Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b(6)f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc(1), cytochrome b(6)f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 A of cytochrome b(6)f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc(1) but also exhibits some unique features, such as binding chlorophyll, beta-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.

Helical membrane protein folding and oligomerization can be usefully conceptualized as involving two energetically distinct stages-the formation and subsequent side-to-side association of independently stable transbilayer helices. The interactions of helices with the bilayer, with prosthetic groups, and with each other are examined in the context of recent evidence. We conclude that the two-stage concept remains useful as an approach to simplifying discussions of stability, as a framework for folding concepts, and as a basis for understanding membrane protein evolution.

Amphipols are a new class of surfactants that make it possible to handle membrane proteins in detergent-free aqueous solution as though they were soluble proteins. The strongly hydrophilic backbone of these polymers is grafted with hydrophobic chains, making them amphiphilic. Amphipols are able to stabilize in aqueous solution under their native state four well-characterized integral membrane proteins: (i) bacteriorhodopsin, (ii) a bacterial photosynthetic reaction center, (iii) cytochrome b 6 f, and (iv) matrix porin.Integral membrane proteins usually are extracted from membranes and are kept soluble in aqueous solutions using detergents (1, 2). Detergent molecules equilibrate between a monolayer covering the transmembrane surface of the protein (3, 4), free monomers, and protein-free micelles. The presence of free micelles is a source of difficulty in membrane protein biochemistry and biophysics. It can induce, for instance, protein inactivation caused by the dissociation of subunits, lipids, or hydrophobic cofactors, phase separation during crystallization attempts, and an increased viscosity of the solutions in NMR experiments. We have endeavored to develop a new class of polymers, ''amphipols,'' that are designed to keep membrane proteins soluble in water in the absence of free surfactant. High molecular weight (MW) congeners of amphipols are known to exhibit a high affinity for hydrophobic particles (5-9). We show here that low MW amphipols can keep soluble under their native state four integral membrane proteins: (i) bacteriorhodopsin (BR), (ii) the photosynthetic reaction center from Rhodobacter sphaeroides R-26 (RC), (iii) the cytochrome b 6 f complex from Chlamydomonas reinhardtii, and (iv) the matrix porin (OmpF) from Escherichia coli.