Effects of phospholipase A2 digestion on the carotenoid and bacteriochlorophyll components of the light-harvesting complexes in Rhodobacter sphaeroides chromatophores

Abstract

The instantaneous electrochromic response of carotenoids associated with the B800-850 light-harvesting complex of Rhodobacter sphaeroides has been used widely as an intrinsic probe of membrane potential. In the present study, the structural basis for this phenomenon was examined by phospholipase A2 digestion of chromatophores from R. sphaeroides strain NF57G, containing B800-850 as the sole pigment-protein complex. The major phospholipase-induced alterations of the overall carotenoid absorption spectrum were characterized by an absorbance loss and a blue shift that were accompanied by a decrease in absorbance at 800 nm and a red shift in the B850 absorbance band. In wild-type chromatophores, the electrochromic carotenoid response induced by both flash illumination and a K+ diffusion potential was diminished by approximately 60% after 1 h of digestion. The initial loss of the carotenoid response was correlated specifically to the hydrolysis of phosphatidylethanolamine, and was shown to arise from effects exerted directly upon the electrochromically active carotenoid pool, possibly by alterations in the spatial relationship between the field-sensitive carotenoids and the polarizing permanent field. In phospholipase A2-digested NF57G preparations in which the B800 band was diminished by nearly half and the carotenoid response was abolished, no significant changes in the efficiency of energy transfer from carotenoids to bacteriochlorophyll were detected at 77 K, suggesting that the electrochromically active carotenoids are not energetically linked to B800 bacteriochlorophyll.

Mutation of a single residue, β‐glutamate‐20, alters protein–lipid interactions of light harvesting complex II

“…It has not been possible to obtain sufficient amounts for the ESI-MS/MS analysis of isolated LH2 αWT/βWT −20A from DG2 strain by detergent extraction from the membrane owing to the mutants' peculiar membrane properties. A possible role of the Car in LH2–lipid interactions has been suggested previously ( Olivera and Niederman, 1993 ). Perhaps in LH2 wt as opposed to αWT/βWT −20A , the surrounding lipids are bound by Glu-20 and the polar moiety of the sphaeroidenone, and therefore not removed by the relatively mild βOG treatment.…”

Isolation, Size Estimates, and Spectral Heterogeneity of an Oligomeric Series of Light-Harvesting 1 Complexes from Rhodobacter sphaeroides

“…All strains were grown on M22+ medium (25); strains NF57, M21, and M2192 were grown semiaerobically (5). In addition, the wild type and strain M21 were grown photoheterotrophically at low light intensity (26). Cells were disrupted in a French pressure cell and pigmented membranes were obtained by rate-zone sedimentation on sucrose density gradients (5); the membranes from strains M21 and M2192, which formed an upper band, were purified further on a second sucrose gradient (27).…”

Selective repression of light harvesting complex 2 formation in Rhodobacter azotoformans by light under semiaerobic conditions

“…The total amount of carotenoids in acetone–methanol extracts was determined as described previously . Relative amounts of spheroidene and spheroidenone were determined as described previously .…”