Proteins, chlorophylls and lipids: X-ray analysis of a three-way relationship Paul K. Fyfe 1,3 , Arwel V. Hughes 1 , Peter Heathcote 2 and Michael R. Jones 1 1 Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, Bristol, UK BS8 1TD 2 School of Biological Sciences, Queen Mary, University of London, Mile End Road, London, UK E1 4NS 3 Present address: Division of Biological Chemistry and Molecular Microbiology, Faculty of Life Sciences, University of Dundee, Nethergate, Dundee, UK DD1 4HN Photosynthetic reaction centres and light harvesting complexes have been at the forefront of crystallographic studies of integral membrane proteins. In recent years, there have been spectacular advances in our under- standing of the structure of (bacterio)chlorophyll- containing membrane proteins from oxygenic and anoxygenic phototrophs. In these complex structures, the protein scaffold encases different combinations of cofactors and interacts with several tightly bound lipid species that play a variety of hitherto unrecognized structural roles. Some of these lipids have relevance to the physiological function of the protein, whereas others are important for the formation of highly ordered crystals. The first site-directed mutagenesis studies of individual lipid binding sites have now underlined the importance of the lipid component for the structural stability of protein-cofactor–lipid complexes. X-ray crystallography of protein–cofactor–lipid complexes The (bacterio)chlorophyll-containing membrane proteins that catalyse the photosynthetic process are naturally abundant and are obvious targets for structural biology. In the ten years following the determination of the X-ray crystal structure of the photochemical reaction centre from Blastochloris (Bcl.) viridis [1,2], many of the reported membrane protein structures were either for photosyn- thetic proteins or for proteins from photosynthetic organ- isms, particularly purple bacteria. In the present context, notable achievements were high-resolution structures for the Rhodobacter (Rb.) sphaeroides reaction centre [3–5] and the light-harvesting 2 (LH2) complexes from Rhodo- pseudomonas acidophila [6] and Rhodospirillum moli- schianum [7]. During the past three to four years, attention has turned to integral membrane proteins from oxygenic phototrophs, with the publication of X-ray crystal structures for the cyanobacterial Photosystem I and Photosystem II complexes [8–11], the cyanobacterial and algal cytochrome b 6 f complexes [12,13], and the light harvesting complex-II (LHC-II) antenna from spinach [14]. As well as showing the beauty and complexity of these protein–cofactor systems, the X-ray data have revealed the structures and interactions of several bound lipids. This crystallographic work has brought atomic-level insights into the question of how lipids ensure the optimum performance of photosynthetic complexes, complementing an extensive literature on the biochem- istry of protein–lipid interactions in photosynthesis [15]. There is also great interest in the role played by lipids in the structure and mechanism of bacteriorhodopsin and related light-driven ion pumps [16–18], but for reasons of space we have limited this review to (bacterio)chlorophyll- containing photosynthetic proteins. Living organisms vary the lipid composition of their membranes, not just between organisms or under differ- ent environmental conditions but also between individual membranes within cells. The precise reasons for this are often unclear, although lipids are generally regarded as conferring additional stability upon the proteins embedded in them [19,20]. Because many embedded membrane pro- teins present a highly irregular surface to the surrounding lipids, the electrically sealed membrane required for sustaining the proton motive force generated by photo- synthetic proteins is often achieved using a mixture of bilayer-forming and non-bilayer lipids. One aim of crystallographic work on these proteins has been to understand the roles played by tightly bound lipids. How do they influence the structure of the protein: do they simply provide the environment of the protein or do they affect structure in a more direct way? Do they interact with the cofactors, perhaps modulating their properties? Do the structurally characterized lipids explain the biochemistry of protein–lipid interactions? Finally, ever pertinent to integral membrane proteins is the question of how individual lipids affect protein packing in membrane protein crystals. The proteins discussed in this review are members of the small minority of membrane proteins that have been successfully crystallized and therefore provide rare and valuable insights into how lipids can contribute to the formation of highly ordered crystals. The purple bacterial reaction centre – a structure– function workhorse The purple bacterial reaction centre is arguably the most intensively studied membrane protein in nature [21] and certainly one of the best understood (Figure 1). The publication of the X-ray crystal structure w20 years ago [1–3] coincided with the development of mutagenesis Corresponding author: Jones, M.R. (m.r.jones@bristol.ac.uk). Available online 10 May 2005 Review TRENDS in Plant Science Vol.10 No.6 June 2005 www.sciencedirect.com 1360-1385/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2005.04.007