Heterogeneity of photosynthetic membranes from Rhodobacter capsulatus: Size dispersion and ATP synthase distribution |
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Authors: | Francesca Gubellini Francesco Francia Daniel Lévy B Andrea Melandri |
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Institution: | a Department of Biology, University of Bologna, Laboratory of Biochemistry and Biophysics, Via Irnerio, 42 I-40126 Bologna, Italy b Institut Curie, UMR-CNRS 168 and LRC-CEA 34V, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France c Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Italy |
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Abstract: | The density distribution of photosynthetic membrane vesicles (chromatophores) from Rhodobacter capsulatus has been studied by isopicnic centrifugation. The average vesicle diameters, examined by electron microscopy, varied between 61 and 72 nm in different density fractions (70 nm in unfractionated chromatophores). The ATP synthase catalytic activities showed maxima displaced toward the higher density fractions relative to bacteriochlorophyll, resulting in higher specific activities in those fractions (about threefold). The amount of ATP synthase, measured by quantitative Western blotting, paralleled the catalytic activities. The average number of ATP synthases per chromatophore, evaluated on the basis of the Western blotting data and of vesicle density analysis, ranged between 8 and 13 (10 in unfractionated chromatophores). Poisson distribution analysis indicated that the probability of chromatophores devoid of ATP synthase was negligible. The effects of ATP synthase inhibition by efrapeptin on the time course of the transmembrane electric potential (evaluated as carotenoid electrochromic response) and on ATP synthesis were studied comparatively. The ATP produced after a flash and the total charge associated with the proton flow coupled to ATP synthesis were more resistant to efrapeptin than the initial value of the phosphorylating currents, indicating that several ATP synthases are fed by protons from the same vesicle. |
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Keywords: | ICM intracytoplasmic membranes Bchl bacteriochlorophyll ATPase ATP synthase (EC 3 6 3 14) F1 soluble portion of the ATP synthase LH1 LH2 light harvesting complex 1 2 RC reaction center PL phospholipid EM electron microscopy QELS quasi elastic light scattering LDAO lauryldimethylamine oxide Δψ transmembrane difference of electrical potential ΔpH transmembrane difference of pH Tricine N-[2-hydroxy-1 1-bis(hydroxymethyl)ethyl] glycine SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis |
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