Non-bilayer structures

Phospholipid(s) 379, 380,382 - 387, 392. See also Specific substances bilayer diagram 391 head groups, functions of 396 inverted hexagonal phase 397 31P NMR 397 non-bilayer structures 397 Phosphomannomutase 654 Phosphomutases 526 Phosphonamidate 626s... 

In contrast to A. laidlawii, E. coli maintains its non-bilayer lipids, CL (in the presence of divalent cations) and PE, within a narrow range and in wild-type cells adjusts the fatty acid content of PE to increase or decrease its non-bilayer potential (S. Morein, 1996). The unsaturated fatty acid content of inner membrane PE is higher than that of the PE on the inner leaflet of the outer membrane (which is 90% PE). The result is that the L to H transition for the inner membrane pool is only 10-15°C above the normal growth temperature of 37°C, while this transition for the outer membrane phospholipids is 10°C higher than the inner membrane phospholipids. This increased potential for the inner membrane lipids to form non-bilayer structures is believed to be biologically significant for the function of the inner membrane. In mutants completely lacking PE, the role of non-bilayer lipid appears... 

Phospholipase C activity is not directly influenced by the formation of non-bilayer structures. However, the presence of lipids (e.g., PE) with a tendency to form such structures stimulates the enzyme even under conditions at which purely bilayer phases exist. Conversely, sphingomyelin, a stabilizer of the bilayer phase, inhibits the enzyme. Thus, phospholipase C appears to be regulated by the overall geometry and composition of the bilayer (M.B. Ruiz-Arguello, 1998) supporting the hypothesis that the collective physical properties of the lipid bilayer can modulate the activities of membrane-associated proteins. 

On the other hand, the neutral non-bilayer-forming lipid MGlcDG can substitute for PE in cell division (M. Wikstrbm, 2004), suggesting that common properties of these two lipids, such as the ability to form non-bilayer structures and dilution of negative charge, are important for cell division [1]. 

Biological membranes, however, also contain lipid components which, in isolation, adopt non-bilayer structures under physiological conditions [114-118]. These non-bilayer structures can be influenced isothermally by a wide variety of physiological factors such as changes in the concentrations of divalent cations, changes in... 

As discussed above, the biogenesis of oat thylakoids is accompanied by a remarkable increase in the potential of the outer monolayer to form non-bilayer structures (Table 2). Thus, incorporation into the greening membrane of large amount of chlorophyll-protein complexes, which are known to interact with MGDG and PG molecules, may be necessary to preserve lamellar structures [10]. In contrast, acyl lipids alone in both prothylakoid monolayers are able to form by themselves stable lamellar structures. In conclusion, incorporation of chlorophyll-protein complexes into the nascent... 

Quinn, P.J. and Williams, W.P. (1983). The structural role of lipids in photosynthetic membranes, Biochim. Biophys. Acta, 737, 223-266. Gounaris, K., Sen, A., Brain, A.P.R., Quinn, P.J. and Williams, W.P. (1983a), The formation of non-bilayer structures in total polar lipid extracts of chloroplast membranes, Biochim. Biophys. Acta, 728, 129-139. 

Concentration of particular lipid types by this phase separation can cause the formation of non-bilayer structures. Lipid adaptations to environmental temperatures are discussed in section 8.2. 

Detailed discussion of membrane properties and functions is beyond the scope of this book - numerous detailed reviews being available. Nevertheless, a few remarks centred on the role of lipids can be made. In Chapter 6 we described current ideas for membrane structure and the concept that, although lipids are capable of adopting non-bilayer structures when isolated, by-and-large they participate in bilayers in vivo. 

An important clue as to the mechanism of lipid membrane fusion has come from the work of de Kruijff and Cullis. They proposed that fusion proceeded because of the ability of lipids to undergo polymorphism (i.e. to adopt different structures). Three types of observation support the hypothesis. First, fusogens (such as monoacylglycerols) induce Hu (inverted micellar structures Figure 6.11) phase structures, consistent with a role for non-bilayer structures in fusion. Second, promotion of fusion of, for example, phosphatidylserine-containing systems by Ca is accompanied by Hji structures. Third, a number of factors such as pH changes or elevated temperatures that cause Hn formation also promote fusion of lipid vesicles. 

Extension of the Cullis/de Kruijff hypothesis to natural membrane events has been difficult to prove. Nevertheless, confirmatory evidence has been obtained with a number of systems such as the release of chromaffin granule contents during stimulation of the adrenal medulla. The exocytosis accompanying this event seems to depend on the ability of Ca to promote non-bilayer structures. 

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