Self-assembly of large, ordered lamellae from non-bilayer lipids and integral membrane proteins in vitro

In many biological membranes, the major lipids are "non-bilayer lipids." wh ich in purified form cannot be arranged in a lamellar structure. The struct ural and functional roles of these lipids are poorly understood. This work demonstrates that the in vitro association of the two main components of a membrane, the non-bilayer lipid monogalactosyldiacylglycerol (MGDG) and the chlorophyll-a/b light-harvesting antenna protein of photosystem II (LHCII) of pea thylakoids, leads to the formation of large, ordered lamellar struc tures: (i) thin-section electron microscopy and circular dichroism spectros copy reveal that the addition of MGDG induces the transformation of isolate d, disordered macroaggregates of LHCII into stacked lamellar aggregates wit h a long-range chiral order of the complexes; (ii) small-angle x-ray scatte ring discloses that LHCII perturbs the structure of the pure lipid and dest roys the inverted hexagonal phase; and (iii) an analysis of electron microg raphs of negatively stained 2D crystals indicates that in MGDG-LHCII the co mplexes are found in an ordered macroarray. It is proposed that, by limitin g the space available for MGDG in the macroaggregate, LHCII inhibits format ion of the inverted hexagonal phase of lipids; in thylakoids, a spatial lim itation is likely to be imposed by the high concentration of membrane-assoc iated proteins.