In many biological membranes, the major lipids are ''non-bilayer lipids,'' which in purified form cannot be arranged in a lamellar structure. The structural 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-a͞b light-harvesting antenna protein of photosystem II (LHCII) of pea thylakoids, leads to the formation of large, ordered lamellar structures: (i) thin-section electron microscopy and circular dichroism spectroscopy reveal that the addition of MGDG induces the transformation of isolated, disordered macroaggregates of LHCII into stacked lamellar aggregates with a long-range chiral order of the complexes; (ii) small-angle x-ray scattering discloses that LHCII perturbs the structure of the pure lipid and destroys the inverted hexagonal phase; and (iii) an analysis of electron micrographs of negatively stained 2D crystals indicates that in MGDG-LHCII the complexes are found in an ordered macroarray. It is proposed that, by limiting the space available for MGDG in the macroaggregate, LHCII inhibits formation of the inverted hexagonal phase of lipids; in thylakoids, a spatial limitation is likely to be imposed by the high concentration of membrane-associated proteins.circular dichroism ͉ chloroplast thylakoid membranes ͉ electron microscopy ͉ light-harvesting complex ͉ lipid-protein interactions T he lamellar organization of biological membranes provides a structural matrix for various proteins and controls the permeability of organic molecules, water, and ions; it also prevents nonspecific protein-protein aggregation, whereas it allows protein diffusion and conformational changes in the membrane. However, biomembranes usually contain substantial amounts of non-bilayer lipids, which in purified form assume nonlamellar structures. In fact, in many membranes, e.g., thylakoid membranes of chloroplasts, membranes of Escherichia coli, rhodopsin, and mitochondria, non-bilayer lipids constitute about half or more of the total lipid content. It is well established that the physical and functional properties of these membranes depend to a large extent on protein-lipid interactions (1, 2). There are a few examples showing that lipid polymorphism can be modulated by proteins, and, in some cases, small unilamellar vesicles can be reconstituted from non-bilayer lipids and membrane proteins (e.g., refs. 3 and 4). However, the structural role of large amounts of non-bilayer lipids has remained enigmatic, and the assembly of extended bilayer lamellae from proteins and predominantly non-bilayer lipids is poorly understood (1, 2, 5). In this work, we use a simple system, the two main constituents of pea thylakoid membranes, purified non-bilayer lipids and isolated protein complexes, to demonstrate that the formation of a large, ordered lamellar structure is possible even in the presence of large amounts of lipids.In chloroplast thylakoid membranes of g...