Lipids and proteins interact in order to determine membrane structure and shape

As discussed in Section 6.5.3 (Fig. 6.12) the shape of complex lipid molecules can give rise to polymorphism. In addition, they may also contribute to curved regions of bilayers. Israelachvili suggested that, for regions of high curvature (such as at the ends of thylakoid stacks in chloroplasts) cone-shaped lipids would be needed in one leaflet and inverted cone-shaped lipids in the other leaflet (Fig. 6.19). This is an attractive idea but examination of the transbilayer distribution of chloroplast lipids by various techniques has shown that the enrichment of monogalactosyldiacylglycerol (inverted cone) is the opposite of that required, i.e. it is more abundant in the outer leaflet. Nevertheless, because it is entirely possible that a small proportion of certain lipids may be concentrated laterally in certain membrane regions, it is still plausible that lipids alone may play a significant role in determining natural membrane shape. Experiments with whole membrane fractions would not pick up such a specific distribution, of course, but would merely provide an averaging of transbilayer composition. Indeed, some of the results with differently shaped lipids during membrane fusion experiments (Section 6.5.10) would support a role for different lipids in helping to stabilize regions of high membrane curvature.

Much more likely is the co-operation of membrane lipids and proteins in controlling membrane shape and properties. For example, membranes containing very large integral protein complexes often contain high amounts of lipid able to form HII phases when purified. Thus, chloroplast thylakoids contain around 50% of the non-bilayer-forming monogalactosyldiacylglycerol while the photo-synthetic membrane of purple non-sulphur bacteria is enriched in unsaturated phosphatidylethanol-amine (also HII-forming). It has been suggested that these inverted cone-shaped lipids may help to

Fig. 6.19 How lipids could cause bilayer curves.

Fig. 6.20 The relationship between lipid composition and erythrocyte shape. Reproduced with kind permission of Professor L.L.M. van Deenen and Elsevier Trends Journals, from Trends in Biochemical Sciences (1985), p. 322, Fig. 3.

package the integral proteins and ensure a good bilayer structure, which must of course be impermeable to ions and other molecules.

A situation where proteins and lipids appear to interact in determining membrane properties is in the erythrocyte. This simple cell can have its membrane composition altered in a non-destructive fashion by the use of phospholipid exchange proteins. The Dutch biochemists op den Kamp and van Deenen and their colleagues introduced specific phosphatidylcholine molecules into the outer leaflet by such means and showed that, depending on the molecular species substituted, the cells became more or less fragile and displayed susceptibility to osmotic shock. They could lose their flexibility and show an altered ion permeability. A surprising finding was that substitution with certain species resulted in a changed cell shape (Fig. 6.20). These results were explained by assuming that replacement of the natural slightly cone-

shaped phosphatidylcholine molecules by the cylindrical dipalmitoyl species would cause the membrane to become convex while enrichment with the cone-shaped dilinoleoyl species would alter the shape to concave (Fig. 6.20).

Sickle cell anaemia is characterized by an amino-acid substitution of the (P-chain of haemoglobin. The erythrocytes have a sickle shape and have changes in their membrane characteristics. Studies with such cells (including experiments on repeated sickling/unsickling) suggest that sickling results from a local uncoupling of the lipid bilayer and the cytoskeleton. This leads amongst other things to a very much increased transbilayer movement (flipflop) of lipids. In agreement with this suggestion it has been noted that membranes that do not contain a strictly organized cytoskeleton, such as the endoplasmic reticulum of rat liver or the plasma membrane of Gram-positive bacteria, also show relatively fast bilayer lipid movement.

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