Many biological membranes adapt in response to environmental conditions. We investigated how the composition and architecture of photosynthetic membranes of a bacterium change in response to light, using atomic force microscopy. Despite large modifications in the membrane composition, the local environment of core complexes remained unaltered, whereas specialized paracrystalline light-harvesting antenna domains grew under low-light conditions. Thus, the protein mixture in the membrane shows eutectic behavior and can be mimicked by a simple model. Such structural adaptation ensures efficient photon capture under low-light conditions and prevents photodamage under high-light conditions.

The Tol–Pal system of Escherichia coli is required for the maintenance of outer membrane stability. Recently, proton motive force (pmf) has been found to be necessary for the co‐precipitation of the outer membrane lipoprotein Pal with the inner membrane TolA protein, indicating that the Tol–Pal system forms a transmembrane link in which TolA is energized. In this study, we show that both TolQ and TolR proteins are essential for the TolA–Pal interaction. A point mutation within the third transmembrane (TM) segment of TolQ was found to affect the TolA–Pal interaction strongly, whereas suppressor mutations within the TM segment of TolR restored this interaction. Modifying the Asp residue within the TM region of TolR indicated that an acidic residue was important for the pmf‐dependent interaction of TolA with Pal and outer membrane stabilization. Analysis of sequence alignments of TolQ and TolR homo‐logues from numerous Gram‐negative bacterial genomes, together with analyses of the different tolQ–tolR mutants, revealed that the TM domains of TolQ and TolR present structural and functional homologies not only to ExbB and ExbD of the TonB system but also with MotA and MotB of the flagellar motor. The function of these three systems, as ion potential‐driven molecular motors, is discussed

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Pheromone-binding proteins (PBPs), located in the sensillum lymph of pheromone-responsive antennal hairs, are thought to transport the hydrophobic pheromones to the chemosensory membranes of olfactory neurons. It is currently unclear what role PBPs may play in the recognition and discrimination of speciesspecific pheromones. We have investigated the binding properties and specificity of PBPs from Mamestra brassicae (MbraPBP1), Antheraea polyphemus (ApolPBP1), Bombyx mori (BmorPBP), and a hexa-mutant of MbraPBP1 (Mbra1-M6), mutated at residues of the internal cavity to mimic that of BmorPBP, using the fluorescence probe 1-aminoanthracene (AMA). AMA binds to MbraPBP1 and ApolPBP1, however, no binding was observed with either BmorPBP or Mbra1-M6. The latter result indicates that relatively limited modifications to the PBP cavity actually interfere with AMA binding, suggesting that AMA binds in the internal cavity. Several pheromones are able to displace AMA from the MbraPBP1-and ApolPBP1-binding sites, without, however, any evidence of specificity for their physiologically relevant pheromones. Moreover, some fatty acids are also able to compete with AMA binding. These findings bring into doubt the currently held belief that all PBPs are specifically tuned to distinct pheromonal compounds.

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SummaryThe Tol±Pal system of the Escherichia coli envelope is formed from the inner membrane TolQ, TolR and TolA proteins, the periplasmic TolB protein and the outer membrane Pal lipoprotein. Any defect in the Tol±Pal proteins or in the major lipoprotein (Lpp) results in the loss of outer membrane integrity giving hypersensitivity to drugs and detergents, periplasmic leakage and outer membrane vesicle formation. We found that multicopy plasmid overproduction of TolA was able to complement the membrane defects of an lpp strain but not those of a pal strain. This result indicated that overproduced TolA has an envelopestabilizing effect when Pal is present. We demonstrate that Pal and TolA formed a complex using in vivo cross-linking and immunoprecipitation experiments. These results, together with in vitro experiments with purified Pal and TolA derivatives, allowed us to show that Pal interacts with the TolA C-terminal domain. We also demonstrate using protonophore, K 1 carrier valinomycin, nigericin, arsenate and fermentative conditions that the proton motive force was coupled to this interaction.

We have examined the role of the environment on the interactions between transmembrane helices using, as a model system, the dimerization of the glycophorin A transmembrane helix. In this study we have focused on micellar environments and have examined a series of detergents that include a range of alkyl chain lengths, combined with ionic, zwitterionic, and nonionic headgroups. For each we have measured how the apparent equilibrium constant depends on the detergent concentration. In two detergents we also measured the thermal sensitivity of the equilibrium constant, from which we derive the van't Hoff enthalpy and entropy. We show that several simple models are inadequate for explaining our results; however, models that include the effect of detergent concentration on detergent binding are able to account for our measurements. Our analysis suggests that the effects of detergents on helix association are due to a pair of opposing effects: an enthalpic effect, which drives association as the detergent concentration is increased and which is sensitive to the chemical nature of the detergent headgroup, opposed by an entropic effect, which drives peptide dissociation as the detergent concentration is raised. Our results also indicate that the monomer-monomer interface is relatively hydrophilic and that association within detergent micelles is driven by the enthalpy change. The wide variations in glycophorin a dimmer, stability with the detergent used, together with the realization that this results from the balance between two opposing effects, suggests that detergents might be selected that drive association rather than dissociation of peptide dimers.

Over the last 9 years, the structures of the various components of the bacterial photosynthetic apparatus or their homologues have been determined by x-ray crystallography to at least 4.8-Å resolution. Despite this wealth of structural information on the individual proteins, there remains an urgent need to examine the architecture of the photosynthetic apparatus in intact photosynthetic membranes. Information on the arrangement of the different complexes in a native system will help us to understand the processes that ensure the remarkably high quantum efficiency of the system. In this work we report images obtained with an atomic force microscope of native photosynthetic membranes from the bacterium Rhodospirillum photometricum. Several proteins can be seen and identified at molecular resolution, allowing the analysis and modeling of the lateral organization of multiple components of the photosynthetic apparatus within a native membrane. Analysis of the distribution of the complexes shows that their arrangement is far from random, with significant clustering both of antenna complexes and core complexes. The functional significance of the observed distribution is discussed. I n photosynthesis, highly efficient multiprotein assemblies convert sunlight into chemical potential energy. This process requires several different membrane proteins that funnel light energy to the primary reaction center (RC) and then ensure a cyclic electron transfer chain that converts this energy into an electrochemical potential (1) and, finally, an ATP synthase that is able to store the energy in the phosphodiester bond of ATP (2). A challenge in structural biology is to analyze the structural basis of this efficiency in native membranes. More precisely, the relationship between the different components of the system that ensure efficient energy and electron transfer needs to be determined (3, 4).In photosynthetic bacteria, a large amount of structural information about the individual components of the photosynthetic unit (PSU) is available. The PSU is an assembly made up of the RC associated with the light-harvesting proteins LH1 and LH2, containing chlorophylls and carotenoids. All components cooperate in absorbing light effectively and channeling energy to the RC. In particular, high-resolution structures of two LH2s from Rhodopseudomonas acidophila and Rhodospirillum molischianum and of two RCs from Rhodopseudomonas viridis and Rhodobacter sphaeroides are available (5-9). Electron crystallography data have revealed a hexadecameric assembly of LH1 around the RC in Rhodospirillum rubrum (10, 11). More recently, atomic force microscope (AFM) topographs of native membranes of R. viridis could be acquired. The data unambiguously reported an elliptical hexadecameric arrangement of the LH1 around each RC in a noncrystalline native environment (12). A 4.8-Å x-ray structure of the core complex of Rhodopseudomonas palustris was also elliptical, but with 15 LH1 subunits and one unidentifiable peptide subunit surrounding the RC (13). This str...