact compensation, the membrane tension is bilayers, an additional short-range attractive interac- Fig. 1. Lateral pressure p in a planar bilayer membrane changes along the membrane depth z and depends on the lipid nature.Ž. a When a membrane is assembled from spontaneously lamellar lipids or lipids with a small spontaneous curvature, the corresponding pressure pro®le in the hydrocarbon tail area is shallow.Ž. b Lipids of higher negative spontaneous curvatures introduce higher pressures in the chain area. S.M. Bezruko¨rCurrent Opinion in Colloid & Interface Science 5() 2000 237᎐243 239 Fig. 2. Two models showing sensitivity of hydrophobic inclusions to the lipid packing stress.Ž. a Changes in hydrophobic mismatches upon conformational transition modify lipid packing around the inclusion. For a negative curvature stress, conformation II is energetically preferred.Ž. b Conformation transition resulting in a changing shape may also change lipid packing around the inclusion. Conformation II relieves elastic stress and is energetically favorable. tion was found. However, this interaction is possibly system exceeds its elastic deformation energyw 28ⅷ x . In due to a hydrogen-bonded water interaction that is the case of strong coupling and short inclusions, lipids speci®c for PE headgroups of the opposing bilayers with negative curvature stress will favor conformatio- wx17 . nal transitions that increase the hydrophobic length of inclusions to a larger degree than lamellar lipids. Length-increasing transitions not only decrease the 2. Hydrophobic inclusions under lipid packing stress elastic stress of compression caused by hydrophobic mismatches, but also reduce the positive curvature of Non-lamellar lipids affect the activity of membrane the surrounding lipidwx 20 . proteins and peptides. Though the physics of this In the second modelŽ. Fig. 2b , lipid packing stress is phenomenon remains largely unclear, the number of relieved by the cylinder-hourglass transition wx ⅷⅷⅷⅷⅷ phenomenological examples is impressive 10 . Among w22 ,23 ,27x . Sensitivity to non-lamellar lipid com- recent ®ndings are the modulation of volume-regu- ponents comes from a redistribution of lateral pres- wx lated anion currents in bovine endothelial cells 18 , sures. Higher lateral pressures in the hydrocarbon where the authors attributed cholesterol-induced ef- chain region are expected for lipids with higher nega- fects to the membrane deformation energy associated tive spontaneous curvaturesŽ. Fig. 1 and, therefore, with channel opening, and the results on the elastic- these lipids promote the hourglass conformation. stress-modi®ed activity of bacteriorhodopsin in a novel According to statistical calculations by several refolding systemwx 19 . groupswx 12,13,15 , the average lateral pressure in the The physical mechanisms by which membrane pro- hydrocarbon chain region can be as high as several teins respond to the elastic stress of lipid packing are ⅷ hundred atm, and, at certain points along the mem- attracting signi®cant interestw 20᎐29x . Obviously, to brane depth, can even peak to above 1000 atmwx 12 . be sensitive to mechanical stress, protein conforma- tional transitions have to be coupled to some mechan- These results are in reasonable agreement with a simple estimate based on the work of Rand et al. ical displacements that change the elastic stress of wx nearby lipids. Two main ideas are illustrated in Fig. 2. 32,33 , which showed that the change in the lateral ᎐ The ®rst modelŽ. Fig. 2a is based on the concept of pressure upon the reentrant hexagonal lamellar tran- hydrophobic mismatchwx 30,31,25
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