Abstract
Most photosynthetic LH1 antennae undergo dissociation into B820 subunits, suggesting their universal character as structural modules. However, dissociation into subunits seems to occur reversibly only in the absence of carotenoids and the subunits were never found to bind carotenoids. The interactions of carotenoids with B820 have been studied in a newly developed reconstitution assay of the LH1 antenna from Rhodospirillum rubrum (Fiedor, L., Akahane, J., and Koyama, Y. (2004) Biochemistry 43, 16487-16496). These model studies show that B820 subunits strongly interact with carotenoids and spontaneously form stable LH1-like complexes with substoichiometric carotenoid content. This is the first experimental evidence that B820 may occur as a short-lived intermediate in the assembly of LH1 in vivo. The resulting complex of B820 subunits with carotenoid, termed iB873, is homogeneous, according to ion exchange chromatography and reproducible pigment composition. The iB873-bound carotenoid is as efficient in energy transfer to bacteriochlorophyll as the one in native antenna. To our knowledge, iB873 is the first complex binding functional carotenoid, with the spectral and biochemical properties intermediate between that of B820 and the fully assembled LH1.
Highlights
Most photosynthetic LH1 antennae undergo dissociation into B820 subunits, suggesting their universal character as structural modules
We have studied the effects of carotenoids on B820 subunits via LH1 reconstitution in a newly developed model system [27] and provide evidence for the latter case, i.e. that Crts strongly interact with the subunits and enhance their aggregation
We report on the trapping, for the first time, of a stable LH1-like complex of substoichiometric Crt content, which is a potential intermediate of LH1 assembly
Summary
Reagents and Solvents—Acetone, benzene, methanol, and tetrahydrofuran, all of analytical grade, were purchased from Merck and used as supplied. Rhodopin (Rhd) and anhydrorhodovibrin (Anv) were extracted from the cells of Allochromatium vinosum (D) Both pigments were purified by column chromatography on alumina and recrystal-. The solubilized fraction of the chromatophores was collected as a dark blue supernatant by centrifugation (6000 ϫ g, 45 min, 4 °C) and was loaded onto a small column (4 cm ϫ 0.5 cm) of DEAE-Sepharose (Fast Flow, Amersham Biosciences), pre-equilibrated in 1% -OG (10 mM NaCl/20 mM Tris-HCl buffer, pH 7.8). The fraction eluting with 45 mM NaCl (4 –5 ml) was discarded, and pure B820 was eluted with 75– 85 mM NaCl (ϳ7 ml) as a dark blue band, with a yield of 70 – 80% with respect to the initial amount of detergent-solubilized chromatophores Under these conditions, the RC remained bound to the column. The circular dichroism (CD) measurements were done with a J-710 (JASCO) spectropolarimeter in 5-mm quartz cells at 15 °C
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
