Proteomic Analysis of the Developing Intracytoplasmic Membrane in Rhodobacter sphaeroides During Adaptation to Low Light Intensity
Although the primary photochemical events in the facultative photoheterotrophic purple bacterium Rhodobacter sphaeroides are now well understood, comparatively little is known about how their photosynthetic apparatus is assembled. Here we present a proteomic analysis of the intracytoplasmic membrane (ICM) assembly process during adaptation to lowered light intensity, in which the size of the photosynthetic units is greatly expanded by addition of the light-harvesting 2 (LH2) peripheral antenna complex. When the isolated ICM-derived chromatophore vesicles were subjected to clear native gel electrophoresis (CNE), four pigmented bands appeared; the top and bottom bands contained the reaction center – light-harvesting 1 (RC–LH1) core complex and LH2 peripheral antenna, respectively, while the two bands of intermediate migration contained associations of the LH2 and core complexes. Proteomic analysis revealed a large array of other proteins associated with the CNE gel bands – in particular, several F1FO-ATP synthase subunits gave unexpectedly high spectral counts, given the inability to detect this coupling factor, as well as the more abundant cytochrome bc 1 complex, by atomic force microscopy. Significant levels of general membrane assembly factors were also found, as well as numerous proteins of unknown function including high counts for RSP6124 that were correlated with LH2 levels. When combined with further AFM and spectroscopic studies, these proteomic approaches are expected to provide a much-improved understanding of the overall assembly process.
KeywordsSpectral Count Reaction Center Complex Peripheral Antenna Lithium Dodecyl Sulfate Bacterium Rhodobacter Sphaeroides
This work was supported by Supported by US Department of Energy Grant No. DE-FG02-08ER15957 from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science. We thank Oluwatobi B. Olubanjo for assistance with membrane purification procedures and Prof. Peter Lobel and Dr. Haiyan Zheng of the Center for Advanced Biotechnology and Medicine, University of Medicine and Dentistry of New Jersey, for conducting the proteomics analysis.
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